Measurement of material anisotropy by dual-probe laser interferometer

A novel dual-probe laser interferometer, which has the advantage that it measures the surface displacements of the same surface wave at two locations, has been used to measure the speeds of surface waves and pseudo-surface waves on a silicon single crystal. Two configurations have been investigated. When the distance between the two points of detection is large relative to the length of the acoustic wave, the two signals detected by the probes are separated in the time domain. Hence the signal travel time can be found by the autocorrelation method. When the distance between the two points of detection is short relative to the acoustic wave, a composite signal is obtained consisting of two overlapping signals. Then the power cepstrum method is employed to extract the signal travel time between the probe points. The speeds for both types of surface waves are shown as functions of direction relative to the [001] crystal axis. The results have been compared with theoretical calculations and line-focus acoustic microscopy measurements.