  Transverse bulk phonons in a multimode integrated quantum acoustic device are
excited and characterized via their free-space coupling to a three-dimensional
(3D) microwave cavity. These bulk acoustic modes are defined by the geometry of
the Y-cut lithium niobate substrate in which they reside and couple to the
cavity electric field via a large dipole antenna, with an interaction strength
on the order of the cavity line-width. Using finite element modeling (FEM) we
determine that the bulk phonons excited by the cavity field have a transverse
polarization with a shear velocity matching previously reported values. We
demonstrate how the coupling between these transverse acoustic modes and the
electric field of the 3D cavity depends on the relative orientation of the
device dipole, with a coupling persisting to room temperature. Our study
demonstrates the versatility of 3D microwave cavities for mediating
contact-less coupling to quantum, and classical, piezoacoustic devices.
