Helical Surface Wave Device for Opto-Acoustic Vortex Generation
Overview
This project was completed as my senior honors thesis in the Bowdoin College Physics Department and received Highest Honors in Physics.
The work investigates the generation of helical surface acoustic waves (SAWs) and their interaction with optical fields. The project combines experimental measurements of fabricated SAW devices with finite element modeling in COMSOL Multiphysics to study the generation of acoustic orbital angular momentum (OAM) and its transfer to light.
The full thesis is access-restricted. A public abstract and summary of the work are provided below.
Abstract
Full optomechanical control of light requires simultaneous modulation of amplitude, phase, and orbital angular momentum (OAM). At GHz frequencies, helical acoustic waves in solid-state platforms offer a potential route to controlling optical OAM on chip. This thesis demonstrates the generation of tunable near-GHz helical surface acoustic waves (SAWs) through the interference of multiple planar SAW beams and uses finite element simulation in COMSOL Multiphysics, validated against interferometric measurements of the fabricated device, to establish that the acoustic OAM of these waves can be transferred to an optical field under experimentally realistic conditions. A unit-cell model maps the dependence of resonance frequency, electromechanical coupling, and surface displacement on ZnO film thickness and interdigital transducer (IDT) electrode geometry. A full-device model of the hexagonal SAW vortex reproduces the measured amplitude and phase field maps, validating the modeling framework. An opto-acoustic extension couples the acoustic solution to a full electromagnetic wave simulation through photoelastic and moving-boundary effects. A Fabry–Pérot microcavity formed between a distributed Bragg reflector and a gold mirror enhances the single-pass interaction. The simulation shows that an acoustic vortex with topological charge \(\ell_{\mathrm{SAW}} = \pm 1\) transfers its angular momentum to the optical field through a dominant scattered sideband at \(\ell_{\mathrm{opt}} = \ell_{\mathrm{SAW}}.\) The target optical mode reaches a peak amplitude approximately 50 times larger than neighboring OAM channels, with a predicted scattered power on the order of 30 nW for a 10 mW optical source. These results indicate that acoustic-to-optical OAM transfer in this geometry is experimentally accessible and identify a path toward future experimental demonstrations.