Surface acoustic waves in the GHz range generated by periodically patterned metallic stripes illuminated by an ultrashort laser pulse

The photoelastic response of periodic arrays of stripes attached to the sur face of a substrate and illuminated by an ultrashort laser pulse were inves tigated. The samples were gold arrays on silicon and aluminum arrays either on crystalline quartz or on silicon. The metallic stripes had submicromete r lateral dimensions and the spatial periods ranged from about 1 mum up to 5 mum. The substrate being transparent (quartz) or slightly absorbing (sili con) at the laser wavelength (lambda =750 nm), a laterally modulated therma l stress is generated near the surface of the substrate when a light pulse illuminates the structure. The studies of vibrations involved by the subseq uent relaxation processes show that surface acoustic waves at frequency as high as about 5 GHz are excited with the samples consisting of aluminum str ipes. In the case of the aluminum samples with the largest lateral spatial periods (aluminum on quartz), the surface acoustic wave propagates outside the illuminated area. In the case of the gold samples, a normal mode of ind ividual bars is observed instead. Experimental evidence shows that these be haviors are mainly governed both by the lateral spatial period of the struc ture and by the density of the metal. (C) 2001 Acoustical Society of Americ a.