High Figure of Merit Fano Resonance in 2-D Defect-Free Pillar Array Photonic Crystal for Refractive Index Sensing

We present a simple and robust structure to realize Fano resonance for refractive index sensing applications. The Fano resonance with ultrahigh $Q$ factors ( $Q$ $>5.0\times 10^4$ ) and high reflections (near 100%) is realized in a 2-D defect-free pillar array photonic crystal (PAPC) with optimized geometry (e.g., pillar array periodicity, height, and radius). We analyze its band structure, polarization, and transmission characteristics in detail with numerical methods (plane wave expansion and 3-D finite-difference time-domain (3-D-FDTD) simulations). The numerical results show that FOM $\sim6.85\times 10^3$ (FOM being the figure of merit) can be achieved, which is an order of magnitude improved over the previous work. In addition, the unique simplicity of this system in which a long-lifetime delocalized electromagnetic field exists and interacts with the surrounding media permits new opportunities for point-of-care applications. Furthermore, the proposed architecture makes the implementation of lab-on-a-chip refractive index sensing possible, which is enabled by integrating PAPC on the tip of an optical fiber.