Refractive index sensing using analytical calculation of angular momentum sidebands of a Laguerre Gaussian beam...

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 [Formula: see text] the only nonzero TCs in the reflected beam are [Formula: see text] and [Formula: see text]. 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.