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

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.