Tailoring LRSPR Penetration Depth Employing 2-D Material at Visible and IR Range

Surface plasmon resonance (SPR) is a prevalent sensing approach for detecting the additional mass concentrations as small as pg/ml and Refractive Index changes up to the orders of $0.1 \; \mu $ RIU. However, SPR possesses a penetration depth of approx. 200 nm limiting its application for microbes such as bacterial cells and viruses. Long-range SPR (LRSPR) possesses more equitable sensing mechanism and deeper penetration depth improving the performance than traditional SPR. In this work, we propose a theoretical design based on transfer matrix method (TMM) for developing LRSPR chip to achieve high penetration depth and figure of merit (FOM). Here, we have used low index prism (CaF2) for high penetration depth with optimized adhesive layer of 1900 and 600 nm for visible and IR wavelengths providing penetration depths of 817 and 1513 nm, respectively. Study revealed that for IR ranges, penetration depth increases comprising the FOM which can further be improved by additional 2-D material between metal-sensing layers. A variety of 2-D materials are used to obtain the best performance of the chip. The inclusion of a monolayer MoS2 improves FOM due to higher absorption of light as it possesses higher imaginary Refractive Index. The simulated findings reveal that the greatest penetration depth of the IR spectrum is 1508.93 nm near surrounding RI of 1.35 aliens with the index of aqueous biomolecular samples. The study demonstrates a potential use of 2-D material with enhanced LRSPR sensor performance with depths comparable to bacterial cells, viruses, and proteins.