Surface Analysis by Laser Ionization

Abstract

The accuracy in measuring the phase velocity with the acoustic microscope is about 0.1 '70 and thus comparable for both methods. However, since the pulsed laser technique is broadband a much higher information content is obtained in the case of dispersion, e.g. in layered systems. This information, however, is averaged over a macroscopic area. Ultimate Limit of Broadband SAW SpectroscopyThe ultimate limit of the pulsed laser SAW method is given by the capability to focus the laser radiation onto the surface. The size of the laser spot or the width of the line focus is limited by the wavelength of the light. If it is, for example, 0.5 pm, the shortest wavelength in the SAW pulse would be about 1 pm, which corresponds to several gigahertz. Thus the pulsed laser method may approach the lower frequency limit of Brillouin scattering.A bandwidth limitation by the laser pulse duration can be completely avoided by choosing a pulse duration in the picosecond range. The penetration depth of light into the solid can also be much smaller than the width of the line focus and therefore is not a limiting factor for the bandwidth in the ideal case. As a consequence of the extremely high absorption coefficients observed in some solids (lo5 -lo6 cm-'), a much higher frequency can be achieved for the excitation of longitudinal bulk phonons, where 60GHz has been reached [30].