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

TL;DR

This study systematically analyzes scattering losses in ultra-low-loss lithium niobate cavities, identifying roughness sources and demonstrating near-intrinsic quality factors through detailed modeling and experimental validation.

Contribution

It provides a comprehensive, material-independent modeling framework for understanding and mitigating scattering losses in lithium niobate photonic cavities.

Findings

Achieved quality factors up to 27 million in waveguides with specific dimensions.

Identified interface roughness as the dominant loss in wider waveguides.

Developed a broadly applicable 3D wave simulation model informed by AFM measurements.

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 $10^7$ have been achieved, they remain below the intrinsic material limit. We present a systematic study of scattering losses due to roughness in TFLN racetrack cavities, isolating contributions from sidewall and interface roughness. Quality factors up to $27 \times 10^6$ are demonstrated in waveguides with widths of $2.2\lambda$ ($\sim3.5\,\mu$m), where interface roughness dominates, and up to $1.2 \times 10^7$ in narrower waveguides $0.8\lambda$ wide ($\sim1.2\,\mu$m), where sidewall roughness is the primary limitation. Our modeling framework, based on 3D wave simulations informed by AFM-measured roughness, is material-independent and broadly applicable across integrated photonic platforms.