Hypersound generation and detection by laser pulses incident on the interface of an opaque anisotropic crystal are theoretically and experimentally investigated in the case where the symmetry is broken by a tilt of its axis of symmetry relative to the interface normal. A nonlocal volumetric mechanism of plane shear sound excitation is revealed and a modification of the temporal shape of the reflectivity signal with variation in probe light polarization is observed, both attributed to asynchronous propagation of the acoustic eigenmodes. Experiments and theory demonstrate the possibility of using polycrystalline materials with an arbitrary distribution of grain orientations for the generation and the detection of picosecond shear ultrasound.

We demonstrate that it is possible to study the modal structures of a vibrating object with digitally recorded holograms by use of the time-averaging principle. We investigate the numerical reconstruction from a theoretical point of view, and we show that the numerically reconstructed object from a digital hologram is modulated by the zeroth-order Bessel function. Results of experiments in time-averaged digital holography are presented.

Features offered by the combination of time averaging and digital Fresnel holography are investigated. In particular, we introduce the concept of the zero-crossing phase of Bessel fringes, which allows a highly contrasted determination of the dark fringes in the hologram. We discuss some particularities of the digital reconstruction and show how time-averaged digital holography can be used to study vibration drifts. Experiment results are presented in the case of a loudspeaker under a sinusoidal excitation; digital and analogical holography are also compared.

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