Self-Coupling of Multiple Long-Range Surface Plasmon Resonance Modes for Ultrasensitive Sensors

In this study, we propose a sensing probe architecture based on a 1-D photonic crystal (1DPC) composed of periodically arranged Cytop/Al structures and realize long-range surface plasmon resonance (LRSPR) through prism coupling. This architecture is characterized by high sensitivity, high detection accuracy, and micrometer-level penetration depth (PD). LRSPR is characterized by narrow resonance peaks, and the combination of multiple LRSPR structures results in strong coupling resonance, leading to the splitting of the resonance signal into multiple peaks. We found that the first resonance peak is the most sensitive and can effectively detect changes in the surrounding environment. The results show that this LRSPR sensing structure based on the Cytop/Al 1DPC demonstrates exceptional sensitivity with a periodicity of two layers (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{\max } =5846$ </tex-math></inline-formula> RIU<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula>). Compared with conventional surface plasmon resonance (SPR) sensors (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{\max } =59$ </tex-math></inline-formula> RIU<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula>), the sensitivity of the proposed LRSPR sensor is increased by about 100 times, achieving an extremely high detection limit of up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\times 10^{-{7}}$ </tex-math></inline-formula> RIU. In addition, the two-layer structure remains optimal across refractive indices from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${n}_{s} =1.328$ </tex-math></inline-formula> to 1.334, underscoring its adaptability. Our research utilizes the 1DPC structure, which, due to its simplicity and stability, exhibits significant potential to enhance both the sensitivity and detection limit of SPR sensors through structural design and simulations. This approach facilitates substantial improvements in sensor sensitivity and resolution within a relatively simple structure, thereby offering potential application value for biomedical and chemical detection.