Phononic Metamaterials for Surface Acoustic Wave Sensing

Abstract

This thesis investigates the sensitivity of phononic metamaterials to the presence of materials and changes in their environment. The behaviour of surface acoustic waves (SAWs) in periodic arrays of holes was investigated with finite element modelling and experimentally. SAW bandstructures and bandgap attenuation were obtained from simulations of arrays of cylindrical and annular holes which were filled with materials with different SAW velocities. Each type of hole array exhibited two distinct scattering regimes (Mie and Bragg scattering). The dependence of the bandgap frequency on the velocity was found to be stronger for annular holes than for cylindrical holes, suggesting that annular holes are potentially a better route to create tuneable phononic metamaterials. Annular holes also displayed a higher bandgap attenuation than cylindrical holes, meaning that annular hole arrays might be exploited for greater sensitivity in applications such as mass loading sensing. SAW attenuation due to mass loading of air was calculated by measuring SAW amplitude on a SAW device using an oscilloscope system and by laser Doppler vibrometry (LDV). An extraordinary increase of 2 to 3 orders of magnitude in mass loading attenuation was observed at the bandgap frequency when a phononic metamaterial was present, with only 4 resonator elements needed to produce this result. The measurements obtained by both experimental systems displayed similar frequency dependencies of mass loading attenuation coefficients. Some mass loading effects were also reproduced using finite element modelling. These approaches show great promise for improving the sensitivity of SAW pressure sensors. Finally, bandstructures were obtained from finite element simulations for an array of annular holes filled with a small sphere comprised of materials with different SAW velocities. The array exhibited similar scattering regimes as before, with an overlapping region. The dependence of the bandgap frequency on the velocity was found to be stronger when the annular holes contained the sphere than when they are fully-filled, suggesting that annular holes are potentially a good candidate for probing biological cells. Higher bandgap attenuation by up to a factor of 2 was exhibited by the single spherical inclusion compared to fully-filled holes. Since annular holes have more degrees of geometrical freedom than conventional phononic crystals, devices with greater sensitivity might be realised for applications such as biological sensing and lab-on-a-chip diagnostics.