Low-loss and high-index contrast ultraviolet-C free-standing waveguides made of thermal silicon oxide

Photonics in the ultraviolet provides an avenue for key advances in biosensing, pharmaceutical research, and environmental sensing. However, despite recent progress in photonic integration, a technological solution to fabricate photonic integrated circuits (PICs) operating in the UV-C wavelength range, namely, between 200 and 280 nm, remains elusive. Filling this gap will open opportunities for new applications, particularly in healthcare. A major challenge has been to identify materials with low optical absorption loss in this wavelength range that are at the same time compatible with waveguide design and large-scale fabrication. In this work, we unveil that thermal silicon oxide (TOX) on a silicon substrate is a potential candidate for integrated photonics in the UV-C, by removing the silicon substrate under selected regions to form single-side suspended ridge waveguides. We provide design guidelines for low-loss waveguide geometries, avoiding wrinkling due to residual intrinsic stress, and experimentally demonstrate waveguides that exhibit optical propagation losses below 3 and 4 dB/cm at a wavelength of 266 nm with claddings of air and water, respectively. This result paves the way for on-chip UV-C biological sensing and imaging.