Extraordinary Gas Loading For Surface Acoustic Wave

Phononic CrystalsBen Ash

Supervisors – G. R. Nash, P. VukusicEPSRC 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, 065601

Previous ApproachesSoundlineRayleigh SAWLeaky 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, 023501

Previous Approaches

- Novel method based on annular holes

- Exciting flexible platform

- Structural integrity

- Applicable for acoustoelectric interaction studies

Our Approach

D

RI

A

RO

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)

Γ Γ

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

Γ Γ

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

Γ Γ

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 SetupPulse

generator

Input RF signal

Output SAW signal

Vacuum chamber

Coupled RF signalRF 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