Mid-infrared sensing with dual-band, tunable, polarization-insensitive L-shaped metasurfaces

Mid-infrared (IR) sensing keeps evolving with developments in semiconductor materials, laser technologies, and quantum optics. Gases with different absorption bands in this range can be detected in the mid-IR area. Furthermore, molecules can be identified as chemical species with excellent selectivity thanks to their distinct vibrational absorption signatures in the mid-IR band. A metasurface is a type of material engineered to have properties that are not found in naturally occurring materials. The metasurface, composed of a thin layer of structured material, has the ability to control electromagnetic waves, particularly those in the mid-infrared spectrum. This work presents a dual-band, tunable, wide-angle, polarization-insensitive mid-IR sensor composed of L-shaped gold metasurfaces on a dielectric spacer and a gold ground surface. A commercial simulator (Comsol Multiphysics) based on the finite element method is used to engineer the metasurfaces in the shape of double and quadruple L-shaped structures. The absorption spectra of the L-shaped metasurfaces show two different peaks, as shown by the numerical findings. The peaks are confirmed to result from magnetic polariton modes produced at two distinct resonant wavelengths by analyzing the electric field distribution. Furthermore, the proposed structure exhibits strong sensing stability across a broad range of incident angles for both TE and TM polarization. Moreover, we show that by altering the separation between two successive L-shaped structures, such a structure may be tailored to desired wavelengths. When the device's top medium is switched from air to water, the suggested metasurfaces provide a sensitivity of 800 nm/RIU. In conclusion, the proposed metasurfaces in mid-IR sensing are a state-of-the-art technology that combines spectroscopy with sophisticated materials engineering to enable sensing devices that are very sensitive, small, and adaptable for various applications.