Control of Fano resonances in photonic crystal nanobeams side-coupled with nanobeam cavities and their applications to refractive index sensing

We study the control of Fano resonances in a 2D photonic crystal nanobeam (PCN) side-coupled with a photonic crystal nanobeam cavity (PCNC) by choosing different cavity modes, the position of the photonic bandgap of PCNs and the displacement between PCNs and PCNCs. By increasing the refractive index of the holes and the surrounding medium, it is found that the air mode cavity with even mirror-reflection symmetry holds the highest sensitivity (538 nm/RIU; RIU, refractive index unit) and maximal figure of merit (FOM  =  516). Our results can be extended to a practical 3D configuration, where an air-suspended silicon PCN is side-coupled with a PCNC. Although the sensitivity is only 192 nm/RIU for our 3D structures, the maximal FOM is as large as 2095 due to the deep transmission valley. The sensitivity of our PCN–PCNC structures can be further improved by designing PCNCs with electric field concentrated in the air region as much as possible. Our PCN–PCNC structures do not require ultrahigh Q and can be fabricated on the silicon-on-insulator platform, which is compatible with CMOS processing. Therefore, our proposed PCN–PCNC structures provide feasible solutions for realizing label-free sensitive integrated refractive index sensors. (paper)