Surface generation and detection of phonons by picosecond light pulses

We report experiments in which picosecond light pulses are used to generate and detect very short stress pulses (coherent longitudinal phonons). We present a theory of the generation process, and describe how the spatial shape of the stress pulse is related to the optical, electronic, and acoustical properties of the material. The stress pulses are detected through a measurement of the changes they induce in the optical reflectivity of the sample surface. We describe the theory of this effect. We present experimental results we have obtained for a-As₂Te₃, a-Ge, a-As₂Se₃, and Ni.