Abstract
The orbital angular momentum (OAM) transformation of optical vortex is realized upon using aluminum metasurfaces with phase distributions derived from the caustic theory. The generated OAM transformation beam has the well-defined Bessel-like patterns with multiple designed topological charges from −1 to +2.5 including both the integer-order and fractional-order optical vortices along the propagation. The detailed OAM transformation process is observed in terms of the variations of both beam intensity and phase profiles. The dynamic distributions of OAM mode density in the transformation are further analyzed to illustrate the conservation of the total OAM. The demonstration of transforming OAM states arbitrarily for optical vortex beams will lead to many new applications in optical manipulation, quantum optics, and optical communication.
Highlights
Optical vortices possess helical phase fronts with azimuthal phase dependency of exp(ilφ), where l is the topological charge (TC) and φ represents the azimuthal angle, carrying orbital angular momentum (OAM) of lħ per photon
Based on the previously demonstrated TC inversion with plasmonic metasurfaces[51], here, the OAM transformation of optical vortex beam is realized by using the ultrathin aluminum plasmonic metasurfaces containing nanoslit antennas with the phase distributions derived from the caustic theory
The dynamic distributions of OAM mode density within the beam inner region (r < 5 μm) and the beam outer region (r > 5 μm) during the transformation process is further studied in Hilbert space constituted by LG modes, illustrating the OAM transformation rule and the conservation of the total OAM
Summary
Optical vortices possess helical phase fronts with azimuthal phase dependency of exp(ilφ), where l is the topological charge (TC) and φ represents the azimuthal angle, carrying orbital angular momentum (OAM) of lħ per photon. The realization of arbitrary OAM transformation including both the high-order and fractional-order optical vortices has not been demonstrated yet This is because the asymmetric deformations of noncanonical vortex structures are unstable for high-order TCs, so that the high-order noncanonical vortex will break apart into its corresponding single-charge constituents along the propagation direction[17]. The dynamic distributions of OAM mode density within the beam inner region (r < 5 μm) and the beam outer region (r > 5 μm) during the transformation process is further studied in Hilbert space constituted by LG modes, illustrating the OAM transformation rule and the conservation of the total OAM Such demonstrated OAM transformation beam will have potential applications related to the high-order and fractional-order optical vortices. OAM transformation beams can enable space-dependent optical tweezers for sorting and transporting particles, while the fractional-order optical vortices are useful for quantum information processing with OAM entanglement and OAM-multiplexed optical communication
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