Crack-free, partial-ring cladding surface waveguides in sapphire fabricated by femtosecond laser direct inscription.

Femtosecond laser direct writing is highly effective for fabricating three-dimensional photonic waveguides deep inside ultra-hard crystals. Extending this capability to the surface realm, where waveguides interact directly with the sample interface, has been challenging due to pronounced susceptibility to structural cracking. To address this, we introduce a strategy that avoids direct laser modification of fragile crystal-air interfaces. Our approach employs a novel, to the best of our knowledge, surface-integrated, partial-ring cladding architecture, unlike the conventional full-ring design for buried waveguides. By precisely controlling the laser focal depth and cladding geometry, we demonstrate crack-free surface waveguides in sapphire with strong optical confinement and a propagation loss of ~2 dB/cm at 1500-1600 nm. Experiments and simulations confirm that buried waveguides with full-ring cladding support near-Gaussian modes, whereas partial-ring surface waveguides exhibit minor modal distortion owing to interfacial effects. This work advances the integration of high-performance surface waveguides in hard optical materials for applications in surface sensing and photonic circuits.