Mid-infrared gas sensor based on high-Q/V point-defect photonic crystal nanocavities

Point-defect nanocavities based on silicon planar photonic crystal (PhC) have been optimized and studied for sensing the refractive index of gases in the mid-infrared wavelength region. The point-defect has been introduced at the center of the triangular lattice of the photonic crystal that is made up of circular air holes, making it suitable for probing the properties of the gas found within the cavity. By optimizing the radius and position of the air holes closest to the defect region precisely, on the order of a few nanometers, the ratio of the quality factor to mode volume (Q/V) for the point-defect PhC nanocavities can be increased considerably. Moreover, a perturbation method has been implemented in order to study the resonant wavelength shift of the optimized point-defect nanocavity modes caused by a small change in the refractive index of the gas. The results obtained show that sensitivity of 270 nm/RIU (Refractive Index Unit) and a detection limit of 10-4 RIU can be achieved for the optimized point-defect PhC nanocavities. These nanocavities have been designed to oscillate at a single mode with a high Q/V thus enabling to sense the refractive index of gases with high sensitivity.