飛秒級激發-探測系統的研究

Abstract

我們已經成功地建立一套飛秒級超短光脈衝激發-探測系統, 並利用自行 組裝的氬離子雷射激發被動鎖模鈦藍寶石雷射為超短脈衝光源, 初步量測 了一些不同氧含量的釔鋇銅氧高溫超導薄膜, 和不同製程方法, 生長溫 度, 退火處理或離子佈植的砷化鎵半導體薄膜樣品, 量得了其反射率變化 的瞬態時間弛緩現像, 證實此套系統的可行性.將來為了發揮其更佳的功 能, 無論是雷射系統或激發-探測系統, 均須大幅改進, 作更完整的量測. We have successfully built up a femtosecond pump-probe system to measure the relaxation behaviors of photoexcited carriers in both superconducting and semiconducting thin films. The laser source is a home-made passively mode-locked Ti: sapphire laser producing pulses with pulsewidth of about 150 fs and average power of about 400 mW. The output beam is guided into a home- made pump-probe system which can detect the change of reflectivity as small as 5*10^-5. Lock-in and differential detection techniques also have been used to obtain the measurement results with better signal to noise ratio. We have performed a series of femtosecond reflectivity measurements on both high Tc superconducting YBCO thin films with different oxygen contents and semiconducting GaAs thin films with different manufacturing processes and annealing conditions. We have found qualitatively different behaviors in the sign, magnitude and temporal response as various manufacturing processes of these thin films. Detailed explanation of the experimental results can be obtained by further improving the resolution of the pump-probe system and by measuring the electrical properties of these thin films.