Extraordinary Gas Loading For Surface Acoustic Wave Phononic Crystals Ben Ash Supervisors – G. R. Nash, P. Vukusic EPSRC Centre for Doctoral Training in Metamaterials

-Introduction, Aims and Motivation -Simulations -Fabrication and Characterization -Conclusions and Future Work Outline

Introduction

-To create phononic crystals (PnCs) that can be used to control the properties of surface acoustic waves (SAW) Motivation -SAW devices are common components used in applications such as sensors and signal processing -PnCs can be used to create new devices with improved performance or functionality -E.g. create acoustic cavities for enhanced sensing Aims

SAW devices -SAWs have transverse and longitudinal displacement -Intensity decays exponentially from the surface -Inter-digital transducers can be used to excite SAWs on piezoelectrics -Oscillating voltage applied over conducting finger pairs

SAW devices -SAWs have transverse and longitudinal displacement -Intensity decays exponentially from the surface -Inter-digital transducers can be used to excite SAWs on piezoelectrics -Oscillating voltage applied over conducting finger pairs

Phononic Crystals -Can be considered an acoustic metamaterial -Consist of arrays of two materials with different elastic constants -Can open phonon bandgaps: -Transmission filters -Waveguiding -Negative refractive index etc.

-Square array of finite depth holes -Bandgap above the soundline S. Benchabane, A. Khelif, J. –Y. Rauch, L. Robert, V. Laude, Phys. Rev. E 2006, 73, Previous Approaches Soundline Rayleigh SAW Leaky SAW

-Square array of cylindrical pillars -Resonances flatten phonon bands M. Addouche, M. A. Al-Lethawe, A. Choujaa, A. Khelif Appl. Phys. Lett 2014, 105, Previous Approaches

-Novel method based on annular holes -Exciting flexible platform -Structural integrity -Applicable for acoustoelectric interaction studies Our Approach D RIRI A RORO

Simulations

Finite Element Method (FEM) -Want to find dispersions of PnCs and create bandgaps -No analytical solutions for piezoelectric surfaces with high anisotropy -Useful tool for optimising geometry Simulations Bloch-Floquet periodic boundary conditions Fixed Constraint Unit Cell

- Complete bandgap from ~ 90MHz – 110MHz - Lower limit of gap determined by depth dependent resonance - Upper limit by depth and radial dependent resonance (Bessel function) ΓΓ A B Complete Bandgaps Simulations

- Complete bandgap from ~ 90MHz – 110MHz - Lower limit of gap determined by depth dependent resonance - Upper limit by depth and radial dependent resonance (Bessel function) Simulations ΓΓ A B Complete Bandgaps A

- Complete bandgap from ~ 90MHz – 110MHz - Lower limit of gap determined by depth dependent resonance - Upper limit by depth and radial dependent resonance (Bessel function) Simulations ΓΓ A B Complete Bandgaps B

Fabrication and Characterisation

‒ FIB etching ‒ 3mm x 80µm area patterned (270 x 7 array) ‒ Holes 6.4µm deep, 11µm pitch Device Fabrication

Measurement Setup Pulse generator Input RF signal Output SAW signal Vacuum chamber Coupled RF signal RF signal generator Oscilloscope

Measurements – Testing bandgap -Dispersion bandgaps at 90MHz – 110MHz and >160MHz

Measurements – Testing bandgap -Dispersion bandgaps at 90MHz – 110MHz and >160MHz

-Extraordinary frequency dependent attenuation due to air gas loading -Potential use as a gas sensor Measurements – Gas Loading

-Used FEM simulations to find that novel annular hole array design can work as a phononic crystal -Fabricated the device using focused ion beam etching and found experimental evidence for simulated dispersions -Found extraordinary frequency dependent gas loading in PnC -Future work to investigate further functionality for annular hole PnC -E.g. SAW waveguiding and combining with acoustoelectric interaction in 2D materials Conclusion and Future Work