Low-Loss and High-Confinement Photonic Platform Based on Germanium-on-Insulator at Mid-Infrared Range for Optical Sensing

We experimentally demonstrate a low-loss Ge-on-insulator (Ge-OI) passive waveguide with low absorptive yttrium oxide (Y
₂
O
₃
) as an insulator for a photonic platform using a wafer-bonding technology at mid-infrared (MIR) wavelength. The lowest propagation losses of 0.84 dB/cm for a channel waveguide (ChW) and 1.79 dB/cm for a slot waveguide with a slab under the slot (s-SlW) were obtained in fundamental transverse-electric (TE) mode at 4.2 μm wavelength. We also designed additional passive circuits such as grating couplers and mode converters for the implementation of the platform. For an optical gas sensor using the platform, the narrow core width of the ChWs and nanoscale of slot gap of the s-SlWs have been derived based on the numerical design toward high optical confinement, and these were fabricated and systematically analyzed to minimize the propagation loss. From the comparison for loss measurement between a nitrogen (N
₂
) ambient and a carbon dioxide (CO
₂
) containing normal atmosphere at 4.2 μm wavelength, as a consequence, narrow ChW and s-SlW showed quite high additional optical loss by the CO
₂
absorption, revealing high potential as an optical gas sensor. In conclusion, we believe that this Ge-OI platform can be a very promising candidate for the optical sensor in terms of low-loss and high-confinement properties.