We have performed theoretical and experimental studies on the optical generation and transmission of picosecond ultrasonic pulses in a-Si/ Mo superlattices using ultrafast lasers. The pulse shapes and spectra of collectively excited folded phonon waves were analyzed. Calculations show that the picosecond strain pulses transmitted through a superlattice have a very complicated waveform due to branch folding and phononic bandgaps, but the transient piezoreflectance ͑TPR͒ response has a rather simple shape, similar to that of a homogeneous thin film. A direct pulse-echo technique was used to measure the waveform of the TPR response and the effective sound velocity of the folded phonons. The experimental results agree well with the theoretical predictions.
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