High quality factor and high sensitivity photonic crystal rectangular holes slot nanobeam cavity with parabolic modulated lattice constant for refractive index sensing

In this paper, we present a novel optical sensor based on photonic crystal slot nanobeam cavity (PCSNC) with rectangular air holes. By introducing a continuous slot and quadratically modulated hole spacing (lattice constant a) structure, the majority of the optical field is localized in the slot region, which enhances the light-matter interaction. With applying the three dimensional finite-difference time-domain (3D-FDTD) simulations, three key geometric parameters (hole width w(x), slot width w(s) and the number of the holes N) are optimized to achieve a high sensitivity (S) while keeping a high quality (Q) factor. The highest S over 1000 nm/RIU (refractive index unit) is achieved when the slot width equals to 200 nm. The highest Q-factor of 2.15x10(7) is obtained when 30 holes are placed on both sides of the host waveguide with the slot width of 80 nm. Considering the transmission efficiency and the trade-off between S and Q-factor, the slot width and the number of the tapered region are chosen as 80 nm and 20, respectively. A high S approximately 835 nm/RIU and a Q-factor about 5.50x10(5) with small effective mode volume of 0.03(lambda/n(air))(3) are achieved simultaneously, resulting in an ultra-high figure-of-merit (FOM) above 2.92x10(5). Furthermore, the active sensing region of the optimized structure occupies only about 12 mu mx0.08 mu m, which makes the device attractive for realizing on-chip integrated sensor arrays.