Refractive Index Sensor Based on a Gradient Grating Period Guided-Mode Resonance

In this letter, we introduce a novel refractive index sensor based on gradient grating period guided-mode resonance (GGP-GMR). Rather than a constant grating period, as in typical GMR, GGP-GMR comprises a grating period varying in increments of 2 nm. For grating variation from 386 to 392 nm, a detection range of 0.109 RIU with a bulk sensitivity of 1433.7 $\mu \text{m}$ /RIU and theoretical and experimental detection limits of $5.58 \times 10^{-3}$ and $10^{-2}$ RIU, respectively, can be achieved. Several tactics that can be readily adopted are suggested to improve the limit of detection by an order of $10^{-4}$ RIU, sufficient for detecting many clinically relevant biomarkers. The readout system requires a narrow band of light and a simple charge-coupled device combined with the compact design of the GGP-GMR sensor, which eases the miniaturization of the system for handheld devices as well as its integration with smartphones.