Photoexcitation in materials generates nonequilibrium electrons, which take hundreds of femtoseconds to picoseconds to thermalize with phonons. Understanding thermal energy transport between electrons and phonons is crucial for various scientific and engineering applications, such as ultrafast demagnetization, nanophotonic and plasmonic phenomena. In my research, I utilized a pump/probe system to perform time-domain thermoreflectance (TDTR) measurements to investigate electron-phonon energy dynamics.In metals, absorption of light generates nonequilibrium electrons. Nonequilibrium electrons can transport heat over distances of tens to hundreds of nanometers before thermalizing with phonons. This diffusion causes a temperature rise in deeper layers of the film. At the metal/substrate interface, the resulting temperature difference induces picosecond acoustics, with the amplitude of the acoustics being proportional to the temperature rise. By measuring the change in acoustic amplitude along the film thickness, we determined the diffusion length of nonequilibrium electrons. To systematically study the role of nonequilibrium electrons in heat transport, we performed front/front and front/back measurements on thickness-gradient metals. These measurements provided insights into the spatial thermalization process within metals. We developed detailed heuristics to understand nonequilibrium electron energy transport. Additionally, we conducted similar experiments on bilayer samples, replacing the bottom ~20 nm metal with Fe or Pt, materials known for strong electron-phonon coupling g_ep. Due to the strong g_ep, there was few nonequilibrium electrons in the bilayer samples, leading to a different energy transport mechanism compared to monolayers. The results from bilayer were identical with monolayer, indicating that nonequilibrium electrons did not significantly affect energy transport. NbN films are promising superconducting materials widely used in various electronic devices, such as Josephson junction, hot electron bolometer and single-phonon detector. We conducted wavelength-dependent pump-probe measurements on different phases of NbN films to determine the electron-phonon coupling parameter g_ep. We studied the correlation between g_ep, density of states D(ε_f ) and transition temperature T_c and found a strong relationship between g_ep and T_c. In addition to characterization of nanomaterial with pump/probe system, I refined the process of nanofabrication. I successfully fabricated high-quality Au nanodisks and tuned their Local Surface Plasmon Resonance (LSPR) from 700 to 1000 nm.

The ability to predict and control thermal/magnetic properties is crucial for numerous applications. Incorporating materials with distinct structure-property relationships could offer tantalizing new possibilities in the design of denser electronic components with efficient thermal management. Hence, it is important to understand how variations in elemental composition and structural inhomogeneities influence thermal transport and/or magnetization. The primary goal for my dissertation research is to use Time-Domain Thermo-Reflectance (TDTR) to generate 2-dimensional thermal conductivity maps of two distinct material systems: (a) ferromagnetic Co-Fe alloys, and (b) Al-PVDF nanocomposites. I also use Time-Resolved Magneto-Optic Kerr Effect (TR-MOKE) to investigate magnetization dynamics in Co-Fe alloys. First, I summarize the experimental results of the magnetization dynamics in Co-Fe alloys. Through TR-MOKE experiments, I show that Co-Fe compositions that exhibit low Gilbert damping parameters (at the nanosecond timescale) also feature prolonged ultrafast demagnetization responses (at the femtosecond timescale) upon photoexcitation. Thus, I report a strong correlation between the dynamics at both timescales, indicating that the same physical mechanisms likely govern both phenomena. Next, I interrogate the thermal conductivity in Co-Fe alloys. I conduct TDTR measurements on Co-Fe thin films and arc-melted alloys, and spatially map the thermal conductivity on a Co-Fe diffusion multiple. I report two main results: (i) the thermal conductivity does not appear to be strongly influenced by crystalline disorder in Co-Fe alloys, and (ii) Co-Fe compositions that feature ultralow magnetic damping also exhibit significantly high non-electronic contribution to thermal transport. I hypothesize that the magnon thermal transport is likely to be very high at these compositions. Finally, I present high-resolution thermal conductivity maps of Al-PVDF nanocomposite films with varied Al volume fractions (0 – 50%). My thermo-reflectance mapping technique has sub-micron resolution and demonstrates how thermal transport properties vary spatially across the polymer, metal, and metal-polymer interfaces. I show that increasing the Al volume fraction to 50% enhances the bulk thermal conductivity of the polymer film by a factor of 2. In certain areas with coalesced Al particles, the local thermal conductivity dramatically increases by a factor of 250. A careful understanding of the spatial variation in the thermal conductivity will aid in the prediction of flame propagation and combustion characteristics in Al-PVDF films.

The goal of this work is to understand spin and heat transfer in metal multilayer systems on nanoscale length scales and ultra-fast time scales due to ultra-fast laser pulse heating. Femto-second laser pulses are absorbed at the surface of the metal system. The absorbed energy is deposited in the metal’s electrons. Energy is then redistributed to vibrational and spin degrees of freedom through electron-electron, electron-phonon, and phonon-phonon interactions. Energy is also dispersed spatially through hot electron transport. The heat transfer in these metal systems can be divided into two temporal regimes with the first spanning from 0 to 1ps and the second from 1ps to 12.5ns

I present Time Domain Thermo-Reflectance (TDTR) and Time-Resolved Magneto Optic Kerr Effect (TR-MOKE) results that further our understanding of how heat is transported in these metal multilayer systems. By conducting various TDTR and TR-MOKE experiments, we are able to experimentally obtain fit parameters in order to prepare model predictions of the heat transfer in the metal systems. I also present my work on spin accumulation in Au due to the Spin dependent Seebeck Effect in Au-iron garnet bilayer systems. By varying the iron garnet used in the bilayer system, we are able to see how different magnetic insulators of the same family behave when demagnetized by ultra-fast laser pulses.

Comprehending the interplay between thermal conductivity and phonon dynamics in semiconductors and insulators holds significance for the field of phonon engineering. While substantial progress has been made in the theoretical understanding of this relationship over the last decade, many of these predictions lack empirical validation. This dissertation endeavors to bridge this experimental gap, focusing particularly on the validation of theoretical insights, with a special emphasis on boron arsenide (BAs), a recently discovered material exhibiting ultrahigh thermal conductivity.Temperature-induced changes in phonon occupation impact thermal conductivity, making it a valuable probe for understanding phonon scattering in materials. Through systematic investigations of BAs samples across the temperature range of 300 to 600 K, we discovered a more pronounced temperature dependence (1/T^2) than theoretical predictions (1/T^1.7) in BAs sample with ambient thermal conductivity of 1500 W m-1 K-1. This discrepancy indicates that existing calculations have underestimated the importance of four-phonon scattering in BAs. Pressure renders a systematic tool to modulate phonon dispersion, offering insights into the correlation between changes in phonon dispersion and thermal conductivity. The Leibfried-Schlömann (LS) equation, a phenomenological model known for its predictive capabilities, has been successfully applied to elucidate the pressure dependence of thermal conductivity in numerous materials. My initial investigations focused on two perovskites, SrTiO3 and KTaO3, revealing that their thermal conductivity variations align with LS equation predictions. The distinct pressure sensitivities observed in SrTiO3 and KTaO3 underscore the pivotal role of phonon lifetime in determining pressure-induced alterations in thermal conductivity. However, when applying the LS equation to BAs, a notable discrepancy emerged. While the LS equation projected a threefold increase in BAs’s thermal conductivity at 30 GPa, our experimental findings demonstrated much less changes. Subsequently, I extended my investigations to GaN, which shares certain phonon dynamics similarities with BAs. Both BAs and GaN have a frequency gap in their phonon dispersion. GaN exhibited a stronger pressure dependence, aligning well with LS equation predictions. Furthermore, I conducted pressure-dependent measurements on diamond, where my data exhibited acceptable agreement with LS predictions. This comparative analysis among BAs, GaN, and diamond underscores the distinctive thermal characteristics of boron arsenide.