长寿命吸收过程对超快动力学过程测量的影响

使用飞秒时间分辨抽运-探测透射光谱技术,实验研究了GaAs体材料中光激发载流子的超快弛豫动力学的波长依赖.在相同的光激发载流子浓度和抽运/探测比时,发现760 nm和780 nm两中心波长处的瞬态透射变化延迟扫描信号出现负的和振荡的信号.与模拟计算结果对比,判定该实验瞬态信号是错误的.分析探测器输出波形,发现是由于反相波形导致的,而引起反相波形的原因在于样品中存在长寿命的吸收过程.指出通过提高探测器上的抽运/探测比能够矫正反相波形,从而获得正确的瞬态透射变化动力学.提高探测器上的抽运/探测比与目前的应尽量减小抽运光对探测器的散射贡献的观点是对立的.文章的研究结果对应用抽运-探测时间分辨光谱技术正确地测量超快瞬态动力学过程具有重要的参考价值. 关键词： 时间分辨抽运-探测透射光谱 饱和吸收 吸收增强 GaAs体材料

Influence of long lifetime absorption process on the measurement of ultrafast carrier dynamics

The wavelength dependence of ultrafast relaxation dynamics of photoexcited carriers in bulk GaAs is studied using femtosecond time-resolved pump-probe transmission spectroscopy under the same photoexcited carrier concentration and the ratio of pump to probe in intensity. Negative and oscillating time-delayed signals are observed at central wavelengths of 760 nm and 780 nm and judged to be incorrect physically by comparson with a simulated computation result. It is found by waveform analysis that they are caused by the phase reversal of the output waveforms from a photodetector, while the phase reversal originates from a long lifetime absorption process existing in GaAs sample. It is pointed out that the phase reversal of the waveform can be corrected by raising the ratio of pump to probe in intensity at the photodetector, so that right transient traces can be obtained. However, raising the ratio is incompatible with the viewpoint that the scattered contribution to the photodetector from pump light should be filtered as much as possible. This result has an important reference value for the acquisition of correct ultrafast dynamics using time-resolved pump-probe spectroscopy.