APL Photon. 4, 026101 (2019)

Silicon, with a bandgap at 1.1 μm, strongly absorbs visible and ultraviolet light and thus is not a suitable material for constructing integrated optical circuits that operate in these regions. However, it now appears that aluminium oxide (alumina) with a bandgap of between 5.1 and 7.6 eV (163–243 nm) could be a promising contender. Gavin West and colleagues from the USA have demonstrated a low-loss integrated photonics platform from alumina that operates in the visible and near-ultraviolet regime. Using atomic layer deposition of alumina followed by reactive ion etching, the team was able to create waveguides with steep, smooth sidewalls. Propagation losses were measured to <3 dB cm–1 and <2 dB cm–1 at 371 nm and 405 nm, respectively. The team also fabricated ring resonator structures with a 500 nm waveguide width and 90 μm radius, and obtained intrinsic quality factors higher than 470,000 at 405 nm. The thermo-optic coefficient of the resonator was 1.68 × 10–5 RIU °C–1, which was comparable to that of silicon nitride at telecom wavelengths. The team pointed out that lower loss is possible by reducing the sidewall roughness.