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 signal transducers (MW-OT) around an FBAR opto-mechanical cavity platform.

While MW-OTs have traditionally been engineered around sub-{\mu}m scale optomechanical cavities, bulk acoustic wave approaches have inherent advantages in phonon injection efficiency, optical power handling and manufacturability.

A key limitation of the FBAR geometry is that the acoustic field is confined under the metal electrode which makes it challenging to engineer the small mode-volume, high quality factor optical cavities which are critical for achieving high transduction efficiency.

Here, we consider lateral field excited suspended overtone bulk acoustic wave resonators (XBARs) as an alternative bulk wave platform for MW-OT, which overcome this limitation, and outline the requirements needed for building efficient MW-OT around this geometry.

As a first step towards viability, we fabricate a small mode-volume phonon trapping acoustic microresonator by shaping the piezoelectric layer into a spherical lens and show an improvement in modal confinement and quality factor ($\approx$ 4$\times$).