Real-Time Gas Mixture Analysis Using Mid-Infrared Membrane Microcavities

Real-time gas analysis on-a-chip was demonstrated using a mid-infrared (mid-IR) microcavity. Optical apertures for the microcavity were made of ultrathin silicate membranes embedded in a silicon chip using the complementary metal-oxide-semiconductor (CMOS) process. Fourier transform infrared spectroscopy (FTIR) shows that the silicate membrane is transparent in the range of 2.5–6.0 μm, a region that overlaps with multiple characteristic gas absorption lines and therefore enables gas detection applications. A test station integrating a mid-IR tunable laser, a microgas delivery system, and a mid-IR camera was assembled to evaluate the gas detection performance. CH₄, CO₂, and N₂O were selected as analytes due to their strong absorption bands at λ = 3.25–3.50, 4.20–4.35, and 4.40–4.65 μm, which correspond to C–H, C–O, and O–N stretching, respectively. A short subsecond response time and high gas identification accuracy were achieved. Therefore, our chip-scale mid-IR sensor provides a new platform for an in situ, remote, and embedded gas monitoring system.