Reduced material loss in thin-film lithium niobate waveguides*

Owing to its large nonlinearity, wide-transparency window, and low optical loss, thin-film lithium niobate (TFLN) is a promising candidate for applications in classical and quantum domains. Several devices including single-photon sources, ultra-high bandwidth modulators with low Vπ, integrated frequency comb sources, and frequency shifters, have been demonstrated on this platform. However, realizing the generation of TFLN devices such as broader electro-optic frequency combs, high-efficiency microwave-to-optical transducers, and single photon nonlinear devices, rely on lowering the optical losses significantly. To date, the lowest optical loss achieved on TFLN microring devices is 2.7 dB/m. Sidewall-induced scattering losses and material absorption can hinder achieving lower losses. One can improve the former by implementing a better fabrication process, while the latter is caused by the material quality and absorption mechanisms. We show that post-fabrication annealing and low-temperature oxide cladding can significantly reduce optical absorption in TFLN waveguides. We quantify the intrinsic absorption losses using a TFLN microring resonator and Kerr-calibrated linear response measurement. Using our newly developed characterization method, we measure the material-limited loss of 0.2 dB/m and nonlinear refractive index of n₂ = 1.61 × 10−19 m² W⁻¹ for our TFLN devices.