High quality factor photonic crystal filter at k ≈0 and its application for refractive index sensing.

We report here the refractive index (RI) sensing using the singly degenerate high quality factor (Q) modes in photonic crystal slabs (PCS) with the free-space coupled incident beam close to normal incidence. Q values of 3.2x10⁴ and 1.8x10⁴ were achieved for the fabricated PCS in air and aqueous solution, respectively. A spectral sensitivity (S) of 94.5 nm/RIU and a detection limit (DL) of 3x10^-5 RIU were achieved with our device. Such a high-Q cavity for the singly degenerate mode close to normal incidence is very promising to achieve a lower DL for RI sensing.