Silicon-on-sapphire waveguides design for mid-IR evanescent field absorption gas sensors

Major trace gases have absorption lines in mid-IR. We propose silicon-on-sapphire waveguides at mid-IR for gas sensing based on evanescent field absorption. This can provide a general platform for multipurpose sensing of different types of gases in a reusable fashion. Three types of waveguides (strip, rib and slot) are investigated on their geometrical dependence of evanescent-field ratio (EFR) and propagation loss to serve as the proposed gas sensor. Slot waveguide provides the highest EFR (>25%) in mid-IR with moderate dimension, but its fabrication can be more challenging and its high loss (~10 dB/cm) impairs the sensing resolution and necessitates higher input power in longer waveguides. Strip and rib waveguides can achieve similar EFR with smaller dimensions. We analyze the detection of CO2 in atmosphere based on its mid-IR absorption peak at ~4.23 µm as a case study. Numerical analysis based on up-to-date commercial mid-IR detector parameters shows that a resolution of 2 ppm, 5 ppm and 50 ppm can be achieved in cooled InSb, room-temperature HgCdTe and room-temperature PbSe detectors respectively by using 1 cm waveguides. Effect of waveguide loss also has been investigated.