Picosecond laser generation of ultrashort acoustic pulses in silicon

Generation and propagation characteristics of picosecond laser-generated ultrashort acoustic pulses in Si were studied by means of a ZnO piezoelectric transducer. The full width at half-maximum of the observed acoustic pulses is about 3 ns, which is determined by the response of the ZnO transducer. Under the condition of no sample ablation, the propagation velocity of laser-generated acoustic pulses agrees well with the sound velocity. In the plasma formation regime, the propagation velocity of acoustic pulses exceeds the sound velocity at the early stage of propagation. The amplitude of acoustic pulses is affected by the formation and expansion of laser-generated plasmas at the silicon surface.