Roughness‐Limited Performance in Ultra‐Low‐Loss Lithium Niobate Cavities

Abstract Achieving low optical loss is critical for scaling complex photonic systems. Thin‐film lithium niobate (TFLN) offers strong electro‐optic and nonlinear properties in a compact platform, making it ideal for quantum and nonlinear optics. While Q factors above 10 ⁷ are achieved, they remain below the intrinsic material limit. A systematic study of scattering losses due to roughness in TFLN racetrack cavities is presented, with isolating contributions from sidewall and interface roughness. Quality factors up to 2.7 × 10 ⁷ are demonstrated in waveguides with widths of 2.2λ (≈3.5 µm), where interface roughness dominates, and up to 1.2 × 10 ⁷ in narrower waveguides 0.8λ wide (≈1.2 µm), where sidewall roughness is the primary limitation. The modelling framework, based on 3D wave simulations informed by roughness measured by AFM, is applicable to any index contrast between waveguide core and cladding and widely applicable on integrated photonic platforms.