Abstract

The acoustic modes guided by thin-film metal superlattices have been investigated using Brillouin spectroscopy. Samples were grown on single crystal sapphire by alternately sputtering two different metals to yield a total thickness of ~0.3 μm. Thermally excited acoustic waves in the metal create a surface ripple which weakly interacts with light incident from a single-mode argon laser. The scattered light was then frequency analyzed with a tandem Fabry-Perot consisting of two synchronized three-pass cavities. The contrast ratio of this interferometer exceeds 10¹⁰ and provides sufficient stray light rejection to detect the surface Rayleigh wave and thirteen higher-order acoustic modes. We have investigated the dependence of bilayer wavelength on the elastic properties of both Cu/Nb and Mo/Ta superlattices over the 8-320-Å range. We note that the behavior of the Rayleigh velocity of our Cu/Nb samples is significantly different from a previous study of this material.¹ An estimate of the elastic constants of the anisotropic superlattices was made by fitting the observed acoustic mode velocities to a parametrized acoustic model. We compare these elastic constants with those predicted from the separate bulk constituents.

© 1986 Optical Society of America