Acoustic Waveguide Properties of a Thin Film of Nanoporous Silica on Silicon

This letter reports picosecond laser-based measurements of acoustic waveguiding in a thin film of nanoporous silica (similar to silica aerogel or xerogel) on silicon for wavelengths between 2.5 and 25 microns. The large mismatch between the acoustic properties of the film and substrate in this system creates pronounced dispersion in the velocities and leads to unusual acoustic behavior: over a relatively large range of wavelengths, the group velocities of the lowest order Rayleigh mode and certain other modes are less than 50 m/s - nearly ten times slower than the intrinsic velocities of the nanoporous silica and more than one hundred times slower than those of silicon. An isotropic model of the waveguide reproduces these and other features. Non-linear least squares fitting of the data to this model determines the intrinsic acoustic velocities of the nanoporous glass.