Laser-written Sb₂Se₃ waveguides for nonvolatile reconfigurable photonics: optimizing the grain morphology on LNOI platform

Phase-change materials (PCMs) are widely utilized for non-volatile optical field manipulation due to their significant optical contrast between amorphous and crystalline states. Direct laser writing (DWL) enables mask-free patterning of phase-change structures on low-loss Sb₂Se₃ films, creating high-refractive-index crystalline waveguide cores surrounded by low-refractive-index amorphous cladding. This non-subtractive fabrication approach allows rewritable waveguides, offering particular advantages for photonic platforms, such as lithium niobate, that are challenging to etch. Here, we demonstrate rewritable hybrid waveguide devices fabricate d via DWL on lithium niobate on insulator (LNOI) coated with a 50 nm-thick Sb₂Se₃ film. While this technique permits free-form patterning, hybrid waveguides typically exhibit high optical loss due to scattering at grain boundaries in crystalline Sb₂Se₃. By precisely controlling crystallization kinetics and grain morphology, as well as synergistically optimizing the waveguide geometry, we achieved a ~11.1 dB improvement in optical transmittance for the laser-written waveguide devices These results underscore the critical role of crystallization morphology in PCM-integrated photonic devices and provide insights for optimizing their performance.