XM2 thesis submitted by Ben Ash: Locally Resonant Metamaterial for Surface Acoustic Waves

Fig 1: CDT in Metamaterials PGRs Erick Burgos-Parra (left) and Ben Ash (right) submitting their PhD thesis on 2 August 2018.

Very many congratulations to our 4th year PGR Ben Ash, who recently submitted his PhD thesis on “Locally Resonant Metamaterial for Surface Acoustic Waves”. Ben informed us that he has accepted an offer for his dream job as a Microsystems engineer which starts on Monday 13th August with Oxford HighQ, http://www.oxfordhighq.com/, a start-up which is developing ‘microcavities’ to sense, trap and measure the properties of nanoparticles and chemicals. His main focus will be the optimisation of the fabrication of these ‘microcavities’.

Previously, Ben published his work in a Nature Communicatiosn article A highly attenuating and frequency tailorable annular hole phononic crystal for surface acoustic waves, Nat Commun, volume 8, no. 1, DOI:10.1038/s41467-017-00278-0.

“It was a pleasure to be a part of the CDT. I appreciated the various training I received, both technical and otherwise, which have made me a more rounded researcher of physics and engineering. To be one of many PhD students within a cohort was fantastic. I have no doubt that the collaborative discussions I had resulted in my research being of a higher quality, which has led to me landing my dream job.” (Ben Ash, 8 August 2018)

Well done Ben! We have no doubt that you will pass your viva splendidly.


Abstract of PhD thesis Locally Resonant Metamaterial for Surface Acoustic Waves

The control of surface acoustic waves (SAWs) using arrays of annular holes was investigated both experimentally and through numerical modelling. Periodic elastic composites, phononic crystals (PnCs), were designed using these annular holes as constituent elements. Local resonances associated with the annular hole structure were found to induce phonon bandgaps of a highly frequency tailorable nature, at frequencies where radiation of acoustic energy into the bulk of the substrate medium is avoided. These bandgaps are numerically demonstrated to exhibit order-of-magnitude improved extinction ratios for finite numbers of PnC elements, relative to the commonly used cylindrical pillar architecture. Devices fabricated on commercially available lithium niobate SAW delay lines verify the predicted behaviour. Through laser knife-edge detector vibrometry, a bandgap attenuation of 24.5 dB at 97 MHz is measured, in excellent agreement with finite element method (FEM) simulations.

Fig 2: Total displacement surface plots for modes A and B for the entire unit cell and for only the locally resonant structure. See publication A highly attenuating and frequency tailorable annular hole phononic crystal for surface acoustic waves, Nat Commun, volume 8, no. 1, DOI:10.1038/s41467-017-00278-0

The first reported experimental evidence of subwavelength confinement of propagating SAWs was realised using the same annular hole PnC concept. Defect holes of perturbed resonant frequencies are included within the PnC to define waveguides and cavities. Confinement within these defects was demonstrated to occur at subwavelength frequencies which was experimentally observed in fabricated cavities using standard SAW transducers, as measured by laser Doppler vibrometry. The success of this result was attributed to the impedance matching of hybridised modes to Rayleigh SAWs in un-patterned substrates at the defect resonance. The work here has the potential to transform the field by providing a method to enhance SAW interactions, which is a route towards the realisation of many lab-on-chip applications.

Finally, the use of annular hole arrays as negative refraction metamaterials was investigated. The symmetry was broken of the unit cells by alternating either the locally resonant frequencies or the distance separating the constituent elements. Both methods, called the bi-dispersive and bi-periodic methods, were numerically demonstrated to exhibit negative group velocity bands within the first Brillouin zone. Preliminary experimental results show that the design has the potential to be used in superlensing, where a SAW spot was imaged over a subwavelength flat lens. Future research looks to demonstrate that this result can be attributed to negative refraction.

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