Optimized Double-Layered Grating Structures for Chem/Biosensing in Midinfrared Range

Double-layered grating structures (DGSs) fabricated of gold (Au) and silver (Ag), respectively, are proposed for chem/biosensing in the midinfrared range. The geometry parameters of the DGS sensors are optimized using a rigorous coupled wave analysis method such that the minimum reflectance is obtained at an infrared wavelength of 4.8 μ or 6.7 μm. The magnetic field patterns and Poynting vector distributions within the optimized grating structures are examined using a finitedifference time-domain method. The simulation results show that for a resonant wavelength of 4.8 μm, the Au and Ag sensors both have a sensitivity of 2000 nm/RIU (refractive index unit) and 3000 nm/RIU theoretical sensing resolution of 5 × 10^-6 RIU and of 1 × 10^-7 RIU, respectively. Similarly, for a resonant wavelength of 6.7 μm, the two sensors have a sensitivity of around 200 nm/RIU and theoretical resolution of 5 × 10^-5 RIU. In other words, the sensitivity of the DGS sensors optimized for an infrared wavelength of 4.8 μm is one order higher than that of the sensors optimized for a wavelength of 6.7 μm. In general, the results presented in this paper show that the proposed DGS sensors provide a simple and versatile solution for performing chem/biosensing at midinfrared wavelengths.