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Lithium niobate (LiNbO3, LN) is a ferroelectric crystal of interest for integrated photonics owing to its large second-order optical nonlinearity and the ability to impart periodic poling via an external electric field. However, on-chip device performance based on thin-film lithium niobate (TFLN) is presently limited by propagation losses arising from surface roughness and corrugations. Atomic layer etching (ALE) could potentially smooth these features and thereby increase photonic performance, but no ALE process has been reported for LN. Here, we report an isotropic ALE process for x-cut MgO-doped LN using sequential exposures of H2 and SF6/Ar plasmas. We observe an etch rate of 1.59±0.02 nm/cycle with a synergy of 96.9%. We also demonstrate that ALE can be achieved with SF6/O2 or Cl2/BCl3 plasma exposures in place of the SF6/Ar plasma step with synergies of 99.5% and 91.5%, respectively. The process is found to decrease the sidewall surface roughness of TFLN waveguides etched by physical Ar+ milling by 30% without additional wet processing. Our ALE process could be used to smooth sidewall surfaces of TFLN waveguides as a postprocessing treatment, thereby increasing the performance of TFLN nanophotonic devices and enabling new integrated photonic device capabilities.
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This content will become publicly available on February 1, 2027
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. WhileQfactors above 107are 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 × 107are demonstrated in waveguides with widths of 2.2λ (≈3.5 µm), where interface roughness dominates, and up to 1.2 × 107in 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.
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- Award ID(s):
- 2402483
- PAR ID:
- 10695801
- Publisher / Repository:
- Advanced Optical Materials
- Date Published:
- Journal Name:
- Advanced Optical Materials
- Volume:
- 14
- Issue:
- 8
- ISSN:
- 2195-1071
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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