Imaging transverse modes in a GHz surface acoustic wave cavity

TL;DR

This paper introduces a fiber-based interferometric method for imaging the spatial distribution of surface acoustic waves in a GHz cavity, revealing overlapping transverse modes undetectable by electrical means.

Contribution

It presents a novel optical measurement technique that enables frequency- and spatially-resolved imaging of SAW fields, uncovering complex mode structures in GHz cavities.

Findings

Detection of frequency-overlapping transverse modes in a 1 GHz SAW cavity

Demonstration of optical imaging revealing mode superpositions

Analysis of complex acoustic fields via quadrature decomposition

Abstract

Full characterization of surface acoustic wave (SAW) devices requires imaging the spatial distribution of the acoustic field, which is not possible with standard all-electrical measurements where an interdigital transducer (IDT) is used as a detector. Here we present a fiber-based scanning Michelson interferometer employing a strongly focused laser beam as a probe. Combined with a heterodyne circuit, this setup enables frequency- and spatially-resolved measurements of the amplitude and phase of the SAW displacement. We demonstrate this by investigating a 1 GHz SAW cavity, revealing the presence of frequency-overlapping transverse modes, which are not resolved with an all-electrical measurement. The frequency overlap of these transverse modes leads to mode superpositions, which we analyze by quadrature decomposition of the complex acoustic field.