Forward Brillouin scattering in non-suspended lithium niobate waveguides at visible wavelengths

Thin-film lithium niobate on sapphire provides an excellent platform for simultaneously confining acoustic and optical modes without suspended structures, enabling efficient acousto-optic modulation through strong piezoelectric coupling. Here, we identify the challenges in realizing the forward Brillouin interaction at visible wavelengths and overcome the limitation by introducing a quasi-phase-matching scheme through periodic waveguide width modulation. We predict a 69.9% inter-modal optical conversion efficiency over 0.36 mm using only 5 mW acoustic power. Our study paves the way for high-performance visible-wavelength acousto-optic devices on integrated platforms.