  By means of a metal opto-acoustic transducer we generate quasi-longitudinal
and quasi-transverse picosecond strain pulses in a (311)-GaAs substrate and
monitor their propagation by picosecond acoustic interferometry. By probing at
the sample side opposite to the transducer the signals related to the
compressive and shear strain pulses can be separated in time. In addition to
conventional monitoring of the reflected probe light intensity we monitor also
the polarization rotation of the optical probe beam. This polarimetric
technique results in improved sensitivity of detection and provides
comprehensive information about the elasto-optical anisotropy. The experimental
observations are in a good agreement with a theoretical analysis.
