| Summary: | 碩士 === 國立中山大學 === 光電工程學系研究所 === 106 === In this thesis, temperature-dependent(78K~290K) coherent phonon dynamics in topological insulator Bi2Te3 are investigated by ultrafast pump-probe system. Four coherent phonon types(A1g1、A1g2、Eg2、coherent acoustic phonon) with strong temperature dependency are observed in transmission pump-probe spectroscopy at low temperature(78K、90K). These could be attributed to temperature dependent gradient force which resulting in the coherent phonons generation. Especially, lifetime of around 2.87ps for Eg2 phonon mode is observed for the first time. Additionally, A1g1 phonon oscillation in low temperature(78K~190K) reflectivity pump-probe spectroscopy is investigated. Unlike behavior in room temperature, A1g1 phonon oscillation is observed as the temperature cooling down to 90K. The depressing signal of reflective pump probe due to the enhancing absorption of surface state for thin film topological insulator will be accordingly increasing as temperature cooling is proposed.
Meanwhile, to enhance the weak A1g1 phonon signal in reflectivity pump-probe spectroscopy, technique of controlling coherent phonon is exploited by Michelson interferometer. The A1g1 phonon signal is successfully enhanced and with the almost same value of phonon lifetime taken from reflection or transmission pump probe measurement further confirm the nature of coherent phonon for thin film topological insulator. Finally, we discuss the possibility for investigating strength of phonon coupling using resonant coherent phonon technique as well.
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