Phonon Trapping Lateral Field Excited Suspended Bulk Acoustic Wave Resonators (XBARs)

TL;DR

This paper introduces a novel phonon trapping FBAR resonator with a spherical lens shape that enhances modal confinement and quality factor, advancing the development of efficient microwave to optical quantum transducers.

Contribution

The authors fabricate a spherical lens-shaped phonon trapping FBAR (XBAR) that significantly improves modal confinement and quality factor compared to traditional designs.

Findings

Approximate 4x increase in quality factor.

Enhanced modal confinement demonstrated.

Potential for improved microwave to optical transduction.

Abstract

Film bulk acoustic wave resonators (FBARs) underpin modern wireless communication by enabling compact, high-performance RF filters in modern smartphones. Traditionally, these FBAR devices work with quasi-plane waves of sound where the transverse extent of the acoustic field $\gg$ the acoustic wavelength ($\lambda_a$). On the other hand, strong modal confinement is needed for achieving the interaction strengths necessary for building efficient microwave to optical quantum photon transducers (MW-OT) around an FBAR opto-mechanical cavity platform. Here, we fabricate a small mode-volume phonon trapping lateral field excited FBAR resonator (XBAR) by shaping the piezoelectric layer into a spherical lens, show an improvement in modal confinement and quality factor ($\approx$ 4$\times$), and discuss the improvements needed for building efficient MW-OTs around this XBAR geometry.