Optical properties of a point-defect nanocavity-based elliptical-hole photonic crystal for mid-infrared liquid sensing

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.