Ultrahigh- and Low-Mode-Volume Parabolic Radius-Modulated Single Photonic Crystal Slot Nanobeam Cavity for High-Sensitivity Refractive Index Sensing

We present a novel optical sensor based on the design of ultrahigh-Q and low-mode-volume 1-D single photonic crystal (PhC) slot nanobeam cavity (SNC) in which the air-hole radius is parabolically tapered. The performance of the device is investigated theoretically. In order to achieve high Q-factor and high sensitivity simultaneously, the slot geometry is exploited to make the optical field strongly localized inside the low index region and overlaps sufficiently with the analytes. With the three-dimensional finite-difference time-domain (3D-FDTD) method, we demonstrate that the proposed single 1-D PhC-SNC sensor device possess an ultrahigh sensitivity (S) up to ~900 nm/RIU (refractive index unit, RIU) and a high Q-factor in air up to > 10⁷ at the telecom wavelength range. The optimized figure of merit is > 10⁷. In addition, an ultrasmall mode volume of V_m ~0.01 (λ/η_air)3 has been achieved, which is more than three orders of magnitude smaller than our previous works [Appl. Phys. Lett. 105, 063118 (2014)] and, thus, is potentially an ideal platform for realizing ultracompact laboratory-on-a-chip applications with dense arrays of functionalized spots for multiplexed gas sensing.