Observation of Anisotropic Wavelength-Dependent Acousto-Optic Scattering on X-Cut Thin-Film Lithium Niobate

We investigate an integrated acousto-optic (AO) device featuring electrically driven anisotropic surface acoustic waves on X-cut thin-film lithium niobate (TFLN). Notably, we observe a novel phenomenon of wavelength-dependent periodic fluctuations in anisotropic AO scattering with different in-plane orientations. This effect is attributed to inter-polarization scattering involving the fundamental modes combined with intramodal AO interaction within the TFLN waveguide. Therefore, the trade-off between efficiency and fluctuation must be carefully considered. When the acoustic excitation direction is 150° (relative to the −Y-axis of LN), the Rayleigh acoustic mode R0 achieves a maximum intramodal scattering efficiency of −20.9 dB at 2.496 GHz, which corresponds to a VπL of 2.34 V·cm. Additionally, the higher-order acoustic mode at 3.544 GHz demonstrates a moderate conversion efficiency of −26.7 dB under a 75° emission angle, with a minimal wavelength-dependent fluctuation of 1.1 dB during AO modulation operation over a bandwidth exceeding 82 nm. By carefully selecting specific orientations or wavelengths, it is possible to tailor diverse applications for anisotropic TFLN AO devices, such as highly flattened AO modulation, enhanced intramodal AO interaction, and efficient inter-polarization conversion. These findings pave the way for developing integrated photonic–phononic interaction devices on widely utilized anisotropic material platforms.