Boundary-Induced Helical Bulk Acoustic Transport in LiNbO_{3} Thin Films

We experimentally demonstrate boundary-induced helical bulk states (BI-HBSs) for rf acoustic transport in LiNbO3 thin-film phononic crystals (∼175-200  MHz). A boundary-symmetry selection rule at an accidental Γ-point fourfold degeneracy creates interior bulk channels that couple to wide-aperture interdigital transducers without edge-aperture mismatch. Near-field vibrometry and two-port rf S parameters confirm low-loss propagation with strongly suppressed backscattering through wavelength-scale defects. The helical band also provides slow-wave, low-dispersion delay and phase control on chip.