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 SiO(2) 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.