Mid-Infrared Sensor Based on a Suspended Microracetrack Resonator With Lateral Subwavelength-Grating Metamaterial Cladding

A mid-infrared (MIR) biochemical sensor based on a one-time Si etching suspended microracetrack resonator with lateral subwavelength-grating metamaterial cladding is theoretically and experimentally demonstrated on a commercial 340-nm-thick-top-silicon silicon-on-insulator platform. The suspended structure offers an increased interaction area between the mode field and the chemicals under investigation, as well as good sensitivity. The one-time Si etching process also eases the fabrication. The suspended waveguide is optimized to obtain a balance between propagation loss and sensitivity. The suspended microracetrack resonator is experimentally measured at 2 μm wavelength with an extinction ratio of 12.1 dB and a full-width at half-maximum of 0.13 nm, which corresponds to a quality factor (Q factor) of 15 300. With the equivalent refractive index method and a specially developed numerical model, the sensing performance based on the waveguide structure has been simulated and analyzed. The simulation results show that the expected sensitivity of fundamental TE mode can achieve 337.5 nm/RIU. This one-time Si etching suspended microracetrack resonator shows great potential for ultrasensitive MIR optical biochemical sensing applications.