Low-loss hydrogen-doped indium oxide thermo-optic phase shifter for visible wavelengths

Thermo-optic phase shifters are the most widely used solution for phase control in integrated photonics, yet conventional designs based on metallic heaters suffer from high optical absorption and limited efficiency at visible wavelengths. Here, we present a thermo-optic phase shifter for the visible spectral range on a silicon nitride platform that supports both transverse electric (TE) and transverse magnetic (TM) polarizations, employing a thin layer of hydrogen-doped indium oxide (IHO) as the heater. Owing to its low extinction coefficient in the visible regime, the IHO layer can be directly integrated on top of the waveguide, enabling strong thermal coupling while maintaining low optical loss. This approach yields a compact, efficient device with reduced power consumption and faster response time. Furthermore, performance is optimized through air-trench engineering, balancing thermal confinement and heat dissipation. The optimized design achieves power consumption of 4.26 mW (TE) and 4.47 mW (TM), with a switching time of 3.3 µs and heater lengths below 40 µm for both polarizations, demonstrating a favorable trade-off between efficiency, speed, and footprint compared to existing Si₃N₄ thermo-optic phase shifters.