Ultrasensitive Mid-Infrared Optical Gas Sensor Based on Germanium-on-Insulator Photonic Circuits with Limit-of-Detection at Sub-ppm Level

Abstract

Optical waveguide structures guiding a strongly localized field in an external region for mid-IR integrated gas sensors using the IR tunable diode laser absorption spectroscopy (TDLAS) technique are significant elements whose role is to improve the performance in a compact device. In recent times, Ge has been adopted as an attractive photonic platform providing better opportunities thanks to Ge being one of the transparent materials over the broad mid-IR range and having a high index contrast with the external region. Herein, we propose a Ge-OI-based slot waveguide (SlW) sensor, where the insulator is Y 2 O 3, with a high confinement factor (45.2%) in the sensing region and low propagation loss (1.93 dB/cm) at 4.23 μm wavelength. The spiral Ge-OI SlW with 2.14 mm 2 area within the compact footprint was successfully fabricated by direct wafer bonding. Our Ge-OI SlW sensor exhibits outstanding sensitivity (0.618%/ppm) and limit-of-detection (LoD) (0.627 ppm) to detect carbon dioxide (CO 2 ) at 4.23 μm. Furthermore, spectroscopic analysis revealed a strong light–matter interaction with excellent repeatability. Finally, our work demonstrates remarkable performance with ultrahigh sensitivity and subppm level LoD in the mid-IR waveguide-based optical sensor.