Temporal dynamics of hot-electrons in metal films and alloys
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When light is coupled into a surface plasmon mode, it can either decay radiatively 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 photogenerated 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 photodetectors, 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.