Optimization of figure of merit in label-free biochemical sensors by designing a ring defect coupled resonator

We propose a high figure of merit (FOM) biochemical sensor by designing a ring defect coupled resonator (RDCR) based on photonic crystal (PhC) slab. The design consists of ring resonant cavity which is coupled in and out with ring and line defect PhC structure. By a three dimensional finite-different time-domain (3D-FDTD) method, we demonstrate that the quality (Q) factor is greatly enhanced by altering the radius of air holes inner the ring resonant cavity and adjusting the width of line defect waveguide. In this paper, we obtain a highest Q up to 107 through numerical calculations. Even though water absorption at telecom wavelength range and random roughness of fabrication is considered, a Q of ~33,517 can be achieved. Simultaneously the proposed sensor possesses sensitivity (S) of 330 nm/RIU (refractive index unit), resulting in FOM of ~8000. Moreover, a minimal detection limit (DL) is obtained as good as 1.24×10−5. Therefore, these suggest that this design is a promising candidate for label-free biochemical sensing in medical diagnosis, life science and environmental monitoring.