  Phononic wire waveguides of subwavelength cross-section support two
orthogonal polarization modes: the out-of-plane motion dominated Rayleigh-like
and the in-plane motion dominated Love-like modes, analogous to
transverse-electric and transverse-magnetic modes in photonic waveguides. Due
to the anisotropic elasticity of the substrate material, the polarization
states of phonons propagating along certain crystallographic orientations can
strongly hybridize. Here we experimentally investigate the
orientation-dependent mode hybridization in phononic wire waveguides patterned
from GaN-on-sapphire thin films. Such mode hybridization allows efficient
actuation of piezoelectrically inactive Love-like modes using common
interdigital electrodes designed for Rayleigh-like modes, and further enables
on-chip polarization conversion between guided transverse modes. Both are
important for on-chip implementation of complex phononic circuits.
