An optimized H1 photonic crystal (PhC) microcavity with a high quality factor ( Q ), small mode volume, wide measurement range, and high sensitivity is proposed, and its possibility in spectroscopic gas sensing is theoretically demonstrated. In the proposed structure, we have taken advantage of the air holes surrounding the defect region to optimize the resonance characteristics so as to detect the target gas, which is required for designing miniaturized spectroscopic gas sensors. By fine-tuning the structural parameters of several ring air holes surrounding the defect region, the results show that the sensitivity can be up to 3303 nm/RIU, along with a high Q of 10 4 and a detection limit as low as 10 − 4 RIU for gas sensing. Also, it is theoretically verified that the optimized H1 PhC microcavity has the ability to maintain RI sensitivity over a wide RI range of gases, enabling detection of H e , N 2 , C O 2 , C 2 H 2 , and C 3 H 8 gases. The main feature of this structure is that the enhanced electrical field is strongly localized in the defect region for any gas sample due to a small mode volume. Therefore, these findings provide useful design rules for applications involving compact and tunable spectroscopic gas sensor devices with high Q and sensitivity.

The optical properties of point-defect nanocavities implemented on elliptical-hole planar photonic crystal (EPhC) have been investigated for mid-infrared liquid sensing application. The EPhC nanocavities are composed of silicon planar photonic crystals with a triangular lattice of elliptical air holes, which do not require sophisticated design and high fabrication resolution. The numerical results show that the quality factor and the mode volume of the EPhC nanocavities increase exponentially with an increase in the ellipticity of the air holes. The Q factor is increased by a factor as large as 38 and 26 for H1 and L3 EPhC nanocavities, respectively, with a small mode volume by finely optimizing the EPhC nanocavities. Furthermore, the transmission spectrum of the optimized EPhC nanocavities shows that the variation in the refractive index that is used in the coating of the EPhC nanocavities induces a shift in the resonant wavelength of the nanocavity modes, thus allowing the precision measurement of liquid. In addition, a point-defect EPhC nanocavity with a sensitivity of 285 nm/refractive index unit (RIU) and a detection limit of 10 ⁻⁴ RIU are demonstrated. Therefore, these results suggest that the designed EPhC nanocavities are considered as a promising platform for mid-infrared liquid sensor devices with small detection volumes.

We present the design and analysis of a high efficiency slotted photonic crystal waveguide for gas sensing applications in the mid-infrared wavelength range. We designed the slotted photonic crystal waveguide structure by engineering the interfaces between the input and output slot waveguides and a resonant coupler. Coupling and transmission spectra of the sensor have been modeled using the three-dimensional finite-difference time domain method. The sensing principle is based on the shift of the upper band edge of the sensor output transmission spectrum which arises due to changes in the ambient refractive index. Transmission spectrum and sensitivity of the proposed sensor are analyzed by tuning the radii of the air holes localized on each side of the slot. The results show that a change of the refractive index of the gas by 10−4 leads to a shift of the upper band edge wavelength by 540 pm which corresponding to a sensitivity of 1720 nm per refractive index unit. The proposed design may be an ideal platform for developing mid-infrared gas sensing devices characterized by high coupling efficiencies and high sensitivities.