Band gaps and Brekhovskikh attenuation of laser-generated surface acoustic waves in a patterned thin film structure on silicon

Surface acoustic modes of a periodic array of copper and SiO₂ lines on a silicon substrate are studied using a laser-induced transient grating technique. It is found that the band gap formed inside the Brillouin zone due to ``avoided crossing'' of Rayleigh and Sezawa modes is much greater than the band gap in the Rayleigh wave dispersion formed at the zone boundary. Another unexpected finding is that a very strong periodicity-induced attenuation is observed above the longitudinal threshold rather than above the transverse threshold.