Highly Q-factor elliptical absorber with cross slot as refractive index sensor in optical spectrum

Highly Q-factor plasmonic absorbers are attractive for achieving the higher figure of merit (FOM) and sensitivity in the optical sensors. In this work, an elliptical plasmonic absorber as metal–insulator–metal element is studied analytically with a cross-slot for increasing the Q-factor to concentrate the energy in gaps as a refractive index sensor for the mid-infrared spectrum. The elliptical absorber shows dual-band characteristic in the range of the 100 to 500 THz with sensitivity to incident wave mode and maximum Q-factor of 462 and 227 for TE and TM modes. The final model is selected based on parametric studies for the elliptical absorber for both TE and TM mode and it is simulated with the time-domain method. Finally, the proposed absorber is utilized as an optical sensor with maximum FOM is 88.2 RIU −1 (refractive index unit) and sensitivity of 476 nm/RIU where the nanoparticle is coated with the material under test with refractive indices of 1 to 1.5. So, this optical sensor can be used for a wide range of applications such as detecting blood groups or chemical material.