Theory of detection of shear strain pulses with laser picosecond acoustics

The principles of the photo-detection of shear acoustic waves in laser pico second acoustics are discussed. The shear strain in an isotropic material i nduces off-diagonal components in the dielectric tensor, that is, the mater ial shows strain induced birefringence. By considering the light reflection from the surface of a birefringent material, it is proved that one needs 1 ) non-normal incidence of probe light, 2) a shear direction perpendicular t o the plane of incidence, and 3) both S and P polarization components of th e probe light, for efficient shear strain detection.