Plasmonically enhanced composite vortex beam generation using ultra-thin dielectric fork gratings

Abstract

We experimentally demonstrate a simple method for the generation of composite vortex beams using resonant ultra-thin dielectric fork gratings (UFGs) of thicknesses an order of magnitude less than the incident wavelength. The degradation in the diffraction efficiency of these gratings at reduced dielectric thicknesses was computationally shown to be compensated for by introducing a thin continuous gold layer (Au, 30 nm) between the grating and the substrate. At the resonance wavelength, the diffraction efficiency of UFGs with Au was ∼ 4 times higher when compared to that of UFGs without Au, which was attributed to plasmon-induced transmission enhancement. UFGs were fabricated with optimized geometric parameters using electron beam lithography. These gratings showed lattice-plasmon resonance at the wavelength corresponding to the grating vector, resulting in vortex beams with a specific wavelength and polarization selectivity. Further, hybrid UFGs were designed by replacing a central concentric circular region of a UFG with another UFG of a different topological charge. These hybrid UFGs resulted in the formation of composite vortex beams, indicating that the relative spatial phase imparted to the lattice plasmon by hybrid gratings was preserved in the leaked radiation. These results can help in designing integrated, ultra-thin, low aspect ratio optical-phase-singularity structures for enhanced optical sources, detectors, and sensing applications.