ABSTRACT
Title of dissertation: TEMPORAL DYNAMICS
OF HOT-ELECTRONS
IN METAL FILMS AND
ALLOYS
Sarvenaz Memarzadeh
Doctor of Philosophy, 2021
Dissertation directed by: Professor Jeremy N. Munday
Department of Electrical Engineering
When light is coupled into a surface plasmon mode, it can either decay radia-
tively by emitting a photon or non-radiatively by transferring its energy to charge
carriers with excessive kinetic energy, also known as the ?hot-carriers.? The pho-
togenerated hot-carriers are promising for applications ranging from optoelectronic
devices to renewable energy. For example, recently, hot-carrier-based solar cells have
emerged as a next generation solar energy converter, which utilizes the photoexcited
hot-carriers and offers simplicity of design and higher power conversion efficiency
when compared to first-generation photovoltaic cells such as the silicon. Over the
past decades, there have been significant efforts to increase the efficiency of hot-
carrier-based devices by introducing novel approaches for generating these energetic
carriers. It has been found that the hot-carrier relaxation time also plays a crucial
role in determining the efficiency of these devices. Further, the fast thermalization
process of hot-carriers is the primary loss mechanism in hot-carrier devices. Thus,
to maximize the device efficiency, we need to prolong the hot-carrier relaxation time
before any thermalization process takes place, which leads to heat generation and
hence efficiency loss of such devices. For other devices, e.g. ultrafast photodetec-
tors, a short lifetime may be beneficial. Thus, the ability to control the hot-carrier
lifetime is important.
In this dissertation, we first focus on measuring the hot-carrier lifetime in
metal films and then offer new approaches for controlling the relaxation time of the
excited hot-carriers. For the measurement, we develop our degenerate pump-probe
spectroscopy setup using a Ti:Sapphire pulsed laser, enabling us to measure the
ultrafast temporal response of the generated hot-carriers in the optical frequency
range. Next, we look at the effect of the propagating surface plasmons on the
relaxation dynamics of the excited carriers in a thin gold film. Furthermore, to
analyze the temporal dynamics and extract the relaxation time from the pump-probe
measurements, we combine the internal electric field profile resulting from surface
plasmon coupling with the conventional two-temperature model. Our results show
that coupling to the propagating surface plasmon enhances the hot-carrier relaxation
time due to the electric field confinement within a gold film. Finally, we explore the
relaxation time of the excited hot-carriers in AuAg and AuCu alloys with different
material compositions. For this purpose, we fabricated thin films with different
Au, Ag, and Cu compositions through the sputtering deposition process. We found
that different alloy compositions affect the relaxation time, and in the case of the
AuAg alloyed thin films, it can vary up to 8 times under constant pump fluency.
These results provide new approaches for controlling the hot-carrier relaxation time
depending on the applications.
TEMPORAL DYNAMICS OF HOT-ELECTRONS IN
METAL FILMS AND ALLOYS
by
Sarvenaz Memarzadeh
Dissertation submitted to the Faculty of the Graduate School of the
University of Maryland, College Park in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
2021
Advisory Committee:
Professor Jeremy N. Munday, Chair/Advisor
Professor Thomas E. Murphy
Professor Mohammad Hafezi
Professor Cheng Gong
Professor John Cumings
? Copyright by
Sarvenaz Memarzadeh
2021
Dedication
Dedicated to my beloved parents, Sima and Mehdi.
ii
Acknowledgments
There are numerous thanks to be given upon the completion of this disser-
tation. First, I would like to give my sincere thanks to my advisor, Prof. Jeremy
Munday, for his support and insightful guidance throughout this dissertation. With-
out his countless support and patience, this dissertation could not be completed. I
would also like to give a very special thanks to Prof. Thomas Murphy for giving me
the opportunity to acquire many skills in his laboratory. Prof. Murphy?s relentless
support goes beyond, and I am deeply grateful for having the opportunity to per-
form the ultrafast measurements in his laboratory. I also would like to appreciate
my other committee members, Prof. Hafezi, Prof. Gong, and Prof. Cumings for
their valuable time and suggestions. I owe a special thanks to Dr. Yigit Aytac
for his valuable advice and help to build the setup, and Dr. Jongbum Kim for his
insightful discussions and comments on the modeling section. I would also like to
express my appreciation to everyone in the ECE graduate and business office, es-
pecially Melanie Prange, Emily Irwin, and Vivian Lu, who made sure I have my
financial support and insurance during my Ph.D. Of course, all my great friends
and colleagues in MundayLab and Photonic Research Laboratory my thanks go out
continually for your support. On a personal note, I would like to sincerely thank my
parents, Mehdi and Sima, and my dearest brother Alireza and sister-in-law Tanaz
for all their love. Finally, I would like to thank everyone in the Maryland Nanocenter
- FabLab, Mark Lecates, Jonathan Hummel, John Abrahams, and Tom Loughran
for their training and helps. This work was supported by the National Science
iii
Foundation under Grant No. (CAREER ECCS-1554503, MMN-2016617, MMN-
1609414), Office of Naval Research YIP (N00014-16-1-2540), Wells Fellowship, and
UMD Graduate School Summer Research Fellowship.
iv
Table of Contents
Preface ii
Foreword ii
Dedication ii
Acknowledgements iii
Table of Contents v
List of Tables vii
List of Figures viii
LIST OF ABBREVIATIONS xii
Publications xiii
Chapter 1: Introduction 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Surface plasmons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.1 Methods of SPP excitation . . . . . . . . . . . . . . . . . . . . 3
1.3 Hot-electron generation with surface plasmon coupling . . . . . . . . 4
1.4 Hot-carrier cooling mechanisms . . . . . . . . . . . . . . . . . . . . . 5
1.5 Dissertation outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Chapter 2: Experimental method for hot-electron relaxation time measure-
ments 8
2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Spot size measurement . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4 Pump-probe measurement procedure . . . . . . . . . . . . . . . . . . 13
2.5 Polarization dependence of degenerate pump-probe signal . . . . . . . 15
2.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Chapter 3: Surface plasmon assisted control of hot-electron relaxation time 19
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
v
3.3 Experimental and numerical measurements of SPP excitation in Au . 22
3.4 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.5 Hot-electron relaxation time analysis . . . . . . . . . . . . . . . . . . 28
3.5.1 Free electron model . . . . . . . . . . . . . . . . . . . . . . . . 29
3.5.2 Modified two-temperature model . . . . . . . . . . . . . . . . 31
3.5.3 Fitting procedure and error bar calculation . . . . . . . . . . . 36
3.6 Effect of electric field enhancement on relaxation time . . . . . . . . . 37
3.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Chapter 4: Control of hot-carrier relaxation time in Au-Ag thin films through
alloying 41
4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.3 Fabrication of AuAg alloyed samples . . . . . . . . . . . . . . . . . . 44
4.4 Optical and material characterization of AuAg alloys . . . . . . . . . 45
4.5 Surface plasmon coupling in the alloyed samples . . . . . . . . . . . . 48
4.6 AuAg alloys and time-resolved differential reflectively measurements . 48
4.7 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Chapter 5: Hot-carrier temporal dynamics in AuCu alloy 56
5.1 Sample fabrication and optical measurements . . . . . . . . . . . . . 56
5.2 Hot-carrier temporal dynamics in AuCu alloys . . . . . . . . . . . . . 57
5.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Chapter 6: Conclusion and future directions 60
6.1 Hot-carrier temporal dynamics in non-metallic materials . . . . . . . 60
6.2 Hot-carrier temporal dynamics in TiN sample . . . . . . . . . . . . . 61
6.3 X-ray diffraction microscopy on AuAg samples . . . . . . . . . . . . . 62
6.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Appendix A: 64
A.1 Fabrication procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
A.2 Ellipsometry data of AuAg alloys . . . . . . . . . . . . . . . . . . . . 64
A.3 Ellipsometry data of AuCu alloys . . . . . . . . . . . . . . . . . . . . 67
A.4 Ellipsometry data of TiN . . . . . . . . . . . . . . . . . . . . . . . . . 69
Bibliography 71
vi
List of Tables
3.1 List of the ellipsometry parameters at room temperature. . . 30
3.2 Double exponential fitting parameters . . . . . . . . . . . . . . 35
4.1 AuAg alloyed fabrication recipes . . . . . . . . . . . . . . . . . 45
A.1 AuCu alloyed fabrication recipes . . . . . . . . . . . . . . . . . 67
vii
List of Figures
1.1 Generation of the hot-electrons upon the decay of the propagating
surface plasmon in a thin metal film. This decay results in the gen-
eration of the excited hot-electrons with a higher temperature than
the ambient temperature. Due to the e-e interactions, electrons equi-
librate among themselves to a hot-electron distribution which is de-
scribed by the Fermi distribution. Subsequently, e-ph interactions
result in the cooling of the hot-electrons to the lattice temperature. . 5
2.1 Experimental setup of the pump-probe measurement. . . . . . . . . . 9
2.2 Pump pulse width measured with an autocorrelator. The Gaussian
fit shows the pulse width of 150 fs. . . . . . . . . . . . . . . . . . . . 11
2.3 Pump power spectrum measured after parabolic mirror and before
the sample. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4 Experimental setup with a gold-coated prism. . . . . . . . . . . . . . 14
2.5 Plasmon linewidth measurement for a 50 nm gold film (blue dots)
and the spectral measurement of the Ti-Sapphire laser (red line). . . 15
2.6 Pump-probe signals recorded for a 50 nm Au film deposited on a right
angle prism while coupled to the propagating surface plasmon at 700
nm wavelength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.7 Coupling to the propagating surface plasmon of a 50 nm Au film
under the Kretschmann configuration using both pump and probe
beams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.8 Polarization dependence of the pump-probe differential reflectivity
signal for a 50 nm gold film deposited on a right angle prism. The
resonance wavelength is 725 nm. . . . . . . . . . . . . . . . . . . . . . 17
3.1 (a) Schematic of light coupling to propagating surface plasmons using
the Kretschmann configuration. (b) Absorption measurement (cir-
cles) and simulation (solid line) after surface plasmon coupling. . . . . 23
3.2 (a) Simulation and (b) measurement absorption for the gold sam-
ple while coupling to the propagating surface plasmon for different
incident wavelengths. In (c), the solid line (simulation) and dots (ex-
periment) are extracted from the computed mesh plots in (a) and (b)
at 745 nm resonance wavelength, respectively. . . . . . . . . . . . . . 24
3.3 Schematic of the pump-probe experimental setup. . . . . . . . . . . . 25
viii
3.4 (a) Schematic diagram showing hot-electron excitation under res-
onance and off-resonance wavelengths while keeping the absorbed
power fixed (120 mW) for both illuminations. (b) Schematic dia-
gram showing a second case where the hot-electron excitation occurs
under the same resonance wavelength (745 nm) but with different ab-
sorbed powers. ?1, ?2 and ?3 are the corresponding electron-phonon
relaxation time for these different cases. . . . . . . . . . . . . . . . . . 26
3.5 (a) Relative reflectivity change for different incident wavelengths rang-
ing from 730 nm to 775 nm measured at fixed absorbed power (120
mW). Resonance wavelength is distinguished by a green frame from
the rest of the wavelengths. (b) Relative reflectivity signals under
the fixed 745 nm resonance wavelength measured with the different
absorbed powers (50 mW, 90 mW, 120 mW, 150 mW). . . . . . . . . 27
3.6 From left to right, dependency of the elecctron effective mass, plasma
frequency, and Drude damping factor on electron temperature in gold. 29
3.7 Numerically calculated real and imaginary part of the permittivity
function at 300 K and 800 K temperature. . . . . . . . . . . . . . . . 31
3.8 (a) Differential reflectivity contour plot computed from the free elec-
tron model and transfer matrix methods. Hot-electron temperature
as a function of the delay time between pump and probe beams under
(b) fixed (120 mW) absorbed power and (c) fixed resonance wave-
length (745 nm). The solid lines are the calculated electron temper-
atures, and the open circles are the electron temperatures obtained
from our differential reflectivity measurements. . . . . . . . . . . . . . 33
3.9 Electric field profiles normalized by the intensity of the input field
and the electric field at resonance wavelength of 745 nm. Profiles are
computed from the FDTD simulation. . . . . . . . . . . . . . . . . . 34
3.10 NMSE plots for different wavelengths. Resonance wavelength is at
745 nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.11 Converted pump-probe data to the electron temperature for 10 differ-
ent wavelengths with their corresponding best two-temperature model
fits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.12 Effect of field enhancement on relaxation time due to the surface
plasmon coupling under the fixed (120mW) and variable (50 mW,
90 mW, 120 mW, 150 mW) absorbed powers. Experimentally mea-
sured hot-electron relaxation time under (a) fixed and (b) variable
absorbed powers. Field enhancement computed from the FDTD sim-
ulation for wavelengths ranging from 730 nm to 775 nm under the (c)
fixed and (d) variable absorbed powers. The electric field profiles are
normalized by the intensity of the input field. . . . . . . . . . . . . . 38
4.1 Chemical and structural properties of thin films. (a) EDX. (b) AFM
topography. (c) Roughness distribution. Insets show RMS roughnesses. 46
ix
4.2 Measured (a) real and (b) imaginary parts of the permittivity, and
(c) computed quality factor of the propagating surface plasmon of
AuxAg1?x alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.3 (a) Experimental and (b) simulated reflectivity for Au, Au98Ag2,
Au65Ag35, and Au25Ag75 alloys under p-polarized illumination with
wavelengths ranging from 680 nm to 740 nm. For FDTD simulations,
we used pulse illumination with 150 fs pulse width. . . . . . . . . . . 49
4.4 Differential reflectivity measurements for Au-Ag alloys with different
chemical compositions. For each sample, the pump power is: 120 mW
(a,f,k,p), 150 mW (b,g,l,q), 180 mW (c,h,m,r), 210 mW (d,i,n,s), and
240 mW (e,j,o,t). Insets are real-color photographs of the alloyed thin
films. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.5 Temperature converted differential reflectivity measurements for Au100,
Au98Ag2, Au65Ag35, and Au25Ag75 alloys under different incident
pump powers of 120 mW (a,f,k,p), 150 mW (b,g,l,q), 180 mW (c,h,m,r),
210 mW (d,i,n,s), and 240 mW (e,j,o,t). The black solid lines in each
plot show the best fits computed from a modified two-temperature
model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.6 Hot-carrier relaxation time as a function of 1/i, the inverse of the
imaginary part of the permittivity, for AuxAg1?x. The solid lines are
the linear fit between the hot-carrier lifetime and 1/i. The colors
represent the range of the pump power between 120 mW (red) to 240
mW (purple). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
5.1 The (a) real, (b) imaginary part of the dielectric function of AuxCu1?x
alloys, and (c) their corresponding surface plasmon quality factor
(Qspp) with x = 100, 70, 57, and 54. The results are determined from
fits to the spectroscopic ellipsometry data. The composition of each
sample is measured by energy-dispersive X-ray spectroscopy (EDX). . 57
5.2 (Top row) Experimental and (bottom row) numerical simulation of
surface plasmon polariton excitation of AuxCu1?x alloys with x =
100, 70, 57, and 54. The wavelength range is from 680 nm to 740 nm. 57
5.3 Transient differential reflectivity measurements of Au70Cu30 alloy at
different pump powers (210 mW to 300 mW) while coupling to the
propagating surface plasmon. Pump and probe wavelengths are 700
nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
6.1 Real (red) and imaginary (blue) parts of the measured dielectric func-
tion of 22 nm TiN fabricated by pulsed laser deposition. The permit-
tivity data is obtained from the ellipsometry fit (see figure A.5). . . . 62
x
A.1 Ellipsometry data: Delta (green line) and Psi (red line) for four dif-
ferent alloy mixtures, (a) Au, (b) Au98Ag2, (c) Au65Ag2, and (d)
Au25Ag75 at five different incident angles with their corresponding
best fits (dashed lines). We use the GenOsc model to fit the data.
Both optical properties and thickness are obtained from the fits. . . 66
A.2 Transmission spectra obtained from the spectroscopic ellipsometry
measurements on AuAg alloyed film on glass. The solid red lines
show the ellipsometry data and the dotted black lines indicate the fit
on data using a B-spline model. . . . . . . . . . . . . . . . . . . . . . 67
A.3 Ellipsometry data and fit on the AuCu alloys. . . . . . . . . . . . . . 68
A.4 Transmission spectra obtained from the spectroscopic ellipsometry
measurements on AuCu alloyed film on a glass. Transimission peak
shift from 550 nm for the pure Au to about 600 nm for the sample
with higher Cu percentage. . . . . . . . . . . . . . . . . . . . . . . . . 69
A.5 Ellipsometry data (solid lines) and a model fit (dashed lines) on a 22
nm TiN film on Si substrate fabricated by pulsed laser deposition. . . 70
xi
List of Abbreviations
CMOS Complementary-metal?oxide?semiconductor
EDS Electron density of states
EDX Energy dispersive X-ray spectroscopy
ENZ Epsilon near zero
FDTD Finite difference time domain
HEB Hot-electron bolometer
LSP Localized surface plasmon
NIR Near infrared
NMSE Normalized mean square error
PML Perfectly match layer
PVD Physical vapor deposition
SNOM Scanning near-field microscopy
SP Surface plasmon
SPP Surface plasmon polariton
TM Transverse magnetic
TMM Transfer matrix methods
TTM Two-temperature model
UV Ultraviolet
XRD X-ray diffraction
xii
List of Publications
? S. Memarzadeh, J. Kim, Y. Aytac, T. E. Murphy, J. N. Munday, ?Surface
plasmon assisted control of hot-electron relaxation time?, Optica 7(6), 608-612
(2020).
? S. Memarzadeh, K. J. Palm, T. E. Murphy, J. N. Munday, ?Control of hot-
carrier relaxation time in Au-Ag thin films through alloying?, Optics Express,
28, 33528-33537 (2020).
? T. Gong, P. Lyu, K. J. Palm, S. Memarzadeh, J. N. Munday, and M. S.
Leite, ?Emergent Opportunities with Metallic Alloys: From Material Design
to Optical Devices?, Advanced Optical Materials, 2001082, 1-21 (2020).
xiii
Chapter 1: Introduction
1.1 Motivation
Absorption of incident photons within a conductive material can result in the
generation of highly energetic, non-thermal carriers, also known as ?hot-carriers?.
In recent years, generation of photo-excited hot-carriers has been extensively inves-
tigated for applications such as photodetection in NIR [1, 2], hot-electron bolometer
(HEB) [3, 4], photothermoelectric effects in graphene for THz detection [5, 6], photo-
catalysis for deriving chemical reactions such as in artificial photosynthesis [7], and
water splitting [8, 9]. However, the efficiency of the hot-carrier-based devices used
in the aforementioned applications extensively relies on the generated hot-carriers?
temporal dynamics. Thus, understanding the temporal response of the hot-carriers
can improve the design of hot-carrier devices. For example, a significant amount
of the incident solar energy in solar cells dissipates quickly due to the rapid decay
of generated hot-carriers, which is not converted to usable electric energy in tra-
ditional photovoltaic cells. This process limits the harvest of the hot-carriers and
results in low power conversion efficiency. By comparison, hot-carrier solar cells
can, in principle, be much more efficient with theoretical values of 66% for uncon-
centrated sunlight, and (? 85%) at the maximum concentration (46,200 suns) [10].
1
The main goal of this dissertation is to search for new approaches to control the
relaxation time of excited hot-carriers. For this purpose, we utilize an ultrafast
time-resolved spectroscopy method to measure the ultrafast response of the excited
hot-carriers.
1.2 Surface plasmons
Surface plasmons (SP) are the coherent oscillations of electrons at the interface
between two materials, typically metal and dielectric, with different signs in their
dielectric functions. Compared to the incident photons, SPs are shorter in wave-
length, more tightly confined spatially, and have a higher field intensity. Generally,
surface plasmons are divided into two classes: localized surface plasmons (LSPs)
and surface plasmon polaritons (SPPs), with the dispersion relation expressed as
[11] ( )1/2
dm
ksp = k0 , (1.1)
d + m
where k0 is the wave vector in free space, d is the permitivitty of the dielectric
material, and m is the metal dielectric function. For LSPs, the incident photons
interact with a conductive nano-structure, leading to strong local fields rather than
propagation. For SPPs, once the light couples into the surface plasmons mode,
it propagates on the metal-dielectric interface and attenuates after a propagation
distance of [11] ( )3/2
1 c ? +  ? 2d ( )
?sp = =
m m
2k?? ?
(1.2)
sp ? md 
??
m
2
with k??spp defined as the imaginary part of the complex surface plasmon wave vector,
? as the real and ??m m imaginary part of the dielectric function of the metal, and d
as the permittivity of the dielectric material. SPPs are surface waves with electro-
magnetic fields that are evanescent on both sides of the metal/dielectric interface.
The surface plasmon propagation distance for gold at 700 nm wavelength is about
5 ?m. They are excited under certain conditions (i.e., specific incident energy for
a particular incident angle), which satisfy the momentum matching between the
incident photons and the propagating SPP. Surface plasmons have applications in
biomolecular studies and biosensors [12, 13], imaging [14], and spectroscopy [15]. In
addition, they are several excellent review articles and books [16, 17, 18, 19, 20].
1.2.1 Methods of SPP excitation
In 1902, Wood made the first observation of the surface plasmon resonance
through the uneven spectrum of the diffracted light reflected from a metallic diffrac-
tion grating [21]. Otto then demonstrated that the drop in the reflectivity from
the attenuated total reflection (ATR) method in a silver film in close proximity to a
prism was due to surface plasmon coupling [22]. In 1968, Kretschmann proposed an-
other attenuated total reflection method in silver, today known as the Kretschmann
configuration (or prism coupling technique), which resulted in a similar drop in the
reflectivity due to surface plasmon coupling in a silver coated prism [23]. Both Otto
and Kretschmann configurations enable coupling through a high index dielectric
prism. This material enables the increase in the wave vector of the incident light
3
to match with the SPP wave vector. The only difference in the Otto configura-
tion is light first tunnels within a small dielectric gap (air) before reaching the SPP
mode. A complementary approach to excite the SPPs can be made by decreasing
the SPP wave vector by replacing a typical dielectric at the metal/dielectric in-
terface with a dielectric that has a real part of the refractive index lower than 1
[24, 25]. This approach results in a prism-free direct coupling to SPPs. There are
other approaches besides these conventional methods, such as using a SNOM probe
technique or using a grating structure [26, 27]. Throughout this dissertation, we
employed the Kretschmann configuration (prism coupling technique) for excitation
of the propagating surface plasmons.
1.3 Hot-electron generation with surface plasmon coupling
Electrons that are not in thermal equilibrium with the material?s atoms are
known as the hot-electrons. In recent years, due to the advancement in nanoscale
system designs and fabrication, hot-carrier generation studies have rapidly expanded
due to the ease. In such cases, the excited surface plasmons decay either radia-
tively by emitting photons or non-radiatively through the generation of the energetic
electron-hole pairs (i.e., hot-carriers) via Landau damping. This hot-carrier excita-
tion can even be followed by the photoemission process if the excited hot-carrier has
higher energy than the work function of the material. In the non-radiative case, the
generated electron-hole pairs have higher energy than the carriers closer to the Fermi
energy, resulting in a broad distribution of the carriers above the Fermi energy, as
4
shown schematically in Figure 1.1.
E E E
?
e- e-e-
Hot e- Hot e-
e- e- E E E
e- F F F
Plasmon excitation and decay to 
hot-electrons thot-electrons e-e and e-ph interactions
 (fs to ps)
Figure 1.1: Generation of the hot-electrons upon the decay of the propagating sur-
face plasmon in a thin metal film. This decay results in the generation of the excited
hot-electrons with a higher temperature than the ambient temperature. Due to the
e-e interactions, electrons equilibrate among themselves to a hot-electron distribu-
tion which is described by the Fermi distribution. Subsequently, e-ph interactions
result in the cooling of the hot-electrons to the lattice temperature.
1.4 Hot-carrier cooling mechanisms
Carrier cooling is a multistep process. In a nanostructured plasmonic system,
this cooling occurs first by plasmon dephasing, which happens on the order of 10s
of femtosecond. The next step is the inelastic electron-electron scattering, typically
in the order of 100s of femtoseconds. The third step, which happens in a longer
time scale, typically on several picoseconds, is the process of carriers scattering
with phonons (electron-phonon scattering). The optical phonons emitted by the
excited carriers then interact with other phonons, which may decay into a low energy
acoustic phonon and result in heat dissipation, which occurs in 10-100 ps [17, 28].
Energy dissipation to the surrounding medium occurs via phonon-phonon coupling
and induces high local temperatures that can destroy cancerous cells [29]or distill
5
organic solvents[30].
Ultrafast transient transmission or reflection spectroscopy is an effective tool
to study the hot-carrier cooling dynamics. Previous studies have been performed
on the ultrafast dynamics of noble metal films [31], single nanoparticles, and en-
sembles. For example, Hu et al. showed that the rate of energy dissipation in Au
nanoparticles depends on their size; smaller particles have faster relaxation time
[32]. Zijlstra et al. presented the first acoustic vibration measurements of a single
gold nanorod with an average size of 90 nm ? 30 nm using pump-probe spectroscopy
[33]. Ultrafast temporal dynamic studies are not only limited to the noble metals.
Other materials, such as aluminum nanostructures have gained a lot of interest for
hot-carrier studies because of their low cost, abundance, CMOS compatibility, and
capability of supporting tunable resonances that span the entire visible spectrum.
Su et al. found that, unlike the gold nanostructures, the ultrafast optical response of
aluminum nanodisks is more sensitive to the lattice temperature than the electron
temperature [34]. Li et al. observed a hot-carrier cooling lifetime as slow as 32 ps
in perovskite nanocrystals; that is much longer than those reported for other semi-
conductor bulk or nanomaterials (e.g., for GaAs thin films, the reported hot-carrier
cooling lifetime is about 2 ps)[35].
1.5 Dissertation outline
This dissertation is divided into six chapters. The first chapter is the introduc-
tion and background information. Chapter 2 discusses the details of the experimen-
6
tal setup developed for the hot-carrier relaxation time measurement. Chapters 3, 4,
and 5 cover the effect of propagating surface plasmons and metallic alloys as the two
studied external factors to control the relaxation dynamics of excited hot-carriers.
Final thoughts and future work is discussed in Chapter 6.
7
Chapter 2: Experimental method for hot-electron relaxation time
measurements
2.1 Overview
Many processes, including the molecular vibration, emission and absorption
of photons, and scattering phenomena, take place in a very fast time scale. Some
of them may occur as fast as a femtosecond (10?15s) temporal range. While these
phenomena are too fast to be observed using conventional cameras or detectors,
one can employ ultrafast lasers to stimulate and probe the response of materials
with femtosecond resolution, which leads to a better understanding of the physics of
light-matter interactions. In this chapter, we discuss the design of our experimental
setup in detail. We also include some of our measurements in this chapter for
completeness.
2.2 Experimental setup
The purpose of time-resolved pump-probe spectroscopy is to measure the
changes in the reflectivity or transmission of a lower power ?probe? pulse induced
from a high-power ?pump? pulse as a function of the time delay between the two.
8
M4 M3 M1
M5 M6M7
M9 M8 BS1 M2
M10 Ti-Sapphire laser
M11
Mechanical delay line
pump probe
PM
M12 BS2
pinhole
Power meter
Figure 2.1: Experimental setup of the pump-probe measurement.
These changes in the differential reflectivity measurements of the probe exhibit
themselves in the form of:
?R(?) Rwith pump ?Rw/o pump
= (2.1)
R Rw/o pump
Our laser source is an ultrafast Ti-Sapphire (Ti : Al2O3) pulsed laser with
an 80 MHz repetition rate, tunable between 680 nm to 1060 nm. In this setup,
shown in Figure 2.1, the Ti-Sapphire output separates into pump and probe paths
right after the beam splitter (BS1). The pump beam is then sent to a mechanical
delay line and modulated by an optical chopper with 600 Hz frequency. The lock-
in amplifier is synchronized with the frequency of the chopper and captures the
9
transient change in the probe beam reflectivity. Furthermore, both pump and probe
pulses need to overlap in space and coincide in time when they reach the sample.
According to the Ti-Sapphire specification, the output of the laser is p-polarized
(TM); however, still, linear polarizers are placed in the path of both beams to make
them fully polarized. We also measured the laser pulse width by directing a portion
of the beam to the APE autocorrelator. The result of this measurement is shown in
Figure 2.2. Gaussian fit to the recorded data demonstrates a pulse width of ?150
fs at 800 nm.
The measurements are performed over ?6 ps delay time produced by moving
the mechanical delay stage for 1mm. The final measurements are reported after
scanning the delay line multiple times and averaging the time traces. The laser
average power at 800 nm is 3.2 W, and the pump power is measured ?630 mW
right before the sample. Figure 2.3 shows the full pump power spectrum at the
same position right before the sample.
10
0.02
0.006
0
-2.35 0 2.48
Tiime  ((pss))
Figure 2.2: Pump pulse width measured with an autocorrelator. The Gaussian fit
shows the pulse width of 150 fs.
700
600
500
400
300
200
100
0
600 700 800 900 1000 1100
(nm)
Figure 2.3: Pump power spectrum measured after parabolic mirror and before the
sample.
11
Intensity (a.u.)
Ppump (mW)
2.3 Spot size measurement
The main challenge in designing the pump-probe optical setup is the spatial
and temporal overlapping of the two beams at the sample surface. A 20-micron
pinhole size is used to overlap the two beams spatially. These beams are first aligned
entirely parallel to each other and directed vertically to a gold-coated parabolic
mirror. The pinhole position is then adjusted using a 3D translational stage to
maximize the light transmitted through the pinhole. The ratio of the intensity
before and after the pinhole can be used to estimate the spot size and can be
computed based on the Gaussian beam profile assumption [36] as follows:
?r2/w2E(r) = E 00e (2.2)
Where E0 is the normalized field and w0 is the radius at which the amplitude
drops to the 1/e of the peak value. The intensity is also Gaussian and is expressed
as follow,
2
? 2r
I(r) = I w0e 0 (2.3)
The intensity before (Ibefore) and after (Iafter) the pinhole can be calculated by
taking an integral from the above equation,
? ?
? 2r
2 ?w2
I w 0before = I0e 0 2?rdr = (2.4)
0 2
? d/2 2 2
? 2r ?w 2 2
I wafter = I0e 0 2?rdr =
0 (1? e?d /2w0) (2.5)
0 2
12
Here d is the diameter of the pinhole. Thus, spot size (w0) can be obtained
using the following equation
2
I dafter ?
= 1? e 2w20 . (2.6)
Ibefore
2.4 Pump-probe measurement procedure
Once the two beams overlap and the smallest spot size is acquired by adjusting
the pinhole?s position, we remove the pinhole and place the prism at the same place.
The prism location is controlled by moving a 3D stage, and the overlap of the two-
beam is confirmed using a microscope objective with a AmScope MU1000 digital
camera. Figure (2.4) shows our experimental setup when the prism is replaced
instead of the pinhole. The prism is also located on a rotational stage to adjust the
surface plasmon coupling?s incident angle. Both rotational stage and mechanical
delay line are controlled with LabVIEW (National Instruments) software. As both
pump and probe beams are p-polarized and collide on the gold-coated prism?s surface
with a small incident angle difference, both beams contribute to the propagating
surface plasmons. However, the final angle is adjusted to the probe beam coupling,
that results in a minimum reflected signal. The reflected probe beam is then directed
to the silicon photodetector, and its output is fed into the lock-in amplifier.
Measurements of the surface plasmon linewidth and the laser spectral width
are shown in Fig. (2.5). We then measured the pump-probe signal from a 50 nm
gold-coated prism by manually sweeping the wavelength from 680 nm to 720 nm
13
M4 M3
M9
M10 M5 M6 M1
M8 M7
M11 BS1
M2
pump ND filters
probe
PM
M12
BS2
To the detector
Prism
Gold side
Rotational stage
camera
Figure 2.4: Experimental setup with a gold-coated prism.
with a 5 nm increment under the fixed pump power (Figure (2.6)). Here, the prism
position is adjusted to have the maximum surface plasmon coupling at 700 nm. The
wavelength adjustment happens through an external laser knob and is confirmed
with the manufacturer software, connected to the built-in laser spectrometer via a
USB. The full-width half maximum of the captured reflectivity signal for the 50 nm
Au film deposited on a prim is ?50 nm, and out of this range, the signal almost
vanishes. For these measurements, the pump power is set to 200 mW and the probe
power is 20 mW.
14
FWHM = 50nm
5 nm
Figure 2.5: Plasmon linewidth measurement for a 50 nm gold film (blue dots) and
the spectral measurement of the Ti-Sapphire laser (red line).
?10R-4!/"10 R at P = 150mW R/R at P pump= 150mW??/?
0 pump
6800 0 680 0
6-9
690
-550 -5 -5
700 700
7--1100 ?! = 700?? -10 710 -10
? = 680??
720 ? = 720?? 720
-4
0 2 4 6 10-4 0 2 4 6 10
tt ((ppss) ) t (ps)
Figure 2.6: Pump-probe signals recorded for a 50 nm Au film deposited on a right
angle prism while coupled to the propagating surface plasmon at 700 nm wavelength.
2.5 Polarization dependence of degenerate pump-probe signal
Only the p-polarized waves will couple to the surface plasmon, and so adjust-
ing the polarization will affect whether the surface plasmon is excited. Thus, we
placed a half-wave plate in front of each beam to investigate the pump polarization
dependency and probe measurements on the transient reflectivity signal. The half-
15
 ?(nR?m//R)?
 (nm)
wave plate controls the pump polarization from fully p to s polarized beam. For
this measurement (?pump = ?probe = 725 nm), the pump and probe powers are 224
mW and 3.8 mW, respectively.
Figure (2.7) shows our gold film normalized reflectivity measurements de-
posited on a right-angle prism upon both pump and probe coupling to the propa-
gating surface plasmon (both p-polarized). Here, we used a manual rotational stage
with an angular resolution of 0.2 degrees. According to this figure, the difference
between the two beams? coupling angle is 6 0.2 degrees at 725 nm. Therefore, it
is a reasonable assumption to consider that both beams couple to the propagating
surface plasmon under approximately the same coupling angle.
1
Rprobe
0.8 Rpump
0.6
0.4
42 44 46 48
 (?)
Figure 2.7: Coupling to the propagating surface plasmon of a 50 nm Au film under
the Kretschmann configuration using both pump and probe beams.
Under the resonance angle, the differential reflectivity signal is recorded for
both p and s polarized pump beam while keeping the probe beam polarization
unchanged (p-polarization). Figure 2.8 illustrates the captured signal for such mea-
16
R (a.u.)
surements. The recorded signal is then fitted with double exponential function;
the result of the longer decay which is due to the electron-phonon interactions are
?p = 1.62?0.19ps and ?s = 1.40?0.14ps, respectively. Here, ?p and ?s are the relax-
ation time of the p and s-polarized pump beam. The hot-electron relaxation time
varies ?0.2 ps by changing the pump polarization direction. Also, the amplitude
of the recorded signal reduced significantly (?10 times smaller), as depicted in Fig.
(2.8). Furthermore, we observed that the probe beam decoupling due to the po-
larization change results in no pump-probe signal in our measurements? sensitivity
range.
0
S
-0.2
-0.4 P
-0.6 Pump polarization
-0.8
-1
0 1 2 3 4 5 6
t(ps)
Figure 2.8: Polarization dependence of the pump-probe differential reflectivity signal
for a 50 nm gold film deposited on a right angle prism. The resonance wavelength
is 725 nm.
17
R (a.u.)
2.6 Conclusion
In summary, we discussed the experimental design of our degenerate pump-
probe set up. We incorporated the surface plasmon coupling stage into our pump-
probe setup to enhance the signal amplitude. In the next chapter, we examine the
effect of the surface plasmon coupling on the hot-electron relaxation time.
18
Chapter 3: Surface plasmon assisted control of hot-electron relaxation
time
3.1 Overview
In the previous chapter, we discussed the detail of our pump-probe experimen-
tal design. This chapter focuses on incorporating the propagating surface plasmon
with our pump-probe measurements and investigating its importance on the hot-
electrons relaxation time.
3.2 Introduction
Recently, the optical generation of hot-carriers in metallic components has at-
tracted interest for applications such as solar energy conversion [37, 38, 39, 40, 41],
non-linear optics [42, 43, 44], sensitive photodetectors [45, 46, 47, 48], nanoscale
heat sources [49], photochemical reactions in biomolecular studies [50, 51, 52], and
biosensors [53, 54]. For the excitation of hot-carriers in metals, the incident photon
energy is typically lower than the energy of the band-to-band transition, thus the
efficiency of hot-carrier generation is reduced as a result of the poor absorption of
light within the metals. To overcome this limitation, surface plasmons have been
19
broadly utilized to enhance absorption through the use of metallic nanostructures
[55, 56, 57, 58, 59], which increase the measurement sensitivity because of the in-
creased absorption [60].
As we discussed in chapter 1, hot-carriers relax to equilibrium through plasmon
dephasing via Landau damping, electron-electron (e-e) scattering, electron-phonon
(e-ph) scattering, and lattice heat dissipation through phonon-phonon (ph-ph) in-
teractions [61]. Throughout these processes, hot-carriers can distribute their energy
to the surrounding environment and in turn thermalize from their excited state to
equilibrium. The temporal duration of hot-carrier relaxation is the key factor to
determine the performance of hot-carrier devices. For example, the efficiency of
hot-carrier injection in energy conversion systems [41, 62] and the operating speed
in optical modulation systems [63, 64] are both strongly linked to hot -carriers?
lifetime. Depending on the geometry of metal nanostructures, the materials? band
structure, and the incident photons? energy [57], the relaxation time can vary from
a few hundred femtoseconds up to a couple of picoseconds. In the case of gold
and aluminum nanostructures, relaxation times on the order of hundreds of picosec-
onds, due to the acoustic vibrations of the lattice, have been reported [65, 66, 67].
The effect of enhanced absorption on hot-carrier relaxation time has been exten-
sively studied in the case of the thin film and nano-structured plasmonic systems
[68, 69, 70]; however, the importance of the strongly confined field inside the metal
thin film induced by surface plasmon coupling on hot-carrier lifetimes is still elusive.
Transient reflectivity measurements using pump-probe spectroscopy are a common
method to characterize carrier dynamics under the intra-band or inter-band tran-
20
sitions. Typically, the measured transient signals for pump-probe spectroscopy are
analyzed with the Two-Temperature Model (TTM), which describes the spatiotem-
poral profile of the electron and the lattice temperature from a coupled nonlinear
partial differential equation [71, 72, 73]. This model is very useful in understanding
relaxation dynamics, but appropriate modification is needed for an accurate mod-
eling of the unique internal electric field profile in metal films due to its coupling to
the propagating surface plasmon.
In this chapter, we experimentally investigate the relationship between the hot-
carrier relaxation time and the characteristics of surface plasmons on gold (Au) thin
films excited under the Kretschmann configuration. For accurate theoretical mod-
eling of the transient reflectivity data resulting from the carrier dynamics in the
conduction band of Au thin film, we employ the free electron model to estimate the
elevated electron temperature due to intra-band optical pumping. From the calcu-
lated electron temperature, we extract the carrier relaxation time with the modified
TTM to better describe the localized electric field distribution inside the Au thin
film. Under fixed absorbed power in the Au film over the spectral range of 730
nm to 775 nm (resonance wavelength at 745 nm), we observe that the hot-electron
relaxation time in the Au film reaches its maximum at the resonance wavelength,
which indicates that the modified intensity and profile of the internal electric field by
the excitation of surface plasmons plays a significant role in hot-carrier relaxation.
21
3.3 Experimental and numerical measurements of SPP excitation in
Au
For experimental measurements, we use a precise motorized rotational mount
with 25 arcsecond angular resolution to couple to the propagating surface plasmon
under the Kretschmann configuration (Fig. 3.1a). The incoming beam from the
glass interface is focused on the Au side of the prism using an off-axis parabolic
mirror. Both reflection and transmission are recorded while rotating the prism
on the stage. Transmission of the gold film is measured to be less than 1% and,
therefore, negligible for determining the absorption (A = 1?R) measurement. We
optimize our absorption measurement using a bare prism first, without any Au
coating, to incorporate the possible scattering effects from every prism interface.
This helps to measure the baseline of the reflection signal. For the range of angles
and wavelengths employed, the bare prism exhibits total internal reflection at 45
degrees. The Au-coated prism then replaces the bare prism on the rotational stage
for the surface plasmon coupling. The reflection signal is recorded over the incident
angle for the various pump wavelengths. The final reported signal is the ratio
between the reflectivity measured using the gold-coated and uncoated prisms (Fig.
3.2b).
We employ the transfer matrix method for the numerical calculation of absorption
(Fig. 3.2a). The thickness of the Au film and the incident angle of light are set
to 44 nm and 44?, respectively. Under these conditions, surface plasmon excitation
22
occurs at 745 nm (1.66 eV), where the photon energy is lower than the d-band
transition of Au, at 2.4 eV [74]. Once the surface plasmon is excited in the Au film,
the electric field is strongly confined at the interface between the Au film and air.
Figure 3.1b shows absorption as a function of wavelength ranging from 730 nm to
775 nm, with resonance wavelength at 745 nm. Coupling to the propagating surface
plasmon results in the maximum absorption of 85%. The numerical (solid line) and
experimental results (dots) of the absorption spectrum in a broader range (from 700
nm to 800 nm) are shown in Figure 3.2c.
(a) (b) 0.9
|E|2
P 2abs= 1/2 ??  |E|?
0.8
z ? 0.7res
0.6
730 740 750 760 770
? (nm)
Figure 3.1: (a) Schematic of light coupling to propagating surface plasmons using
the Kretschmann configuration. (b) Absorption measurement (circles) and simula-
tion (solid line) after surface plasmon coupling.
23
Absorption
(a) Simulation (b) Experiment
700
0.8 700 00..88
800 0.6 ?!"# 0.6
LOREM 
750
0.4
900 ? 0.4!"#
0.2 0.2
1000 800
42 44 46 48 42 44 46 48
(L?o)rem ipsum (?)
(c) 1 42
0.8
0.6
0.4
0.2
0
700 800 900 1000
 (nm)
Figure 3.2: (a) Simulation and (b) measurement absorption for the gold sample
while coupling to the propagating surface plasmon for different incident wavelengths.
In (c), the solid line (simulation) and dots (experiment) are extracted from the
computed mesh plots in (a) and (b) at 745 nm resonance wavelength, respectively.
3.4 Experimental procedure
As discussed in chapter 2, we utilize a degenerate pump-probe technique for the
time-resolved differential reflectivity measurements, once coupled to the propagating
surface plasmon of gold at 745 nm resonance wavelength. The simplified schematic
of the setup is shown in Fig. 3.3. Here, transverse-magnetic (TM) polarized pulses
are produced from a femtosecond high-power Ti-Sapphire laser with an 80 MHZ
repetition rate. Using a beam splitter, the incoming pulses are then separated into
pump and probe paths. Both beams are directed to coincide on the gold surface after
24
 (nm)
Absorption
 (nm)
Absorption
reflecting off the off-axis parabolic mirror to a spot size of approximately 40 ?m.
After spatially separating the two beams, the probe beam is then directed to the
Si photodetector for differential reflectivity measurements. The time delay between
the pump and probe pulses are produced by passing the pump beam through the
mechanical delay stage.
Si detector
O-axis parabolic Pabs
mirror Lens 0.5
LP
BS
Ti-Sapphire Probe
Pump mirror
Right angle prism
Mechanical 
delay stage 
Figure 3.3: Schematic of the pump-probe experimental setup.
To rule out the effect of absorbed light power in the control of the hot-carrier
relaxation temporal dynamics, we designed two different experimental conditions:
1) sweeping the wavelength (? = 730 ? 775 nm) with the fixed absorbed power
(Pabs=120 mW),
2) varying the absorbed laser power (Pabs=50 ? 150 mW) with the fixed
wavelength (?=745 nm).
25
Z 
1
0
Figure 3.4a and 3.4b schematically illustrate hot-electron excitation under
these two conditions.
(a) (b)
?1  = ?res ? ?
?
2  = ?res 2 ? 31  = ?res ?2  = ?res
?1 ?1
Ec Ec
E Ef f
Pabs(?1)=Pabs(?2) Pabs(?1)?Pabs(?2)
Figure 3.4: (a) Schematic diagram showing hot-electron excitation under resonance
and off-resonance wavelengths while keeping the absorbed power fixed (120 mW)
for both illuminations. (b) Schematic diagram showing a second case where the
hot-electron excitation occurs under the same resonance wavelength (745 nm) but
with different absorbed powers. ?1, ?2 and ?3 are the corresponding electron-phonon
relaxation time for these different cases.
Transient reflectivity (?R/R0) measurements as a function of time delay (?t)
between the pump and probe for both conditions are shown in Figure 3.5. When
the wavelength was varied, we adjust the incident pump intensity according to the
absorption spectra (Fig. 3.1b to ensure that the absorbed power remains the same
over the entire incident wavelength range. We observe that the transient reflectivity
(?R/R0) reaches the maximum at resonance, and signal modulation is gradually
reduced as the wavelengths tend away from resonance. For the case of fixed wave-
length illumination, the input power is varied (59 mW, 105 mW, 141 mW and 176
mW) at the resonance wavelength.
26
(a) -4
10 5
0 ? = 730 nm ? = 735 nm ? = 740 nm ?res = 745 nm ? = 750 nm
-5
-10
-15
-4-20
10 5
0 ? = 755 nm ? = 760 nm ? = 765 nm ? = 770 nm ? = 775 nm
-5
-10
-15
-20
0 2 4 0 2 4 0 2 4 0 2 4 0 2 4
?t (ps) ?t (ps) ?t (ps) ?t (ps) ?t (ps)
(b) -4
10 5
Pabs= 50 mW Pabs= 90 mW Pabs= 120 mW Pabs= 150 mW0
-5
-10
-15
-20
0 2 4 0 2 4 0 2 4 0 2 4
?t (ps) ?t (ps) ?t (ps) ?t (ps)
Figure 3.5: (a) Relative reflectivity change for different incident wavelengths rang-
ing from 730 nm to 775 nm measured at fixed absorbed power (120 mW). Resonance
wavelength is distinguished by a green frame from the rest of the wavelengths. (b)
Relative reflectivity signals under the fixed 745 nm resonance wavelength measured
with the different absorbed powers (50 mW, 90 mW, 120 mW, 150 mW).
27
Pabs= 120 mW
R/R R/R
?res = 745nm
R/R
3.5 Hot-electron relaxation time analysis
To extract the hot-electron relaxation time, we develop a model using a com-
bination of the free electron and modified two-temperature models. Our model is
based on converting the transient reflectivity measurements (?R/R0) to electrons
temperature under the intra-band optical pumping assumption, which results in a
non-equilibrium hot electron distribution that can modify the optical properties of
the Au film. The Au band structure is modelled using a simplified parabolic electron
density of states [75].
Considering that the carrier density is a temperature independent quantity and
the intra-band excitation does not generate extra carriers in the conduction band
(N 22 ?3epump?=Nenopump= 4.92?10 cm ), we can calculate the Drude plasma frequency
(? = (e2p Ne)/(0m)) and damping coefficient (?p = ~e/(m?e)) as a function of
the electron temperature (detail in the following section). Here, Ne is the carrier
concentration,  = 8.854 ? 10?120 F/m is the permitivity of free space, m is the
dimensionless effective electron mass, and ?e is the electron mobility. Figure 3.6
shows the dependency of the above parameters on electron temperature. Based on
our calculation, we obtained the plasma frequency of gold equal to 7.845 eV at room
temperature.
As numerically depicted in figure (3.6), and experimentally shown in Reddy et
al.[76], when the inter-band transition is insignificant, the temperature dependencies
of the optical properties are mainly due to the change in the plasma frequency (?p),
effective mass (m), and Drude damping (?) parameters. As the temperature is
28
1.1002 7.8475 0.16554
1.1 7.847 0.16552
1.0998 7.8465 0.1655
1.0996 7.846 0.16548
1.0994 7.8455 0.16546
1.0992 0.16544
500 1000 1500 500 1000 1500 500 1000 1500
Te (k) Te (k) Te (k)
Figure 3.6: From left to right, dependency of the elecctron effective mass, plasma
frequency, and Drude damping factor on electron temperature in gold.
raised the effective mass in metals decreases which is in agreement with previous
reports [77].
3.5.1 Free electron model
To calculate the changes in the permittivity function due to the intra-band
optical pumping, we use a free electron model assuming a parabolic density of states.
Starting with a constant value of the carrier density at room temperature, Ne(T =
300K) = 4.92? 1022(1/cm3), which is obtained from the ellipsometry fits, using the
following equation [78]
?
1 ?
( ) 1
m 2T=300K 2mT=300KE
Ne(T = 300K) = fo(?T=300K, T )dE, (3.1)
?2 ~2 ~20
the chemical potential at room temperature can be computed as ?T=300K = 4.4526eV.
In the above equation, fo is the Fermi-Dirac distribution, and mT=300K = 1.1eV is
the effective electron mass at room temperature.
Under the assumption of a fixed chemical potential level and the same carrier den-
sity as for the intra-band optical pumping (i.e.Ne(T ) = Ne(T = 300K)), we can
29
meff
p(eV)
 (eV)
implicitly extract the effective electron mass at a higher temperature m(T ) from
[78]: ?
1 ?
( ) 1
m(T ) 2m(T )E 2
Ne(T ) = fo(?, T )dE. (3.2)
?2 ~20 ~2
Finally, permittivity as a function of the electron temperature can be calculated in
terms of the summation between the Drude term (w.r.t to the carrier density (Ne)
and mobility (?) as the fitting parameters) and the Lorentz term according to [79]:
?~2e2N ? ?e n AnBnEnn
(?, T ) = ? + + , (3.3)
o(?nm(T ) + iq~E) En2 2n ? E ? iE.Bn n
in which ? is the high-frequency dielectric constant, 0 is the vacuum dielectric
constant, ~ is the reduced Planck?s constant, e is the electron charge, ? is the
carrier mobility, A is the amplitude of oscillation, En is the center energy, B is
the broadening amplitude, and n is the number of oscillators. Here, the number of
oscillators used in the Drude and Lorentz terms are 1 and 2, respectively. All the
fitting parameters obtained from the ellipsometry measurement at room temperature
are listed in table 3.1.
Parameters Values
A1 2.0093
A2 6.1582
B1 0.6250 (eV)
B2 2.8199 (eV)
En1 2.9580 (eV)
En2 4.1990 (eV)
2
?1 6.353 (
cm )
V.S
? 3.1990
Table 3.1: List of the ellipsometry parameters at room temperature.
30
Subsequently, the change in the reflectivity with electron temperature over
different incident wavelengths can be determined from the above permittivity func-
tion and by using the Transfer Matrix Methods (TMM) calculation. According to
our numerical modeling, temperature variation causes less than 0.1% variation in
the real and imaginary part of the permittivity function (i.e. ?r = 0.03% and
?I = 0.08%) (Fig. 3.7).
-10 4.1
3.71
4 3.705 T = 800 K
3.7
-12 3.9 3.695 T = 300 K3.69
3.685
3.8
-14 745 746 747?(?(n?m))
-13.88 3.7T = 300 K
-13.9
-16 3.6
-13.92 T = 800 K
745 745.1 745.2 745.3 3.5
?((?nm?) )
-18
680 700 720 740 760 780 800 3.4680 700 720 740 760 780 800
?((?n?m)) ?(?(?nm))
Figure 3.7: Numerically calculated real and imaginary part of the permittivity func-
tion at 300 K and 800 K temperature.
3.5.2 Modified two-temperature model
Quantitative theoretical research on modeling the nonequilibrium dynamics
started after demonstrating the first generation of the mode-locked lasers in the early
60s. Soon after, and through rapid growth in the femtosecond lasers? applications in
material studies, Anisimov et al. [72] proposed the conventional Two-Temperature
Model (TTM), which describes the interaction of the short lasers pulse with two
subsystems, electron and lattice. This model introduces the spatiotemporal (2D)
temperature distribution of electron and lattice through solving two coupled non-
31
?r!
?r?
?I"
I?"
linear differential equations. Here, the main difference between the conventional
two-temperature model and the modified two-temperature model is that in the sec-
ond case, the absorbed power profile of the coupled surface plasmon is included in
the source term definition of the coupled equations.
The general format of the two-temperature model is as follow:
?Te
C 2e(Te) = Ke? Te ?G(Te ? Tl) + S(z, t) (3.4)
?t
?Tl
Cl = G(Te ? Tl)
?t
2
where Te and Tl are electron and lattice temperature [80, 81], C (T ) =
? Nekb
e e (k T /E )2 b e f
and Cl = 2.5? 106Jm?3K?1 are the electron and lattice heat capacities [80, 82], Ef
and kb are the Fermi level and Boltzmann constant, K = 315 Wm
?1
e K
?1 is the
electron thermal conductivity, G = Ce(Te)/?e?ph is the electron-phonon coupling
coefficient within the weak perturbation approximation with ?e?ph as the electron-
phonon relaxation time. Under the weak perturbation regime (Te  T ? 104f ),
Tf is the Fermi temperature, the electrons? heat capacity is much smaller than the
lattice heat capacity; this makes the lattice temperature relatively constant with
respect to the electrons? temperature.
Figure 3.8b and 3.8c show the converted electron temperature as a function
of time delay for both fixed absorbed power with varied wavelengths, and for fixed
resonance wavelength with varied absorbed powers. The converted electron temper-
atures is modelled using the modified TTM.
In the conventional two temperature model, the skin depth of a material is
32
?R/R
(a) -310 -1.5 -1 -0.5 0
760
740
720
700
300 400 500 600 700 800
Te(K)
(b) 1000 ? = 775 nm ?res = 745 nm ? = 765 nm ? = 775 nm
800 Pabs= 120 mW P  = 120mW Pabs= 120 mW Pabs  = 120 mWabs
600
400
(c) 1000 Pabst= ( p5s0) mW Pabs= 90 mW Pabst= ( p1s2)0 mW Pabst= ( p1s5)0 mW
800 ?res = 745 nm ?res = 745 nm ?res = 745 nm ?res = 745 nm
600
400
0 2 4 0 2 4 0 2 4 0 2 4
?t (ps) ?t (ps) ?t (ps) ?t (ps)
Figure 3.8: (a) Differential reflectivity contour plot computed from the free elec-
tron model and transfer matrix methods. Hot-electron temperature as a function
of the delay time between pump and probe beams under (b) fixed (120 mW) ab-
sorbed power and (c) fixed resonance wavelength (745 nm). The solid lines are the
calculated electron temperatures, and the open circles are the electron temperatures
obtained from our differential reflectivity measurements.
33
Te(K) Te(K)
?(nm) 
simply applied to the laser heating source term (S(z, t)) to model the laser interac-
tion with the material as a function of depth. Instead, in our modified version, we
change the source term by using the decaying length of the confined electric field
of the surface plasmon at both sides of the interface instead of skin depth of the
Au. Furthermore, using the Finite Difference Time Domain (FDTD) simulation,
the electric field profile is numerically computed within the sample throughout the
range of the wavelengths. To keep the total absorbed power constant, we vary the
input power accordingly. Figure 3.9 shows the results of the normalized electric
field profiles over the range of the incident wavelengths. In this figure, the black
line corresponds to the electric field profile under no surface plasmon excitations
(? = 745 nm, ? = 0o).
1
0.8  = 730 nm = 735 nm
 = 740 nm
0.6  = 745 nm = 750 nm ? = 44o
 = 755 nm
0.4  = 760 nm = 765 nm
 = 770 nm
0.2  = 775 nm = 745 nm ? = 0o
?
0 10 20 30 40
z(nm)
Figure 3.9: Electric field profiles normalized by the intensity of the input field and
the electric field at resonance wavelength of 745 nm. Profiles are computed from
the FDTD simulation.
The calculated field is fitted with double exponential terms, including the de-
caying field at the Au/prism interface and the decaying field at the Au/air interface.
34
|E |2 2max / |E o|
|E |2res
The modified source term (S(z,t)) to incorporate the absorbed power profile inside
the Au film can be described as:
S(z, t)?= (3.5)
? (1?R)? a1
( e?z/b
a
1 2+ e(z?d)/b2)e??((t?2tp)/tp)
2
? tp b1 b2
where tpis the laser pulse width, ? is the laser fluence, d is the sample thickness and
? = 4 ln(2) [81]. a1 and a2 correspond to the intensity of electric field at Au/air
and Au/prism, and b1 and b2 correspond to the decaying length of electric field at
Au/air and Au/prism, respectively.
Excitation ?(nm) a1 a2 b1(nm) b2(nm)
730 0.997 0.137 15.191 12.061
735 1.014 0.149 15.370 13.222
740 1.018 0.163 15.544 14.349
745 1.019 0.179 15.692 15.412
750 1.007 0.195 15.827 16.346
755 0.985 0.210 15.958 17.147
760 0.955 0.224 16.075 17.815
765 0.919 0.236 16.075 18.333
770 0.880 0.248 16.302 18.730
775 0.835 0.257 16.425 19.028
Table 3.2: Double exponential fitting parameters
Using our experimental conditions with the modified TTM, we numerically
calculate the electron temperature as displayed in Fig. 3.8b and 3.8c. For the
case of constant absorbed power, we show four wavelengths and their corresponding
best fits on the relaxation time to preserve space. We also incorporate the spatial
dependence of the electron temperature by averaging the temperature profiles along
the z direction. The result of the fits is shown in Fig. 3.8b and 3.8c based on
35
the Normalized Minimum Squared Error (NMSE) calculation for the hot-electron
relaxation time (Fig. 3.10).
3.5.3 Fitting procedure and error bar calculation
Best fits are selected according to the calculation of the Normalized Mean
Square Error (NMSE) between the measured and calculated temperature data ob-
tained from the modified two-temperature model. The fitting parameter is the
electron-phonon relaxation time (?e?ph = Ce/G). Here, the error bars in the hot-
electron relaxation time are derived from the 95% confidence bounds calculation on
the fitted coefficient. Complete set of the fits for the on- and off-resonance wave-
lengths are shown in Fig. 3.11.
?!"# = 0.75 ?? ?!"# = 1.05 ?? ?!"# = 1.4 ?? ?!"# = 1.4 ?? ?!"# = 0.75 ??
?!"# = 0.7 ?? ?!"# = 0.7 ?? ?!"# = 0.7 ??
?!"# = 0.35 ?? ?!"# = 0.3 ??
!!"#$ (#$) !!"#$ (#$) !!"#$ (#$) !!"#$ (#$) !!!!""##$$((##$$))
Figure 3.10: NMSE plots for different wavelengths. Resonance wavelength is at 745
nm.
36
 Pabs= 120 mW
800
700 ? = 730 nm ? = 735 nm ? = 740 nm ? = 745 nm ? = 750 nm
600
500
400
300
800
700 ? = 755 nm ? = 760 nm ? = 765 nm ? = 770 nm ? = 775 nm
600
500
400
300
0 2 4 0 2 4 0 2 4 0 2 4 0 2 4
t (ps) t (ps) t (ps) t (ps) t (ps)
Figure 3.11: Converted pump-probe data to the electron temperature for 10 different
wavelengths with their corresponding best two-temperature model fits.
3.6 Effect of electric field enhancement on relaxation time
Figure 3.12a and 3.12b present the extracted hot-carrier relaxation time for
both cases of fixed absorbed power and fixed illumination wavelength. When the
incident power is varied while coupling to the surface plasmon, the hot-carrier re-
laxation time increases linearly with increasing incident pump power (Fig. 3.12b).
However, when the absorbed power is held constant and the internal field intensity
profile is varied (i.e. the amount of surface plasmon coupling is varied), we find
that the hot-carrier relaxation time is strongly dependent on the intensity of the
electric field (see the trend of hot-carrier relaxation time in Fig. 3.12a and the
normalized maximum intensity of electric field in Fig. 3.12c). This result confirms
that the surface plasmon coupling can enhance the hot-carrier relaxation time in
37
T(k) T(k)
Figure 3.12: Effect of field enhancement on relaxation time due to the surface plas-
mon coupling under the fixed (120mW) and variable (50 mW, 90 mW, 120 mW, 150
mW) absorbed powers. Experimentally measured hot-electron relaxation time un-
der (a) fixed and (b) variable absorbed powers. Field enhancement computed from
the FDTD simulation for wavelengths ranging from 730 nm to 775 nm under the (c)
fixed and (d) variable absorbed powers. The electric field profiles are normalized by
the intensity of the input field.
38
the Au film with high field confinement as well as the increase of the light ab-
sorption in the Au film. Notably, we can more effectively increase the hot-carrier
relaxation time with the local electric field enhancement than with increasing the
input power. We achieve approximately a doubling of the hot-carrier relaxation
time with only a ?3.5% increase in electric field intensity (normalized to the input
field) at the metal/air interface through SP coupling. Although, the hot-electron
relaxation time and the corresponding electron-phonon coupling factor have exten-
sively been studied as a function of the elevated electrons? temperature [83, 84],
the effect of the electric field confinement on the relaxation time has not fully been
determined. Furthermore, we hypothesize that the electric field confinement could
affect the reabsorption rate of the non-equilibrium phonon population due to a bot-
tleneck effect. The increase in the reabsorption rate leads to the reduction of the
thermalization rate and enhances the hot-electron?s relaxation time, which has also
been observed in case of other high density materials [85]. Consequently, this feature
suggests that electric field confinement helps to excite free electrons to higher energy
states, and these non-equilibrium hot-electrons take longer to relax via a series of
electron-phonon scattering processes.
3.7 Conclusion
In summary, we have experimentally demonstrated the impact of propagating
surface plasmon excitation on the hot-carrier relaxation time through the use of a
degenerate pump-probe technique under the Kretschmann configuration. We intro-
39
duce an approach to analyse the unique internal field confinement in Au thin films
with surface plasmon coupling by modifying the two-temperature model. It?s worth
mentioning that our heat equation does not account for the propagating portion of
the surface plasmon, which can in principle deposit optical energy outside of the
illuminated area. From the comparison study between the constant absorbed pump
power and the constant electric field, we determine that the electric field confine-
ment results in the generation of long-lived hot electrons in the Au thin film. Our
results provide a foundation for the design of efficient plasmonic systems to tailor
hot-carrier lifetime with low power consumption in hot-carrier based optoelectronic
devices.
40
Chapter 4: Control of hot-carrier relaxation time in Au-Ag thin films
through alloying
4.1 Overview
The plasmon resonance of a structure is primarily dictated by its optical prop-
erties and geometry, which can be modified to enable hot-carrier photodetectors
with superior performance. Recently, metal-alloys have played a prominent role in
tuning the resonance of plasmonic structures through chemical composition engi-
neering. However, it has been unclear how alloying modifies the time dynamics of
the generated hot-carriers. In this chapter, we elucidate the role of chemical compo-
sition on the relaxation time of hot-carriers for the archetypal AuxAg1?x thin film
system. Through time-resolved optical spectroscopy measurements in the visible
wavelength range, we measure composition-dependent relaxation times that vary
up to 8x for constant pump fluency. Surprisingly, we find that the addition of 2% of
Ag into Au films can increase the hot-carrier lifetime by approximately 35% under
fixed fluence, as a result of a decrease in optical loss. Further, the relaxation time is
found to be inversely proportional to the imaginary part of the permittivity. Our re-
sults indicate that alloying is a promising approach to effectively control hot-carrier
41
relaxation time in metals.
4.2 Introduction
Pure metals, such as gold (Au) and silver (Ag), have long been the most
commonly used plasmonic materials due to their high electron densities and desirable
optical and chemical properties [16, 86, 87, 88, 89, 90, 91]. However, when using pure
metals, applications are limited to a narrow range of optical frequencies stemming
from the fixed resonances of the metals. Alloying these metals together presents a
promising alternative by allowing the opportunity to tune the plasmonic resonances
without altering the geometry of the system. The optical properties of the Au-Ag
alloys can be tailored throughout the visible spectrum by modifying the atomic ratio
of the two metals [92, 93, 94].
Additionally, by varying the alloy's chemical composition, one can modify
the electronic band structure, which results in interband transitions over differ-
ent incident photon energies. It was recently reported that as the concentration
of Au increases in Au-Ag alloyed films, the position of the d-band shifts closer to
the Fermi level[95]. This reduces the energy gap for interband transitions, leading
to transitions occurring with lower incident photon energies. Similar modification
of the threshold of the interband transitions has also been studied in other types
of materials such as metal nitrides [96], semiconductors [97], and transition metal
dichalcogenides [98]. The resonance tunability and band structure engineering of al-
loys proves useful in a variety of applications including superabsorbers [99], imaging
42
probes in biomolecular studies [100], implant devices [101], catalysis [102, 103, 104],
photovoltaics [105], and hydrogen sensing [106, 107, 108, 109].
Many of the aforementioned applications rely on significant light absorption
within the films or nanostructures. One common approach for absorption enhance-
ment is through coupling the incident photons into surface plasmons, i.e. coherent
oscillations of free electrons at the metal-dielectric interface. This process results in
the generation of highly energetic non-thermal carriers, also known as hot-carriers.
Particularly, hot-carriers are generated after nonradiative decay of the localized or
propagating surface plasmons through either direct or phonon-assisted intraband
transitions [84, 110]. Once these carriers are excited, they thermalize to create a
population of electrons that can be described as a Fermi-Dirac distribution at an
elevated temperature. They start to equilibrate with the lattice temperature via
a series of scattering processes including the electron-phonon and phonon-phonon
scattering[111, 112]. These highly energetic carriers have been utilized in appli-
cations such as water splitting [113], artificial photosynthesis [7], medical therapy
[114], and drug delivery [115]. However, efficient generation and extraction of these
carriers depends on the choice of material, and their corresponding hot-carrier relax-
ation time. In particular, understanding of the hot-carrier relaxation time plays a
significant role in modulation speed [116], power conversion efficiency enhancement
[117, 118], determining the hot-electron flux [119, 120], and nanoscale photothermal
heat control [121]. Thus, due to the broad spectral tunability associated with de-
vices exploiting hot-carrier physics, their temporal study in planar Au-Ag structures
would benefit a variety of applications.
43
In this chapter, we focus on Au-based hot-carrier devices due to their chemical
stability and incorporate different ratios of Ag to create Au-Ag alloys. We use
ultrafast pump-probe optical spectroscopy to measure the hot-carrier relaxation
time. The pump wavelength is nominally set to 700 nm wavelength (1.77 eV) to
ensure that the relaxation time is due to intraband transitions rather than interband
ones (2.4 eV in Au and 4.0 eV in Ag) [95, 122]. We employ the Kretschmann
geometry to couple into the propagating surface plasmon mode, which has the added
benefit of increasing the measurement sensitivity as a result of increased photon
absorption. To determine the hot-carrier lifetime, we use a free-electron model
and convert the differential reflectivity measurements to the corresponding elevated
electron temperature [123]. Our results show that the hot-carrier relaxation time
depends upon the Ag mole fractions. We further find that the lifetime is inversely
proportional to the imaginary part of the permittivity for different Au-Ag alloys.
Finally, considering the pure Au film as the baseline of the lifetime measurements,
we observe that the slight addition of Ag (2%) can increase the hot-carrier relaxation
time, while higher fractions of Ag (e.g. 35% and 75%) yield smaller lifetimes.
4.3 Fabrication of AuAg alloyed samples
We used a co-sputtering system (AJA International sputtering system) for
the alloyed Au-Ag thin film depositions. All sample fabrications, and optical and
material testing are performed at the Maryland NanoCenter-FabLab.
Sputtering is a Physical Vapor Deposition (PVD) method in which the energetic
44
Film composition Deposition time Voltage (Au, Ag) Chamber pressure Thickness
Au 45 sec 200 4? 10?6 35 nm
Au98Ag2 45 sec (200,100) 3.8? 10?6 42 nm
Au65Ag
?6
35 20 sec (200,105) 3.7? 10 25 nm
Au25Ag75 15 sec (100,200) 3.8? 10?6 21 nm
Table 4.1: AuAg alloyed fabrication recipes
ionized Argon gas generated by applying a high voltage between the cathode (target)
and anodes (substrate) accelerates toward the targets. The surface atoms of the
targets are then ejected and form a thin film on the substrate surface. The three
incorporated targets within this system enable the deposition of the three different
materials, metal or dielectric, simultaneously. We only used two of the available
targets with gold and silver source pockets.
For each deposition round, the applied vacuum pump runs for ?3Hours to drop the
chamber pressure down to ? 4? 10?6 Torr. The glass substrate cleaning procedure
for each deposition round is carried out using acetone, IPA, and blow-dry with
nitrogen gas. In each deposition, both prism and glass substrate are placed within
the same chamber. The glass substrate is then used for further material and optical
characterizations. We change the alloy?s composition by varying the applied voltage
on the Au and Ag targets during each deposition. Table 4.1 summarized the applied
voltage, deposition time, initial chamber pressure, and the alloyed samples? final
thickness.
4.4 Optical and material characterization of AuAg alloys
The chemical composition of the alloyed samples is determined with energy-
dispersive X-ray spectroscopy (EDX) (Fig. 4.1). Figure 4.1b and 4.1c show the AFM
45
topography and roughness distribution of Au-Ag alloys. Subsequently, we measure
the optical properties of our samples with spectroscopic ellipsometry ranging from
200 nm to 1000 nm (See appendix A for the ellipsometry data and their fits).
Au100 Au98 Ag2 Au65 Ag35 Au25 Ag75
(a)
6.0 nm 4.0 nm
(b) 1.0 nm 2.5 nm
-1.0 -2.5 -6.0 -4.0
(c) 0.5 nm 1.4 nm 3 nm 2 nm
Figure 4.1: Chemical and structural properties of thin films. (a) EDX. (b) AFM
topography. (c) Roughness distribution. Insets show RMS roughnesses.
(a) 0 (b)15 (c) 0.8
Au100 Au100
Au98Ag2 Au Ag
-10 98 2
10 Au65Ag
0.6
35 Au65Ag35
Au25Ag75 Au25Ag75
-20 Au100 Au100(Ref[48]) 0.4 Au100(Ref[48])
Au98Ag2
-30 Au65Ag
5
35 0.2
Au25Ag75
Au100(Ref[48])
-40 0 0
200 400 600 800 1000 200 400 600 800 1000 200 400 600 800 1000
(nm) (nm) (nm)
Figure 4.2: Measured (a) real and (b) imaginary parts of the permittivity, and (c)
computed quality factor of the propagating surface plasmon of AuxAg1?x alloys.
We use a Drude-Lorentz model including two Drude and one Lorentz terms to
46
r
i
Q 2 -3spp = ( r/ )x10i
fit the ellipsometry data. The modelled permittivity is shown in Fig. 4.2a and 4.2b
for our fabricated samples. We also compute the surface plasmon polariton (SPP)
quality factor, Qspp (?) = 
2
r(?)/i(?) [124], for the different Au-Ag alloys (Fig.
4.2c). In general, many experimental factors such as the chamber pressure, substrate
temperature, deposition rate, etc. can affect the films? quality factors due to the
change in the dielectric functions [125]. Our experiments keep all of these other
factors the same, thus isolating the effects of changing the alloy composition. At 700
nm, the wavelength used for our pump-probe measurements, the 100% Au and 98%
Au samples show a higher Qspp when compared to the other alloys, predominantly
due to the lower i. The dielectric functions can also be affected by a disordered
mixture of Au and Ag at a certain molar combination, leading to the reduction of
electron scattering and plasma frequency [95, 126]. Additionally, it has been shown
that the co-sputtering of a small amount of metal suppresses the island growth,
leading to a film with low optical and electrical losses [105]. Furthermore, the
imaginary part of the dielectric function of a thin film, which is responsible for the
optical losses, generally increases with the decrease of the film thickness for gold
films below 80 nm [127]. We have compared the optical properties of our pure gold
sample with that of a pure gold film obtained from [127] (gray lines in Fig. 4.2),
showing good agreement. We further hypothesis that by varying the thickness of
the different AuAg alloys, the relaxation time of the excited hot-carriers could be
further tuned due to the variation in the optical losses in these films.
47
4.5 Surface plasmon coupling in the alloyed samples
Before measuring the relaxation dynamics of the excited hot-carriers, we mea-
sured the propagating surface plasmon mode using the Kretschmann configuration
[128]. Figure 4.3b shows the experimental results of the reflection measurements for
all four samples as a function of incident angle near the plasmon coupling angle for
incident wavelength from 680 nm to 740 nm with 5 nm spectral bandwidth.
We utilized the Finite Difference Time Domain (FDTD) method (Lumerical
Inc.) for the reflection calculations (Fig. 4.3b). The optical properties of the
samples are extracted from the ellipsometry measurements and used as inputs for
the simulations. A perfectly match layer (PML) boundary condition with 64 layers
is used for the boundaries along with a non-uniform mesh setting with an accuracy
of 4. The source is a plane wave and the incident angle is swept from 40 to 48 degrees
with 0.1-degree increments for each wavelength between 680 nm and 740 nm. There
is a good agreement between the FDTD simulation results and the experimental
measurements. As expected, the surface plasmon resonance is sharper for samples
with higher Qspp and broader for the samples with lower values.
4.6 AuAg alloys and time-resolved differential reflectively measure-
ments
Similar to the previous chapter, the non-equilibrium hot-carrier dynamics of
the alloys are investigated using degenerate (?pump = ?probe) time-resolved differ-
48
(a)
Exp
740 nm
680 nm
(b) Sim
740 nm
680 nm
Figure 4.3: (a) Experimental and (b) simulated reflectivity for Au, Au98Ag2,
Au65Ag35, and Au25Ag75 alloys under p-polarized illumination with wavelengths
ranging from 680 nm to 740 nm. For FDTD simulations, we used pulse illumination
with 150 fs pulse width.
ential reflectivity measurements at the surface plasmon resonance angle. We use a
Ti-Sapphire laser system with 700 nm wavelength and 80 MHz repetition rate to
generate both the pump and probe beams. A fraction of the laser beam is split off
to serve as the probe beam and the other portion is passed through a mechanical
delay stage to set the time delay between the two beams. We use nearly co-linear
pump and probe beams, which are adjusted to couple into the propagating surface
plasmon mode but can also be spatially separated in the reflected field. The overlap
of the beams is achieved using an off-axis parabolic mirror with a measured spot size
of approximately 90 ?m. Pump-probe measurements are conducted at the surface
plasmon resonance angle under five different incident pump powers (i.e. 120 mW,
150 mW, 180 mW, 210 mW, and 240 mW) with a fixed probe power of 19.8 mW,
as shown in Fig.4.4.
49
  120mW    150mW    180mW     210mW    240mW
-4
10 0
-10
Au
100 (a (b (c) (d) (e)
-20
-4
10 0
-10
Au Ag
98 2 (f (g (h) (i) (j)
-20
-4
10 0
-10
Au Ag
65 35 (k (l (m) (n) (o)
-20
-4
10 0
-10
Au Ag
25 75 (p (q (r) (s) (t)
-20
0 2 4 0 2 4 0 2 4 0 2 4 0 2 4
t(ps) t (ps) t (ps) t (ps) t (ps)
Figure 4.4: Differential reflectivity measurements for Au-Ag alloys with different
chemical compositions. For each sample, the pump power is: 120 mW (a,f,k,p), 150
mW (b,g,l,q), 180 mW (c,h,m,r), 210 mW (d,i,n,s), and 240 mW (e,j,o,t). Insets
are real-color photographs of the alloyed thin films.
As expected, in all cases, increasing the pump power produces a larger change in
the transient reflectivity (?R/R). Because the temporal pulse width employed here
is longer than the electron-electron scattering time, on the order of 100 fs [129],
the relaxation time for the optically excited hot-carriers is mostly governed by the
electron-phonon relaxation time.
It is also worth mentioning that both hot-electrons and hot-holes can con-
tribute to device performance, see for example the use of hot-holes for photochemical
reactions [130]. Gong et al. showed how the energy of the hot-carrier distribution
50
R/R R/R R/R R/R
depends not only on the Electron Density of States (EDS) but also on the energy
of the incident photons and how it can be modified for a variety of structures [131].
For the 700 nm wavelength illumination used in our study, hot-hole extraction is
more efficient than the hot-electrons extraction in pure Au, as the distribution of
hot-holes is peaked further away from the Fermi level. However, for the case of
the Au-Ag mixtures, the distributions of both hot-holes and hot-electrons become
more uniform as the illumination wavelength approaches the Near-IR range [132];
thus, both of these excited hot-carriers will have similar contributions to the over-
all hot-carrier effects. We also note that the differences between the hot-hole and
hot-electron distributions in Au and Au-Ag alloys can provide additional tunability
for the carrier extraction depending upon the materials and functionality of the rest
of the device. Further, the threshold for the interband transition for Au-Ag alloys
shifts to a longer wavelengths as the Au content increases, allowing for additional
control of these processes [95, 132].
4.7 Data analysis
To find the excited hot-carrier relaxation time from the transient reflectivity
measurements, we employ the combination of a free-electron model [133] and the
modified two-temperature model [123]. In this model, the effect of the surface
plasmon's electric field profile is incorporated into the absorbed laser power density
within the conventional two-temperature model, which accounts for variation of the
field in the vertical (surface normal) direction. This combination allows us to convert
51
the pump-probe reflectivity signal to the relevant electron temperature, which results
in more accurate theoretical modeling due to the nonlinear relationship between
the reflectivity signal and the electron temperature. The model uses the optical
parameters extracted from our ellipsometry measurements at room temperature for
each alloy. Finally, best fits to the temperature converted reflectivity signals are
computed by minimizing the Normalized Mean Squared Error (NMSE) of the hot-
carrier relaxation time. Figure 4.5 shows the results of the temperature converted
data (filled circles) and their corresponding best fits (solid lines) to the hot-carrier
relaxation time of the alloyed Au-Ag films at pump powers of 120 mW, 150 mW,
180 mW, 210 mW, and 240 mW under the resonance condition, i.e. upon coupling
to the surface plasmon mode.
The pump-probe measurements also reveal an additional short decay compo-
nent that only appears immediately after excitation for the 2% Ag composition. We
attribute this decay component to electron-electron interactions, which are typically
too fast to be detected in the pure Au. The plasmon dephasing time (i.e. the rate
at which electron's collective oscillations cease) is longer in Ag as a result of differ-
ent radiative or non-radiative plasmon damping mechanisms, and so the addition
of Ag to the Au alloy may increase this decay component to a measurable amount
in the 2% Ag alloy. For the higher Ag concentration alloys, this decay mechanism
is not distinguishable from electron-phonon interactions based on our measurement
sensitivity.
Analysis of the temperature converted differential reflectivity shows that the
hot-carrier relaxation time (?) of the Au98Ag2 sample is 8? larger than for Au65Ag35
52
   120mW     150mW     180mW     210mW     240mW
800
Au (a) (b) (c) (d) (e)
100
600
400
800
Au Ag (h) (i)
98 2 (f) (g) (j)
600
400
800
Au Ag (k) (l) (m) (n) (o)
65 35
600
400
800
Au Ag (p) (q) (r) (s) (t)
25 75
600
400
0 2 4 0 2 4 0 2 4 0 2 4 0 2 4
t(ps) t(ps) t(ps) t(ps) t(ps)
Figure 4.5: Temperature converted differential reflectivity measurements for Au100,
Au98Ag2, Au65Ag35, and Au25Ag75 alloys under different incident pump powers of
120 mW (a,f,k,p), 150 mW (b,g,l,q), 180 mW (c,h,m,r), 210 mW (d,i,n,s), and 240
mW (e,j,o,t). The black solid lines in each plot show the best fits computed from a
modified two-temperature model.
and Au25Ag75 for a fixed laser fluence. Additionally, we find that the film with
Au98Ag2 has the longest lifetime of any of the samples measured (3.20 ? 0.15 ps
with 240 mW pump power), even including pure Au. To further investigate this
phenomenon, we consider the optical properties of each Au-Ag alloy at 700 nm
pump wavelength and compare the result with our measured hot-carrier relaxation
time, and find ? to be inversely proportional to the imaginary part of the permittivity
(See Fig. 4.6).
53
T (k) T (k) T (k) T (k)
e e e e
4
Au Ag
98 2
Au
3 100
Power
2
Au Ag
1 25 75
Au Ag
65 35
0
0.2 0.3 0.4 0.5 0.6 0.7
1/
i
Figure 4.6: Hot-carrier relaxation time as a function of 1/i, the inverse of the
imaginary part of the permittivity, for AuxAg1?x. The solid lines are the linear
fit between the hot-carrier lifetime and 1/i. The colors represent the range of the
pump power between 120 mW (red) to 240 mW (purple).
Our results suggest that the addition of a small fraction (2%) of Ag to a
Au film increases the hot-carrier lifetime. This is consistent with previous findings
that showed particular Ag-Au alloys having higher Qspp than pure metals [94] and
that doping one metal with another can improve film quality and decrease optical
loss [105, 134]. However, all alloyed films that we measured have similar surface
roughnesses, suggesting that the decreased loss may come from changes in the band
structure or other changes to the material rather than simply smoothing of the films.
Because we are probing relaxation times >10s of fs, the main mechanism leading
to the increase in the hot-carrier lifetime is likely a suppression of the electron-
phonon scattering, which could result for decreased lattice defects, grain boundaries,
54
 (ps)
etc., but further work will be necessary to isolate the individual contributions. In
addition, the lifetime is inversely proportional to i and increases with pump power
(see Fig. 4.6 for a comparison with all pump powers), which is in agreement with
previously reported studies [135, 136]. However, this is an empirical observation,
and not a strict mathematical proportionality that is grounded in theory. Our
measurements show that the optical loss is an important and potentially controllable
internal parameter compared to the other external factors, such as pump power. Our
observation further opens a new route to alter the hot-carrier relaxation time for
plasmonic applications through alloying.
4.8 Conclusion
In summary, we measured the hot-carrier relaxation time of Au-Ag thin film
alloys under visible excitation and found that adding a small fraction of Ag to Au
increases the hot-carrier relaxation time. Our experimental results suggested that
the relaxation time depends on the alloy's composition and is inversely proportional
to i. Surprisingly, some alloys can have loss factors that are less than their pure
counterparts, which leads to improved hot-carrier performance. By comparing the
relaxation time of the fabricated alloys with the pure Au sample, we determined that
the measured relaxation time increases with slight addition of Ag and then drops
significantly for alloys with higher Ag content. Overall, this work demonstrated that
the relaxation time of hot-carriers can be engineered through alloying.
55
Chapter 5: Hot-carrier temporal dynamics in AuCu alloy
5.1 Sample fabrication and optical measurements
To probe the alloying effect on the hot-electrons? temporal dynamics, we fur-
ther fabricate AuCu alloys with different compositions. The same fabrication process
as in the case of AuAg (Chapter 4) is also performed here. EDX and ellipsometry
measurements are used to determine the material compositions and dielectric func-
tions of the AuCu alloys (Fig. 5.1). The samples? thicknesses are determined by
fitting the B-spline model on the ellipsometry data, resulting in 43 nm, 47 nm, and
49 nm for samples from high to low Au concentration (i.e. Au70Cu30, Au57Cu43, and
Au54Cu46), respectively. Figure (5.2) shows the results of the surface plasmon cou-
pling experiment and simulation on these alloys repeated for wavelengths ranging
from 680 nm to 740 nm. Considering approximately the same thickness for these
alloys, the surface plasmon coupling reduces as the copper composition increases.
Among these samples, the only detectable pump-probe signal is for the Au70Cu30
sample with ?70% absorption at resonance.
56
(a) (b) (c)
Figure 5.1: The (a) real, (b) imaginary part of the dielectric function of AuxCu1?x
alloys, and (c) their corresponding surface plasmon quality factor (Qspp) with x
= 100, 70, 57, and 54. The results are determined from fits to the spectroscopic
ellipsometry data. The composition of each sample is measured by energy-dispersive
X-ray spectroscopy (EDX).
(a)
Exp
740 nm
680 nm
(b)
Sim
740 nm
680 nm
Figure 5.2: (Top row) Experimental and (bottom row) numerical simulation of
surface plasmon polariton excitation of AuxCu1?x alloys with x = 100, 70, 57, and
54. The wavelength range is from 680 nm to 740 nm.
5.2 Hot-carrier temporal dynamics in AuCu alloys
Figure (5.3) illustrates the recorded transient response of the Au70Cu30 alloys
with pump power ranging from 210 mW to 300 mW and a fixed probe power of 17
mW. These measurements are performed at resonance and under a wavelength of
700 nm for both pump and probe beams.
57
?"
?!
? = (?$/? )?10&'!"" # %
A coherent interference artifact appears near zero-time delay, especially for the
case of lower pump power when the signal is small. This effect happens due to the
spatial overlapping of the two pump and probe beams. The two-beam interference
generates a spatial modulation of the refractive index on the sample surface results
the diffraction of the pump pulse into the direction of the probe beam[137]. The
diffracted pump has an opposite phase compared to the probe and leads to a decrease
in the probe beam?s amplitude. Thus, the recorded pump-probe signal can be
affected by the interference patterns for the case of the degenerate pump-probe
measurements. Interference artifacts are more detectable in the AuCu alloys under
lower fluences. As the absorbed pump power increases, the signal gets enhanced
and overcomes these artifacts.
In these alloys, the random atomic structure destroy the periodicity of the
crystal structure [122]. This effect results in the higher imaginary part of the dielec-
tric response above 500 nm (Fig. 5.1). Using the Drude model, the imaginary part
of the dielectric function is proportional to the Drude damping factor, which itself
is inversely proportional to the electron mean free path. Thus, we can conclude
that the higher loss in these samples results in the lower electron mean free path,
and consequently higher scattering phenomena, and a reduction of the hot-electron
relaxation time.
58
10-4
0
-2
Ppump = 210 mW Ppump = 240 mW Ppump = 270 mW Ppump = 300 mW-4
0 5 0 5 0 5 0 5
t(ps) t(ps) t(ps) t(ps)
Figure 5.3: Transient differential reflectivity measurements of Au70Cu30 alloy at
different pump powers (210 mW to 300 mW) while coupling to the propagating
surface plasmon. Pump and probe wavelengths are 700 nm.
5.3 Conclusion
Just as we observed in the previous two chapters, faster hot-carrier relaxation
times occur under low pump power. Here, the relaxation time under the maximum
pump fluence for the Au70Cu30 sample reaches 0.9 ps which is almost half of what
we measured in the Au98Ag2 alloy. This result could be due to the lower absorption
(?70%) compared to the Au98Ag2 sample. However, for a better comparison, mea-
surements of the hot-electron relaxation time are needed over a larger compositional
range.
59
R/R
Chapter 6: Conclusion and future directions
6.1 Hot-carrier temporal dynamics in non-metallic materials
This dissertation specifically investigated the temporal dynamics of the hot-
electrons in metal films and metallic alloys. The reason is, in fact, much of the
current experimental work on nanophotonic and plasmonic systems have either uti-
lize gold or silver due to their simple fabrication process and relatively low losses in
the visible and NIR frequencies. However, in some plasmonic systems, such losses
are still detrimental to the overall performance of the devices. Furthermore, Au and
Ag are not compatible with the CMOS technologies used for the integrated circuits
industry [138]. Another point that could make gold less desirable for hot-carrier col-
lection is its high work function. As a result, there is a large barrier height between
the gold-semiconductor junction. Therefore, it makes the collection of hot-electrons
generated with lower energy photons more challenging. Thus, alternative plasmonic
materials such as intermetallics (a mixture of metals with non-metallics such as
nitrides), ceramics, and semiconductor-based materials have emerged to overcome
these limitations. For instance, titanium nitride (TiN) is a ceramic material that
is considered as an alternative to the conventional plasmonic metals such as gold
in the visible and near-infrared frequencies because of the similarity of its optical
60
properties to those in gold beyond 500 nm[139]. It also overcomes drawbacks of
conventional plasmonic metals? based on its lower cost, higher melting point, and
chemical stability. Additionally, it is appealing for manufactured electronics devices
due to its compatibility with the CMOS technology. Further, its broad range tun-
ability using different fabrication techniques adds to its advantages. Thus, studying
hot-carrier temporal dynamics in non-metallic materials with possibly lower loss
would be another interesting direction.
6.2 Hot-carrier temporal dynamics in TiN sample
Besides low loss (around 500 nm) and similar optical properties to gold, an-
other advantage of TiN sample (Fig. 6.1) is that it supports an ENZ (Epsilon-near-
zero) property in the visible range.
ENZ materials exhibit a near-zero real part of the dielectric function at a wavelength
known as the zero-permittivity wavelength [140] with major applications in optical
switching devices. Also, not all materials with ENZ properties can support an ENZ
mode. Such a mode can be excited when the film?s thickness is sufficiently thin [141];
thus, enabling a high photon absorption and electromagnetic field confinement at
the resonance. Recent studies have measured the a hot-electron relaxation time of
? 350 ps for TiN samples [142], which is without considering any ENZ mode. The
ENZ mode excitation in such materials is another promising direction for modifying
the hot-electron time dynamics.
61
0.5
4
0
3
-0.5
2
-1
1
-1.5
500 1000 1500
 (nm)
Figure 6.1: Real (red) and imaginary (blue) parts of the measured dielectric func-
tion of 22 nm TiN fabricated by pulsed laser deposition. The permittivity data is
obtained from the ellipsometry fit (see figure A.5).
6.3 X-ray diffraction microscopy on AuAg samples
The hot-electron relaxation enhancement observed for the Au98Ag2 film is
something that requires additional material analysis. To understand the origin of
such enhancement, the next step could be to perform X-ray diffraction measurements
(XRD). This would help us to determine the crystalline features of the film; as the
reduced grain sizes can result in a reduction of the electron scattering. Also, it?s
important to understand the material properties of the prism itself. The prism used
for these measurements is an N-BK7 right-angle prism with an anti-reflection coating
on the hypotenuse. However, the ellipsometry measurements are performed on the
glass slide which is mounted in the same deposition chamber as the prism. The
surface of the prism will also affect the metal film and should be further analyzed
in future experiments.
62
r
i
6.4 Conclusion
In this dissertation, we measured the relaxation time of the excited hot-
electrons in metal films and metallic alloys by employing a degenerate pump-probe
spectroscopy setup. With the advantage of coupling to the propagating surface
plasmon, we were able to increase the absorption and further tune the hot-electron
relaxation dynamics within metallic films and alloys. Results from hot-carrier tem-
poral measurements on different alloys (AuAg and AuCu) show that the relaxation
time heavily depends on the material compositions and can be controlled by select-
ing a proper ratio. Finally, We anticipate that this study could lead to the efficient
design of future hot-electron-based devices.
63
Appendix A:
A.1 Fabrication procedure
All sample fabrications are performed at the Maryland NanoCenter-FabLab.
Angstrom e-beam evaporator is utilized, which is configured for the metal deposi-
tions. In this system, electron beams are emitted off of the tungsten filament at a
very high temperature. Both prism and glass substrate are loaded within the same
chamber. For each deposition round, the applied vacuum pump runs for 3 hours to
drop the chamber pressure down to ? 4 ? 10?6 Torr. The glass substrate cleaning
procedure for each deposition round is carried out using acetone, IPA, and blow-dry
with nitrogen gas.
A.2 Ellipsometry data of AuAg alloys
Spectroscopic ellipsometry data measured for the alloy samples are depicted
in Fig.A.1. Peak shift in the transmission spectrum (Fig.A.2 of the alloyed sample
deposited on glass substrate from pure gold at ?520 nm to Au25Ag75 at ?400
nm) shows a progressive decrease in the inter-band transition due to lower gold
concentrations. These results agree with the Gong et al. [94] study of transmission
64
spectra of noble metal alloyed thin films.
65
66
(a) (b)
Au Au!"Ag#
(c) (d)
Au$%Ag&% Au#%Ag'%
Figure A.1: Ellipsometry data: Delta (green line) and Psi (red line) for four different alloy mixtures, (a) Au, (b) Au98Ag2,
(c) Au65Ag2, and (d) Au25Ag75 at five different incident angles with their corresponding best fits (dashed lines). We use the
GenOsc model to fit the data. Both optical properties and thickness are obtained from the fits.
0.2
0.2 Au100 0.15 Au98 Ag2
0.15
0.1 0.1
0.05 0.05
0 0
500 1000 1500 500 1000 1500
 (nm)  (nm)
0.3 Au65Ag35 0.4 Au25 Ag75
0.2 0.3
0.2
0.1
0.1
0 0
500 1000 1500 500 1000 1500
 (nm)  (nm)
Figure A.2: Transmission spectra obtained from the spectroscopic ellipsometry mea-
surements on AuAg alloyed film on glass. The solid red lines show the ellipsometry
data and the dotted black lines indicate the fit on data using a B-spline model.
Film composition EDS (%) Deposition time Voltage (Au, Cu) Chamber pressure Thickness
AuCu 70,30 45 sec (200,100) 4.2? 10?6 43 nm
AuCu 57,43 45 sec (200,150) 4.4? 10?6 47 nm
AuCu 54,46 45 sec (100,180) 4.9? 10?6 49 nm
Table A.1: AuCu alloyed fabrication recipes
A.3 Ellipsometry data of AuCu alloys
67
Transmission Transmission
Transmission Transmission
Au70Cu30
Au57Cu43
Au54Cu46
Figure A.3: Ellipsometry data and fit on the AuCu alloys.
68
0.2 Au 0.1 Au 70Cu100 30
0.08
0.15
0.06
0.1
0.04
0.05 0.02
0 0
500 1000 1500 500 1000 1500
 (nm)  (nm)
0.06
Au 57Cu0.06 43
Au 54Cu46
0.04
0.04
0.02 0.02
0 0
500 1000 1500 500 1000 1500
 (nm)  (nm)
Figure A.4: Transmission spectra obtained from the spectroscopic ellipsometry mea-
surements on AuCu alloyed film on a glass. Transimission peak shift from 550 nm
for the pure Au to about 600 nm for the sample with higher Cu percentage.
A.4 Ellipsometry data of TiN
69
Transmission Transmission
Transmission Transmission
Variable Angle Spectroscopic Ellipsometric {VASE) Data 
30-----------------------180 
????????????
???? --????? . 
20 120 
ro 
....., 
en 15 90 
a.. 
10 60 
Psi (55.00, 65.00, 75.00?) 
5 30 
Delta (55.00, 65.00, 75.00?) 
Model 
0 0 
0 300 600 900 1200 1500 1800 
Wavelength (nm) 
Figure A.5: Ellipsometry data (solid lines) and a model fit (dashed lines) on a 22
nm TiN film on Si substrate fabricated by pulsed laser deposition.
70
Bibliography
[1] Mark W Knight. Photodetection with Active. Science, 702(2011):702?704,
2011.
[2] Ali Sobhani, Mark W Knight, Yumin Wang, Bob Zheng, Nicholas S King,
Lisa V Brown, Zheyu Fang, Peter Nordlander, and Naomi J Halas. Narrow-
band photodetection in the near-infrared with a plasmon-induced hot electron
device. Nature Communications, 4:1?6, 2013.
[3] Boris S. Karasik, Andrei V. Sergeev, and Daniel E. Prober. Nanobolome-
ters for THz photon detection. IEEE Transactions on Terahertz Science and
Technology, 1(1):97?111, 2011.
[4] A. Shurakov, Y. Lobanov, and G. Goltsman. Superconducting hot-electron
bolometer: From the discovery of hot-electron phenomena to practical appli-
cations. Superconductor Science and Technology, 29(2), 2015.
[5] Xinghan Cai, Andrei B. Sushkov, Ryan J. Suess, Mohammad M. Jadidi, Gre-
gory S. Jenkins, Luke O. Nyakiti, Rachael L. Myers-Ward, Shanshan Li, Jun
Yan, D. Kurt Gaskill, Thomas E. Murphy, H. Dennis Drew, and Michael S.
Fuhrer. Sensitive room-temperature terahertz detection via the photothermo-
electric effect in graphene. Nature Nanotechnology, 9(10):814?819, 2014.
[6] Xinghan Cai, Andrei B. Sushkov, Mohammad M. Jadidi, Luke O. Nyakiti,
Rachael L. Myers-Ward, D. Kurt Gaskill, Thomas E. Murphy, Michael S.
Fuhrer, and H. Dennis Drew. Plasmon-Enhanced Terahertz Photodetection
in Graphene. Nano Letters, 15(7):4295?4302, 2015.
[7] Syed Mubeen, Joun Lee, Nirala Singh, Stephan Kra?mer, Galen D. Stucky, and
Martin Moskovits. An autonomous photosynthetic device in which all charge
carriers derive from surface plasmons. Nature Nanotechnology, 8(4):247?251,
2013.
[8] Alberto Naldoni, Urcan Guler, Zhuoxian Wang, Marcello Marelli, Francesco
Malara, Xiangeng Meng, Lucas V. Besteiro, Alexander O. Govorov, Alexan-
der V. Kildishev, Alexandra Boltasseva, and Vladimir M. Shalaev. Broadband
71
Hot-Electron Collection for Solar Water Splitting with Plasmonic Titanium
Nitride. Advanced Optical Materials, 5(15):1?11, 2017.
[9] Luca Mascaretti, Aveek Dutta, S?te?pa?n Kment, Vladimir M Shalaev, Alexan-
dra Boltasseva, Radek Zbor?il, Alberto Naldoni, L Mascaretti, S? Kment,
R Zbor?il, A Naldoni, A Dutta, V M Shalaev, and A Boltasseva. Plasmon-
Enhanced Photoelectrochemical Water Splitting for Efficient Renewable En-
ergy Storage. Advanced Optical Materials, 1805513:1?23, 2019.
[10] Gavin Conibeer. Third-generation photovoltaics. Materials Today, 10(11):
42?50, 2007.
[11] Alain Dereux & Thomas W. Ebbesen William L. Barnes. Surface plasmon
subwavelength optics. Nature, 424:824?830, 2003.
[12] Ji?r?? Homola. Surface Plasmon Resonance Sensors for Detection of Chemical
and Biological Species. Chem. Rev, 108:462?493, 2008.
[13] Maung Nyan Win, Joshua S. Klein, and Christina D. Smolke. Codeine-binding
RNA aptamers and rapid determination of their binding constants using a
direct coupling surface plasmon resonance assay. Nucleic Acids Research, 34
(19):5670?5682, 2006.
[14] Satoshi Kawata, Yasushi Inouye, and Prabhat Verma. Plasmonics for near-
field nano-imaging and superlensing. Nature Photonics, 3(7):388?394, 2009.
[15] Christy L. Haynes, Adam D. McFarland, and Richard P. Van Duyne. Surface-
enhanced: Raman spectroscopy. Analytical Chemistry, 77(17):339?346, 2005.
[16] Stefan A. Maier. Plasmonics: Fundamentals and Applications. Springer, 2018.
ISBN 5856420187.
[17] L William and W Thomas. Surface plasmon subwavelength optics. Nature,
424(14), 2003.
[18] Lukas Novotny and Bert Hecht. Principles of Nano-Optics, page 378?418.
Cambridge University Press, 2006.
[19] Vincenzo Amendola, Roberto Pilot, Marco Frasconi, Onofrio M. Marago?, and
Maria Antonia Iat??. Surface plasmon resonance in gold nanoparticles: A re-
view. Journal of Physics Condensed Matter, 29(20):1?48, 2017.
[20] William L. Barnes. Surface plasmon-polariton length scales: A route to sub-
wavelength optics. Journal of Optics A: Pure and Applied Optics, 8(4):S87?
S93, 2006.
[21] R W Wood. On a Remarkable Case of Uneven Distribution of Light in a
Diffraction Grating Spectrum. Proc. Phys. Soc. London, 18:269?275, 1902.
72
[22] Andreas Otto. Excitation of Nonradiative Surface Plasma Waves in Silver
by the Method of Frustrated Total Reflection. Zeitschrift ffir Physik, 216:
398?410, 1968.
[23] E Kretschmann and H Raether. Radiative Decay of Non Radiative Surface
Plasmons Excited by Light. Naturforsch, 23:2135?2136, 1968.
[24] Alireza Shahsafi, Yuzhe Xiao, Jad Salman, Bradley S Gundlach, Chenghao
Wan, Patrick J Roney, and Mikhail A Kats. Mid-infrared Optics Using Di-
electrics with Refractive Indices Below Unity. Physical Review Applied, 10:
34019, 2018.
[25] Alireza Shahsafi, Jad Salman, Bryan E Rubio Perez, Yuzhe Xiao, Chenghao
Wan, and Mikhail A Kats. Infrared Polarizer Based on Direct Coupling to
Surface Plasmon Polaritons. Nano Letters, 20:8483?8486, 2020.
[26] C. Ropers, C. C. Neacsu, T. Elsaesser, M. Albrechty, M. B. Raschke, and
C. Lienau. Grating-coupling of surface plasmons onto metallic tips: A
nanoconfined light source. Nano Letters, 7(9):2784?2788, sep 2007.
[27] Anatoly V. Zayats, Igor I. Smolyaninov, and Alexei A. Maradudin. Nano-
optics of surface plasmon polaritons. Physics Reports, 408(3-4):131?314, 2005.
doi: 10.1016/j.physrep.2004.11.001.
[28] Jacob B Khurgin. How to deal with the loss in plasmonics and metamaterials.
Nature Nanotechnology, 10:2?6, 2015. doi: 10.1038/nnano.2014.310.
[29] Ekaterina Y. Lukianova-Hleb, Xiaoyang Ren, Rupa R. Sawant, Xiangwei Wu,
Vladimir P. Torchilin, and Dmitri O. Lapotko. On-demand intracellular ampli-
fication of chemoradiation with cancer-specific plasmonic nanobubbles. Nature
Medicine, 20(7):778?784, 2014.
[30] Oara Neumann, Albert D. Neumann, Edgar Silva, Ciceron Ayala-Orozco, Shu
Tian, Peter Nordlander, and Naomi J. Halas. Nanoparticle-mediated, light-
induced phase separations. Nano Letters, 15(12):7880?7885, 2015.
[31] C.-K Sun, F Vallee, L H Acioli, E P Ippen, and J G Fujimoto. Femtosecond-
tunable measurement of electron thermalization in gold. Physical Review B,
50(15337-15348), 1994.
[32] Min Hu and Gregory V. Hartland. Heat dissipation for au particles in aqueous
solution: Relaxation time versus size. Journal of Physical Chemistry B, 106
(28):7029?7033, 2002.
[33] Peter Zijlstra, Anna L. Tchebotareva, James W.M. Chon, Min Gu, and Michel
Orrit. Acoustic Oscillations and Elastic Moduli of Single Gold Nanorods. Nano
Letters, 8(10):3493?3497, 2008.
73
[34] Man-Nung Su, Pratiksha D Dongare, Debadi Chakraborty, Yue Zhang,
Chongyue Yi, Fangfang Wen, Wei-Shun Chang, Peter Nordlander, John E
Sader, Naomi J Halas, and Stephan Link. Optomechanics of Single Aluminum
Nanodisks. Nano Letters, 17:2575?2583, 2017.
[35] Mingjie Li, Saikat Bhaumik, Teck Wee Goh, Muduli Subas Kumar, Na-
talia Yantara, Michael Gra?tzel, Subodh Mhaisalkar, Nripan Mathews, and
Tze Chien Sum. Slow cooling and highly efficient extraction of hot carriers
in colloidal perovskite nanocrystals. Nature Communications, 8(May):3?12,
2017.
[36] Gaussian beam optics. https://www.newport.com/n/
gaussian-beam-optics. Accessed: 2021-03-07.
[37] Mark L. Brongersma, Naomi J. Halas, and Peter Nordlander. Plasmon-
induced hot carrier science and technology. Nature Nanotechnology, 10(1):
25?34, 2015.
[38] A J Nozik. Quantum dot solar cells. Physica E, 14:115?120, 2002.
[39] Syed Mubeen, Joun Lee, Nirala Singh, Stephan Kra?mer, Galen D Stucky,
and Martin Moskovits. An autonomous photosynthetic device in which all
charge carriers derive from surface plasmons. Nature Nanotechnology, 8:247?
251, 2013.
[40] Philipp Reineck, Delia Brick, Paul Mulvaney, and Udo Bach. Plasmonic Hot
Electron Solar Cells: The Effect of Nanoparticle Size on Quantum Efficiency.
J. Phys. Chem. Lett., 7:4137?4141, 2016.
[41] Andrew J Leenheer, Prineha Narang, Nathan S Lewis, and Harry A Atwater.
Solar energy conversion via hot electron internal photoemission in metallic
nanostructures: Efficiency estimates. J. Appl. Phys, 115:1?7, 2014.
[42] Silvia Peruch, Andres Neira, Gregory A. Wurtz, Brian Wells, Viktor A. Podol-
skiy, and Anatoly V. Zayats. Geometry Defines Ultrafast Hot-Carrier Dynam-
ics and Kerr Nonlinearity in Plasmonic Metamaterial Waveguides and Cavi-
ties. Advanced Optical Materials, 5(15):1?12, 2017.
[43] Mohammad M. Jadidi, Jacob C. Konig-Otto, Stephan Winnerl, Andrei B.
Sushkov, H. Dennis Drew, Thomas E. Murphy, and Martin Mittendorff. Non-
linear Terahertz Absorption of Graphene Plasmons. Nano Letters, 16(4):2734?
2738, 2016.
[44] M Mehdi Jadidi, Kevin M Daniels, Rachael L Myers-Ward, D Kurt Gaskill,
Jacob C Ko Nig-Otto, Stephan Winnerl, Andrei B Sushkov, H Dennis Drew,
Thomas E Murphy, and Martin Mittendorff. Optical Control of Plasmonic
Hot Carriers in Graphene. ACS Photonics, 6:302?307, 2019.
74
[45] Lisa J Krayer, Elizabeth M Tennyson, Marina S Leite, and Jeremy N Munday.
Near-IR Imaging Based on Hot Carrier Generation in Nanometer-Scale Optical
Coatings. ACS Photonics, 5:306?311, 2018.
[46] Hamidreza Chalabi, David Schoen, and Mark L. Brongersma. Hot-electron
photodetection with a plasmonic nanostripe antenna. Nano Letters, 14(3):
1374?1380, 2014.
[47] Ali Sobhani, Mark W Knight, Yumin Wang, Bob Zheng, Nicholas S King,
Lisa V Brown, Zheyu Fang, Peter Nordlander, and Naomi J Halas. Narrow-
band photodetection in the near-infrared with a plasmon-induced hot electron
device. Nature Communications, 4:1?6, 2013.
[48] Peter Nordlander Mark W. Knight, Heidar Sobhani and Naomi J. Halas. Pho-
todetection with Active Optical Antennas. Science, 332:702?704, 2011.
[49] Guillaume Baffou and Romain Quidant. Thermo-plasmonics: Using metallic
nanostructures as nano-sources of heat. Laser and Photonics Reviews, 7(2):
171?187, 2013.
[50] Pierfrancesco Zilio, Michele Dipalo, Francesco Tantussi, Gabriele C Messina,
and Francesco De Angelis. Hot electrons in water: injection and pondero-
motive acceleration by means of plasmonic nanoelectrodes. Light: Science &
Applications, 6:1?8, 2017.
[51] Minho Kim, Mouhong Lin, Jiwoong Son, Hongxing Xu, and Jwa Min Nam.
Hot-Electron-Mediated Photochemical Reactions: Principles, Recent Ad-
vances, and Challenges. Advanced Optical Materials, 5(15):1?21, 2017.
[52] Emiliano Corte?s, Wei Xie, Javier Cambiasso, Adam S Jermyn, Ravishankar
Sundararaman, Prineha Narang, Sebastian Schlu?cker, and Stefan A Maier.
Plasmonic hot electron transport drives nano-localized chemistry. Nature
Communications, 8:1?10, 2017.
[53] Maria Solera Gerardo A. Lopez, M.-Carmen Estevez and Laura M. Lechuga.
Recent advances in nanoplasmonic biosensors: applications and lab-on-a-chip
integration. Nano photonics, 6(1):123?136, 2016.
[54] Jeffrey N. Anker, W. Paige Hall, Olga lyandres, Nilam C. Shah, Jing Zhao,
and Richard P. Van Duyne. Biosensing with plasmonic nanosensors Recent.
Nature Materials, 7:442?453, 2008.
[55] Phillip Christopher and Martin Moskovits. Hot Charge Carrier Transmission
from Plasmonic Nanostructures. Annu. Rev. Phys. Chem., 68:379?398, 2017.
[56] Giulia Tagliabue, Adam S. Jermyn, Ravishankar Sundararaman, Alex J.
Welch, Joseph S. DuChene, Ragip Pala, Artur R. Davoyan, Prineha Narang,
and Harry A. Atwater. Quantifying the role of surface plasmon excitation and
75
hot carrier transport in plasmonic devices. Nature Communications, 9(1):1?8,
12 2018.
[57] Prineha Narang, Ravishankar Sundararaman, and Harry A. Atwater. Plas-
monic hot carrier dynamics in solid-state and chemical systems for energy
conversion. Nanophotonics., 5(1):96?111, 2016.
[58] A. Deviz?is, V. Vaic?ikauskas, and V. Gulbinas. Ultrafast pump-probe surface
plasmon resonance spectroscopy of thin gold films. Applied Optics, 45(11):
2535?2539, 2006.
[59] A. Deviz?is and V. Gulbinas. Ultrafast dynamics of the real and imaginary
permittivity parts of a photoexcited silver layer revealed by surface plasmon
resonance. Applied Optics, 47(10):1632?1637, 2008.
[60] Rogier H. M. Groeneveld, Rudolf Sprik, and Ad Lagendijk. Ultrafast Relax-
ation of Electrons Probed by Surface Plasmons at a Thin Silver Film. Physical
Review Letters, 64(7):784?787, 1990.
[61] Gregory V. Hartland. Optical Studies of Dynamics in Noble Metal Nanostruc-
tures. Chemical Reviews, 111(6):3858?3887, 2011.
[62] Fuming Wang and Nicholas A Melosh. Plasmonic Energy Collection through
Hot Carrier Extraction. Nano Letters, 11:5426?5430, 2011.
[63] Mohammad Taghinejad, Hossein Taghinejad, Zihao Xu, Yawei Liu, Sean P.
Rodrigues, Kyu Tae Lee, Tianquan Lian, Ali Adibi, and Wenshan Cai. Hot-
Electron-Assisted Femtosecond All-Optical Modulation in Plasmonics. Ad-
vanced Materials, 30(9):1?7, 2018.
[64] Jongbum Kim, Enrico G Carnemolla, Clayton Devault, Amr M Shaltout,
Daniele Faccio, Vladimir M Shalaev, Alexander V Kildishev, Marcello Fer-
rera, and Alexandra Boltasseva. Dynamic Control of Nanocavities with Tun-
able Metal Oxides. Nano Letters, 18:740?746, 2018.
[65] Man-Nung Su, Christopher J Ciccarino, Sushant Kumar, Pratiksha D Don-
gare, Seyyed Ali Hosseini Jebeli, David Renard, Yue Zhang, Behnaz Ostovar,
Wei-Shun Chang, Peter Nordlander, Naomi J Halas, Ravishankar Sundarara-
man, Prineha Narang, and Stephan Link. Ultrafast Electron Dynamics in
Single Aluminum Nanostructures. Nano Letters, 19:3091?3097, 2019.
[66] Man-Nung Su, Pratiksha D Dongare, Debadi Chakraborty, Yue Zhang,
Chongyue Yi, Fangfang Wen, Wei-Shun Chang, Peter Nordlander, John E
Sader, Naomi J Halas, and Stephan Link. Optomechanics of Single Aluminum
Nanodisks. Nano Letters, 17:2575?2583, 2017.
[67] Feng He, Nathanial Sheehan, Seth R. Bank, and Yaguo Wang. Giant elec-
tron?phonon coupling detected under surface plasmon resonance in Au film.
Optics Letters, 44(18):4590?4593, 2019.
76
[68] Rogier H M Groeneveld, Rudolf Sprik, and Ad Lagendijk. Femtosecond spec-
troscopy of electron-electron and electron-phonon energy relaxation in Ag and
Au. Physical Review B, 51:11433?11445, 1995.
[69] Stephan Link and Mostafa A. El-Sayed. Spectral Properties and Relaxation
Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nan-
odots and Nanorods. Journal of Physical Chemistry B, 103(40):8410?8426,
1999.
[70] Sungnam Park, Matthew Pelton, Mingzhao Liu, Philippe Guyot-Sionnest, and
Norbert F. Scherer. Ultrafast resonant dynamics of surface plasmons in gold
nanorods. Journal of Physical Chemistry C, 111(1):116?123, 2007.
[71] Michele Ortolani, Andrea Mancini, Arne Budweg, Denis Garoli, Daniele Brida,
and Francesco De Angelis. Pump-probe spectroscopy study of ultrafast tem-
perature dynamics in nanoporous gold. Physical Review B, 99:1?6, 2019.
[72] S I Anisimov, B L Kapeliovich, T L Perel?man, and L D Landau. Electron
emission from metal surfaces exposed to ultrashort laser pulses. Journal of
Experimental and Theoretical Physics, 39(2):375?377, 1974.
[73] J. Hohlfeld, S.-S. Wellershoff, J. Gu?dde, U. Conrad, V. Ja?hnke, and
E. Matthias. Electron and lattice dynamics following optical excitation of
metals. Chemical Physics, 251(1-3):237?258, 2000.
[74] C.-K Sun, F Vallee, L Acioli, E P Ippen, and J G Fujimoto. Femtosecond
investigation of electron thermalization in gold. Physical Review B, 48:12365?
12368, 1993.
[75] Neil W Ashcroft and N David. Mermin. Solid state physics. New York : Holt,
Rinehart and Winston, 1976.
[76] Harsha Reddy, Urcan Guler, Alexander V Kildishev, Alexandra Boltasseva,
Vladimir M Shalaev, J A Schuller, E S Barnard, W Cai, Y C Jun, J S White,
and M L Brongersma. Temperature-dependent optical properties of gold thin
films. Optics Express, 6(9):2776?2802, 2016.
[77] The frequency and temperature dependence of the optical effective mass of
conduction electrons in simple metals. Journal of Physics and Chemistry of
Solids, 30(12):2765?2769, 1969.
[78] Peijun Guo, Richard D Schaller, John B Ketterson, and Robert P H Chang.
Ultrafast switching of tunable infrared plasmons in indium tin oxide nanorod
arrays with large absolute amplitude. Nature Photonics, 10:267?274, 2016.
[79] J A Woollam Co. CompleteEASE? Data Analysis Manual. Technical report,
2004. URL http://www.jawoollam.com.
77
[80] Lan Jiang and Hai-Lung Tsai. Improved Two-Temperature Model and Its
Application in Ultrashort Laser Heating of Metal Films. Journal of Heat
Transfer, 127:1167?1173, 2005.
[81] J. K. Chen and J. E. Beraun. Numerical study of ultrashort laser pulse inter-
actions with metal films. Numer. Heat TR. A-Appl., 40(1):1?20, 2001.
[82] Zhibin Lin, Leonid V Zhigilei, and Vittorio Celli. Electron-phonon coupling
and electron heat capacity of metals under conditions of strong electron-
phonon nonequilibrium. Physical Review B, 77(075133):1?17, 2008.
[83] Nicki Hogan, Shengxiang Wu, and Matthew Sheldon. Photothermalization
and hot electron dynamics in the steady state. Journal of Physical Chemistry
C, 124:4931?4945, 2020.
[84] Ana M Brown, Ravishankar Sundararaman, Prineha Narang, William A God-
dard Iii, and Harry A Atwater. Ab initio phonon coupling and optical response
of hot electrons in plasmonic metals. Physical Review B, 94:75120, 2016.
[85] Ye Yang, David P Ostrowski, Ryan M France, Kai Zhu, Jao Van De Lage-
maat, Joseph M Luther, and Matthew C Beard. Observation of a hot-phonon
bottleneck in lead-iodide perovskites. Nature Photonics, 10:53?59, 2016.
[86] S. J. Oldenburg, R. D. Averitt, S. L. Westcott, and N. J. Halas. Nanoengi-
neering of optical resonances. Chemical Physics Letters, 288:243?247, 1998.
[87] S. A. Maier, M. L. Brongersma, P. G. Kik, S. Meltzer, A. A.G. Requicha, and
H. A. Atwater. Plasmonics - A route to nanoscale optical devices. Advanced
Materials, 13(19):1501?1505, 2001.
[88] Vivian E. Ferry, Jeremy N. Munday, and Harry A. Atwater. Design consid-
erations for plasmonic photovoltaics. Advanced Materials, 22(43):4794?4808,
2010.
[89] Xiang Zhang Nicholas Fang, Hyesog Lee, Cheng Sun. Sub?Diffraction-Limited
Optical Imaging with a Silver Superlens. Science, 308:534?538, 2005.
[90] Vladimir M. Shalaev, W. Cai, U. Chettiar, H. K. Yuan, A. K. Sarychev,
V. P. Drachev, and A. V. Kildishev. Negative index of refraction in optical
metamaterials. Optics letters, 30(24):3356?3358, 2005.
[91] David O. S. Melville and Richard J. Blaikie. Super-resolution imaging through
a planar silver layer. Optics Express, 13(6):2127, 2005.
[92] Yaocai Bai, Chuanbo Gao, and Yadong Yin. Fully alloyed Ag/Au nanorods
with tunable surface plasmon resonance and high chemical stability. The Royal
Society of Chemistry, 9:14875 ? 14880, 2017.
78
[93] Xiaotian Wang, Guanshui Ma, Anran Li, Jian Yu, Zhao Yang, Jie Lin, Ang Li,
Xiaodong Han, and Lin Guo. Composition-adjustable Ag-Au substitutional
alloy microcages enabling tunable plasmon resonance for ultrasensitive SERS.
Chemical Science, 9:4009 ? 4015, 2018.
[94] Chen Gong and Marina S. Leite. Noble Metal Alloys for Plasmonics. ACS
Photonics, 3(4):507?513, 2016.
[95] Chen Gong, Alan Kaplan, Zackery A. Benson, David R. Baker, Joshua P. Mc-
Clure, Alexandre R. Rocha, and Marina S. Leite. Band Structure Engineering
by Alloying for Photonics. Advanced Optical Materials, 6(17):1?7, 2018.
[96] Mukesh Kumar, Satoshi Ishii, Naoto Umezawa, and Tadaaki Nagao. Band
engineering of ternary metal nitride system Ti1?xZrxN for plasmonic appli-
cations. Optical Materials Express, 6(1):29, 2016.
[97] Cun Zheng Ning Peidong Yang, Letian Dou, and Peidong Yang. Bandgap
engineering in semiconductor alloy nanomaterials with widely tunable compo-
sitions. Nature Reviews Materials, 2:1?15, 2017.
[98] Tao Gong and Jeremy N Munday. Angle-Independent Hot Carrier Generation
and Collection Using Transparent Conducting Oxides. Nano Letters, 15:147?
152, 2015.
[99] Mariama Rebello Sousa Dias, Chen Gong, Zackery A. Benson, and Marina S.
Leite. Lithography-Free, Omnidirectional, CMOS-Compatible AlCu Alloys
for Thin-Film Superabsorbers. Advanced Optical Materials, 6(2):1700830, jan
2018.
[100] Jun Ando, Akihiko Nakamura, Mayuko Yamamoto, Chihong Song, Kazuyoshi
Murata, and Ryota Iino. Multicolor High-Speed Tracking of Single
Biomolecules with Silver, Gold, and Silver-Gold Alloy Nanoparticles. ACS
Photonics, 6:2870?2883, 2019.
[101] Mitsuo Niinomi, Masaaki Nakai, and Junko Hieda. Development of new metal-
lic alloys for biomedical applications. Acta Biomaterialia, 8(11):3888?3903,
2012.
[102] Xiangwen Liu, Dingsheng Wang, and Yadong Li. Synthesis and catalytic
properties of bimetallic nanomaterials with various architectures. Nano Today,
7:448?466, 2012.
[103] Dingsheng Wang and Yadong Li. Bimetallic Nanocrystals: Liquid-Phase Syn-
thesis and Catalytic Applications. Advanced Materials, 23(9):1044?1060, mar
2011.
[104] P. Herna?ndez-Ferna?ndez, S. Rojas, P. Oco?n, J. L.Go?mez De La Fuente, J. San
Fabia?n, J. Sanza, M. A. Pen?a, F. J. Garc??a-Garc??a, P. Terreros, and J. L.G.
79
Fierro. Influence of the preparation route of bimetallic Pt-Au nanoparticle
electrocatalysts for the oxygen reduction reaction. Journal of Physical Chem-
istry C, 111(7):2913?2923, 2007.
[105] Cheng Zhang, Dewei Zhao, Deen Gu, Hyunsoo Kim, Tao Ling, Yi Kuei Ryan
Wu, and L. Jay Guo. An ultrathin, smooth, and low-loss Al-doped Ag film and
its application as a transparent electrode in organic photovoltaics. Advanced
Materials, 26(32):5696?5701, 2014.
[106] Carl Wadell, Ferry Anggoro Ardy Nugroho, Emil Lidstro, Beniamino Iandolo,
Jakob B Wagner, and Christoph Langhammer. Hysteresis-Free Nanoplasmonic
Pd-Au Alloy Hydrogen Sensors. Nano Letters, 15:3570, 2015.
[107] Ferry A.A. Nugroho, Iwan Darmadi, Lucy Cusinato, Arturo Susarrey-Arce,
Herman Schreuders, Lars J. Bannenberg, Alice Bastos da Silva Fanta, Shima
Kadkhodazadeh, Jakob B. Wagner, Tomasz J. Antosiewicz, Anders Hell-
man, Vladimir P. Zhdanov, Bernard Dam, and Christoph Langhammer.
Metal?polymer hybrid nanomaterials for plasmonic ultrafast hydrogen detec-
tion. Nature Materials, 18(5):489?495, may 2019.
[108] Kevin J Palm, Joseph B Murray, Joshua P Mcclure, Marina S Leite, and
Jeremy N Munday. In Situ Optical and Stress Characterization of Alloyed
PdxAu1?x Hydrides. ACS Appl. Mater. Interfaces, 11:45057?45067, 2019.
[109] Yoshiaki Nishijima, Shogo Shimizu, Keisuke Kurihara, Yoshikazu Hashimoto,
Hajime Takahashi, Armandas Balcytis, Gediminas Seniutinas, Shinji Okazaki,
Jurga Juodkazyt, Takeshi Iwasa, Tetsuya Taketsugu, Yoriko Tominaga,
Saulius Juodkazis, S K Earl, T D James, T J Davis, J C McCallum, R E
Marvel, R F Haglund, and A Roberts. Optical readout of hydrogen storage in
films of Au and Pd. Optics Express, 7(25):24081, 2015.
[110] Ravishankar Sundararaman, Prineha Narang, Adam S Jermyn, William A
Goddard III, and Harry A Atwater. Theoretical predictions for hot-carrier
generation from surface plasmon decay. Nature Communications, 5:1?8, 2014.
[111] Jose H Hodak, Arnim Henglein, and Gregory V Hartland. Photophysics of
Nanometer Sized Metal Particles: Electron-Phonon Coupling and Coherent
Excitation of Breathing Vibrational Modes. Journal of Physical Chemistry B,
104:9954?9965, 2000.
[112] Christophe Voisin, Natalia Del Fatti, Dimitris Christofilos, and Fabrice Valle?e.
Ultrafast Electron Dynamics and Optical Nonlinearities in Metal Nanoparti-
cles. Journal of Physical Chemistry B, 105:2264?2280, 2001.
[113] Alberto Naldoni, Francesca Riboni, Urcan Guler, Alexandra Boltasseva,
Vladimir M Shalaev, and Alexander V Kildishev. Solar-Powered Plasmon-
Enhanced Heterogeneous Catalysis. Nanophotonics, 5:112?133, 2016.
80
[114] Prashant K. Jain, Ivan H. ElSayed, and Mostafa A. El-Sayed. Au nanoparticles
target cancer. Nano Today, 2(1):18?29, 2007.
[115] Gang Han, Partha Ghosh, and Mrinmoy M De Vincent Rotello. Drug and
Gene Delivery using Gold Nanoparticles. Nanobiotechnol, 3:40?45, 2007.
[116] Otto L Muskens, Luca Bergamini, Yudong Wang, Jeffrey M Gaskell, Nerea
Zabala, C H De Groot, David W Sheel, and Javier Aizpurua. Antenna-assisted
picosecond control of nanoscale phase transition in vanadium dioxide. Light:
Science and Applications, 5:1?9, 2016.
[117] Fuming Wang and Nicholas A Melosh. Plasmonic Energy Collection through
Hot Carrier Extraction. Nano Letters, 11:5426?5430, 2011.
[118] Wei Li and Jason Valentine. Metamaterial Perfect Absorber Based Hot Elec-
tron Photodetection. Nano Letters, 14:3510?3514, 2014.
[119] Anshu Pandey and Philippe Guyot-Sionnest. Slow Electron Cooling in Col-
loidal. Science, 322(November):929?932, 2008.
[120] G. J. Conibeer, D. Ko?nig, M. A. Green, and J. F. Guillemoles. Slowing of
carrier cooling in hot carrier solar cells. Thin Solid Films, 516:6948, 2008.
[121] Zachary J Coppens, Wei Li, D Greg Walker, and Jason G Valentine. Probing
and Controlling Photothermal Heat Generation in Plasmonic Nanostructures.
Nano Letters, 13:1023?1028, 2013.
[122] David Rioux, Simon Vallie?res, Se?bastien Besner, Philip Mun?oz, Eric Mazur,
and Michel Meunier. An analytic model for the dielectric function of Au, Ag,
and their Alloys. Advanced Optical Materials, 2(2):176?182, 2014.
[123] Sarvenaz Memarzadeh, Jongbum Kim, Yigit Aytac, Thomas Murphy, and
Jeremy Munday. Surface Plasmon Assisted Control of Hot-Electron Relaxation
Time. Optica, 7(6), 2020.
[124] Paul R. West, Satoshi Ishii, Gururaj V. Naik, Naresh K. Emani, Vladimir M.
Shalaev, and Alexandra Boltasseva. Searching for better plasmonic materials.
Laser and Photonics Reviews, 4(6):795?808, 2010.
[125] Kevin M Mcpeak, Sriharsha V Jayanti, Stephan J P Kress, Stefan Meyer,
Stelio Iotti, Aurelio Rossinelli, and David J Norris. Plasmonic Films Can
Easily Be Better: Rules and Recipes. ACS Photonics, 2:326?333, 2015.
[126] Yoshiaki Nishijima and Shunsuke Akiyama. Unusual optical properties of the
Au/Ag alloy at the matching mole fraction. Optical Materials Express, 2(9):
1226?1235, 2012.
[127] Dmitry I. Yakubovsky, Aleksey V. Arsenin, Yury V. Stebunov, Dmitry Yu.
Fedyanin, and Valentyn S. Volkov. Optical constants and structural properties
of thin gold films. Optics Express, 25(21):25574, 2017.
81
[128] Olivier Pluchery, Romain Vayron, and Kha-Man Van. Laboratory experiments
for exploring the surface plasmon resonance. Eur. J. Phys., 32:585, 2011.
[129] C. K. Sun, F. Valle?e, L. H. Acioli, E. P. Ippen, and J. G. Fujimoto.
Femtosecond-tunable measurement of electron thermalization in gold. Physical
Review B, 50(20):15337?15348, 1994.
[130] Evangelina Pensa, Julian Gargiulo, Alberto Lauri, Sebastian Schlu?cker, Emil-
iano Corte?s, and Stefan A. Maier. Spectral Screening of the Energy of Hot
Holes over a Particle Plasmon Resonance. Nano Letters, 19(3):1867?1874,
2019.
[131] Tao Gong and Jeremy N Munday. Materials for hot carrier plasmonics. Optical
Materials Express, 5(11):2501?2512, 2015.
[132] Lisa J. Krayer, Kevin J. Palm, Chen Gong, Alberto Torres, Cesar E.P. Ville-
gas, Alexandre R. Rocha, Marina S. Leite, and Jeremy N. Munday. Enhanced
near-Infrared Photoresponse from Nanoscale Ag-Au Alloyed Films. ACS Pho-
tonics, 7(7):1689?1698, 2020.
[133] Peijun Guo, Richard D Schaller, John B Ketterson, and Robert P H Chang.
Ultrafast switching of tunable infrared plasmons in indium tin oxide nanorod
arrays with large absolute amplitude. Nature Photonics, 10(4):267?274, 2016.
[134] Deen Gu, Cheng Zhang, Yi Kuei Wu, and L. Jay Guo. Ultrasmooth and ther-
mally stable silver-based thin films with subnanometer roughness by aluminum
doping. ACS Nano, 8(10):10343?10351, 2014.
[135] Jose Hodak, Ignacio Martini, and Gregory V Hartland. Ultrafast study of
electron-phonon coupling in colloidal gold particles. Chemical Physics Letters,
284:135?141, 1998.
[136] Marcos M Alvarez, Joseph T Khoury, T Gregory Schaaff, Marat N Shafigullin,
Igor Vezmar, and Robert L Whetten. Optical Absorption Spectra of Nanocrys-
tal Gold Molecules. Journal of Physical Chemistry B, 101:3706?3712, 1997.
[137] H. J. Eichler, D. Langhans, and F. Massmann. Coherence peaks in picosecond
sampling experiments. Optics Communications, 50(2):117?122, 1984.
[138] Gururaj V. Naik, Vladimir M. Shalaev, and Alexandra Boltasseva. Alternative
plasmonic materials: Beyond gold and silver. Advanced Materials, 25(24):
3264?3294, 2013.
[139] Alexandra Boltasseva, Atwater, and Harry A. Low-Loss Plasmonic Metama-
terials. Science, 331(January):290?292, 2011.
[140] Orad Reshef, Israel De Leon, M. Zahirul Alam, and Robert W. Boyd. Non-
linear optical effects in epsilon-near-zero media. Nature Reviews Materials, 4
(8):535?551, 2019.
82
[141] Salvatore Campione, Igal Brener, and Francois Marquier. Theory of epsilon-
near-zero modes in ultrathin films. Physical Review B - Condensed Matter
and Materials Physics, 91(12):1?5, 2015.
[142] Benjamin T. Diroll, Soham Saha, Vladimir M. Shalaev, Alexandra Boltasseva,
and Richard D. Schaller. Broadband Ultrafast Dynamics of Refractory Metals:
TiN and ZrN. Advanced Optical Materials, 8(19):1?9, 2020.
83