Dynamics of energy carriers in solids during interaction with ultrafast laser pulses
Abstract
The dynamics of energy carriers, including mainly electrons, holes, and phonons, determine important physical properties of solids such as thermal conductivity and electrical conductivity, which affect the design and the performance of solid-state devices. Solids can be classified as metal, semimetal, semiconductor, and insulator according to the band structure. Different kinds of solids have some common features but are generally distinct in terms of the energy-carrier dynamics. Ultrafast spectroscopy is a powerful tool to examine the dynamics of energy carriers, which excites a transient response of the material by a short laser pulse and detects the response by optical observables such as reflectance and transmittance. This technique allows observation of the optical response, which is closely related to the dynamics of energy carriers, in the time domain with a high temporal resolution on the order of 10 fs. In this thesis, ultrafast spectroscopy is used to investigate the behavior of electrons, holes, and phonons in metals and semiconductors. For metals, the dynamics of energy carriers is relatively well understood. Our work focuses on energy transport in gold as demonstration. By measuring the transient reflectance of gold films and analyzing the measured signals with the two-temperature model (TTM), it is found that during the transient heating process, when electrons and phonons are in thermal nonequilibrium state, electrons in metal thin films can directly transfer energy to phonons in dielectric substrates. In addition, the effect of the probe wavelength on the transient reflectance signal of gold is analyzed, which provides comprehensive information about the carrier dynamics and helps to optimize the experimental configuration for facilitating the analysis. For semiconductors, there is much less detailed work on the energy-carrier dynamics due to the typically complicated and diverse band structures. In our work, the carrier dynamics of gallium arsenide (GaAs) is investigated using ultrafast spectroscopy due to its well-known band structure, which serves as a reference for materials with more complex band structures. Based on the information obtained from the analysis of GaAs, we explore the carrier dynamics of filled-skutterudites, which is an important thermoelectric material. It is found that the transient reflectance signals of skutterudites are mainly attributed to bandfilling and lattice heating, which can reflect the relaxation of photoexcited carriers. In addition to the dynamics of electrons and holes, lattice vibration can also be manifested in the time domain by ultrafast spectroscopy. Our study shows that the guest atoms added to the voids in the lattice structure of skutterudites can interact with the host lattice and form lattice vibration modes by resonant interaction and different species of guest atoms lead to such vibration modes with different frequencies, which can scatter phonons in a broad spectral span. Therefore, filling skutterudites with multiple species of atoms can reduce the lattice contribution to the thermal conductivity more effectively and improve the thermoelectric performance. For future work, the acquired information from the studied materials can be applied to investigate materials with potential and important application such as black phosphorene and molybdenum disulfide (MoS2). Ultrafast spectroscopy can be carried out by different pump and probe wavelengths at varying temperatures and fluences to provide comprehensive information about the properties of energy carriers. The ultimate goal is to fully understand the energy-carrier dynamics in solid materials and to guide the design of solid-state devices.
Degree
Ph.D.
Advisors
Xu, Purdue University.
Subject Area
Mechanical engineering|Condensed matter physics|Optics
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