Generation of hollow Gaussian beams by restoring structured light with meta-optics

Abstract

Hollow Gaussian beams (HGBs) with annular intensity distributions but uniform phase and polarization distributions have aroused enormous interest due to their advantages in optical trapping, optical limiting, etc. Although extensive HGB generation methods have been performed previously, there are still existing limitations in polarization independence and broadband response, which hinder the application of HGBs. Here, we propose and experimentally demonstrate an HGB generation method by restoring structured light with a pair of cascaded dielectric metasurface q-plates. The metasurface q-plates are based on Pancharatnam-Berry (PB) phase and work as elements of optical spin-to-orbital angular momentum conversion. The first metasurface q-plate is used to introduce orbital angular momentum to an incident circularly polarized beam, and the output light field possesses both annular intensity distribution and spiral phase with reversed circular polarization. Since the phase retardation of the metasurface q-plates isπ, the reversed circularly polarized beam output from the first metasurface q-plate will obtain opposite spiral phase after the second metasurface q-plate. Then, the spiral phase of the output light field is counteracted, but its annular intensity distribution is retained as a new HGB. Arbitrarily polarized Gaussian beams are available for incident beams because they can be decomposed into orthogonal circularly polarized components. Furthermore, this method is applicable to a broad spectrum of visible light owing to the dispersion-free characteristic of the PB phase. This work suggests a polarization-independent and broadband HGB generation method, which may benefit HGB-based fundamental research.