Acoustic and phononic metamaterials for surface waves
Date: 7 June 2021
University of Exeter
PhD in Physics
The research in this work is along two main themes. The first involves the generation of acoustic surface waves in aluminium plates with lattices of periodic sub-wavelength perforations, the experimental characterisation of the bound modes supported by the honeycomb and hexagonal lattices, and the verification of results through numerical ...
The research in this work is along two main themes. The first involves the generation of acoustic surface waves in aluminium plates with lattices of periodic sub-wavelength perforations, the experimental characterisation of the bound modes supported by the honeycomb and hexagonal lattices, and the verification of results through numerical modelling. The energy is driven into the system via a point source positioned over a hole in the centre of the sample and a needle-tip probe microphone positioned on the opposite side of the sample measures local pressure field. The full band-structure of bound modes is obtained via Fourier transformations of the acoustic signal in time and space, and agrees well with the numerically obtained dispersion relations, with one exception; for the honeycomb sample the upper branch of the Dirac cone in the Γ-K direction is present in the model, but is not observed experimentally in the first Brillouin zone. Conversely, the existence of the upper branch is shown in the M-K direction for the honeycomb sample. Although losses increase monotonically as the mode travels through the K point at the Dirac frequency on the honeycomb lattice we have been able to measure pressure fields at distance 116 mm from the source. The second theme studied in this work involves using periodic arrays of spiral resonators to produce bandgaps for Rayleigh surface acoustic waves propagating on a piezoelectric substrate, lithium niobate, which is a solid crystal. In this numerical study, the surface acoustic wave source is an interdigital transducer and the frequency of the generated Rayleigh wave is dependent on the distance between the transducer’s metallic fingers. Dispersion relations and transmission spectrums were obtained using the COMSOL® eigenfrequency and frequency domain models respectively. Dispersion relationships for arrays of such resonators showed that they can used to produce bandgaps for Rayleigh waves, but at lower frequencies than those achieved for other phononic crystals with the same unit cell size. In addition, negative slopes in the dispersion curve of the spiral structure indicate that a square array of spiral resonators, supports negative group velocity SAW modes. Transmission analysis has shown that the bandgap attenuation is large (~25 dB) at both high and low frequencies and displacement field profiles highlight confinement of the acoustic energy throughout the height of the oscillators. Considering the small size of the proposed device and the aforementioned propagation characteristics, we stress that the spiral metamaterial shows very good potential for sound manipulation and filtering in a plethora of lab-on-chip applications.
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