Femtosecond time-resolved investigation of coherent acoustic modes of free-standing Si membranes

This work reports the time-resolved experimental observation of the acoustic vibrational dynamics in the thin freestanding silicon membranes. The femtosecond time resolved pump-probe experiments are performed using the newly developed high-speed asynchronous optical sampling (ASOPS) method. 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.