Hypersonic spectroscopy of porous silicon for acoustic devices

Abstract

I will review our work on porous silicon (pSi) presenting achievements while highlighting underlying physical questions that remain to be answered. pSi is produced by the electrochemical etching of crystalline silicon. It is typically mesoporous, having pores of 10–30nm diameter. The etching current density determines the final porosity, the volume fraction of air, with a wide range of porosities, 25%–95%, achievable. For wavelengths much greater than the pore size, pSi gives a tunable effective medium for light and sound waves. We have characterized pSi acoustic properties using transmission spectroscopy with matched transducer pairs working at 0.5–2.5 GHz. The results for velocity, v, have fitted to a general law of v=v0(1−φ)k, where v0 is the velocity in bulk silicon, φ is porosity, and k is the fitting parameter. We have investigated the variation of k with the direction of propagation and the etching conditions used to extract the dependence of the elastic constants on porosity. The measurement of velocity has enabled us to produce and characterize pSi Bragg mirrors and rugate filters that have a smoothly varying acoustic impedance. This has demonstrated the potential use of pSi in acousto-optic phoxonic crystal devices that have both phononic and photonic bandgaps.