Refractive index sensing using analytical calculation of angular momentum sidebands of a Laguerre Gaussian beam reflected from a plasmonic structure

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2025年7月1日

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Mojtaba Baniasadi,
Abbas Ghasempour Ardakani

摘要

When a Laguerre Gaussian beam carrying orbital angular momentum (OAM) with a specific topological charge (TC) is incident onto an interface, the reflected beam will be a superposition of different OAM states. In this study, we analytically calculate the OAM spectrum of the reflected beam from a plasmonic structure and determine the weight of each TC within the beam. Our results confirm that for incident TC \(\:{l}_{i}\) the only nonzero TCs in the reflected beam are \({-l}_{i},-\left({l}_{i}\pm1\right)\) and \(-\left({l}_{i}\pm2\right)\) . Furthermore, the weight of these sidebands depends on how much the system is far from the optimal excitation of surface plasmons. We utilize these weights to introduce a novel method for detecting subtle changes in the refractive index of the sensing medium in plasmonic biosensors, where conventional methods based on measuring the reflectance of a Gaussian beam incident onto the sensor versus wavelength or incident angle fail to detect such small changes.