Ultrafast electron-phonon coupling at the metal-dielectric interface
The pump-probe technique is an ultrafast spectroscopy method of detecting the dynamics of energy carriers such as electrons, phonons, and holes with transient thermal reflectance measurement. A laser beam is divided into a pump beam and probe beam with different wavelength or polarization and time delay. According to the transient reflectance result, this method could be applied to investigate the interaction between electron-phonon and electron-electron coupling with a high temporal resolution on the order of 10 femtoseconds. Energy transfer of photo-excited electrons in a metal film to the dielectric substrate at the metal-dielectric interface is important for understanding the ultrafast heat transfer process across the two materials. Many researches have been conducted in finding this energy transfer process in different materials. In this thesis, by measuring the transient reflectance variation, the two-temperature model (TTM) is used to analyze the interface metal electron and dielectric substrate coupling. In order to relate temperature to the reflectance change, a temperature and wavelength dependent Drude-Lorentz model was developed which represents the temperature dependent dielectric constant and can be used to calculate reflectance variation. Ultrafast pump-and-probe interband transition measurements on Au-Si samples were carried out, where the probe photon energy was chosen to be close to the interband transition threshold (ITT) of gold to minimize the influence of non-equilibrium or non-thermalized electrons on the optical response, and to increase the signal to noise ratio for reflectance change. In the experiment, different pump fluences have been used to test the transient reflectance variation on Au-Si samples of different thicknesses. The pump wavelength is taken as 800 nm while the probe wavelength is taken as 490 nm. A thick gold of 1000 nm thickness has been used to determine the electron-phonon coupling strength represented by a constant G 0, and thinner films have been tested by fitting the transient reflectance change with this G0 and the electron-phonon thermal resistance across interface. Interface thermal conductance (inverse of thermal resistance) at different pump laser fluences was obtained, and was found to increase with the interface electron and phonon temperature. For future work, with the model and measurement method implemented in this thesis, more gold film samples with more types of substrate, such as Au-Glass, Au-Quartz, can be tested to see the difference when the non-equilibrium electrons factor has been reduced. Also, more proper probe wavelengths which maximize the signal-noise ratio for different materials can be concluded from the Drude-Lorentz model. The contribution of non-thermalized electron compared with thermalized electron at different wavelength for other samples shall be investigated in the future.
Xu, Purdue University.
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