Abstract
The detailed theoretical description of how picosecond plane shear acoustic fronts can be excited by ultrafast lasers at the interface of two isotropic media, a transparent medium and an opaque medium, is presented. The processes leading to the emission of inhomogeneous plane bulk shear acoustic modes from the interaction at the interface of plane inhomogeneous compression/dilatation modes thermoelastically generated by laser interference gratings are analyzed. The theory describes the basic features of the spectral transformation function of the laser light conversion into shear modes and predicts an interval of frequencies where it is possible to achieve the emission into the transparent medium of propagating shear inhomogeneous modes only, while the compression/dilatation inhomogeneous modes will be evanescent and will be localized at the interface. The guidelines for optimal choice of the materials, with the goal of improving the amplitude of the photoexcited picosecond shear acoustic fronts are proposed. All-optical monitoring, i.e., excitation and detection, by fs-ps laser pulses of picosecond plane inhomogeneous shear acoustic fronts propagating in thin films and substrates can be applied for the noncontact determination of shear rigidity of materials.
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