LASER EXCITATION OF SURFACE PHONONS AND WAVE PULSES

Excitation of surface phonons or surface acoustic wave (SAW) pulses with a broad frequency spectrum up to 200-300 MHz using focused laser pulses is discussed. Several optical and piezoelectric methods for broadband detection of the resulting surface displacements are considered. It is shown that for an anisotropic single crystal the SAW velocity can be measured with an accuracy of 0.1% as a function of the crystal plane and direction due to the short wave pulses achieved. In a layered system strong dispersion occurs, since the waves with different wavelengths probe different depths corresponding to their wavelength and thus travel with different velocities in the. heterogeneous solid. From a non-linear dispersion effect several film properties can be obtained, such as thickness, density and elastic constants (e.g, Young's modulus and Poisson's ratio). Results are presented for fullerite and diamondlike carbon films. The characteristics of the pulsed laser method are compared with Brillouin scattering, interdigital transducers and the acoustic microscope.