Abstract

We present a time-resolved investigation of the acoustic vibrarional dynamics in thin freestanding Silicon membranes. The confinement of the longitudinal acoustic modes caused by the small thickness of the membranes leads to a discretization of the vibrational acoustic spectrum [1]. Within the time domain, this behaviour is evidenced by the superposition of oscillations of a fundamental mode and the different higher odd harmonics. In this work we report the first time-resolved experimental observation of these oscillations. The femtosecond time resolved pump-probe experiments are performed using the newly developed high-speed asynchronous optical sampling (ASOPS) method [2]. This method is based on two asynchronously linked femtosecond unidirectional ring lasers of-1GHz repetition rate. The time delay between the pump and the probe beams is realized through an off-set in the repetition rates of the two lasers, which lies in the kHz frequency range. The absence of mechanical moving parts in the set-up allows a very fast scanning time and a high signal-to-noise ratio, impossible to archive with the usual pump-probe set-ups [3].

© 2009 IEEE