Radially and azimuthally pure vortex beams from phase-amplitude metasurfaces

TL;DR

This paper demonstrates a novel dielectric phase-amplitude metasurface capable of generating high-purity Laguerre-Gaussian vortex beams with controlled azimuthal and radial components, outperforming traditional phase-only methods.

Contribution

It introduces a single-step on-axis transformation metasurface that precisely controls both phase and amplitude to produce high-purity vortex beams with unprecedented azimuthal and radial purity.

Findings

Achieved 98% purity for vortex beams with =50, p=0.

Outperformed phase-only metasurfaces in vortex beam purity.

Discovered formation of 'ghost' orbital angular momentum orders.

Abstract

To exploit the full potential of the transverse spatial structure of light using the Laguerre-Gaussian basis, it is necessary to control the azimuthal and radial components of the photons. Vortex phase elements are commonly used to generate these modes of light, offering precise control over the azimuthal index but neglect the radially dependent amplitude term which defines their associated corresponding transverse profile. Here we experimentally demonstrate the generation of high purity Laguerre-Gaussian beams with a single step on-axis transformation implemented with a dielectric phase-amplitude metasurface. By vectorially structuring the input beam and projecting it onto an orthogonal polarisation basis, we can sculpt any vortex beam in phase and amplitude. We characterize the azimuthal and radial purity of the generated vortex beams, reaching a purity of 98% for a vortex beam with $\ell=50$ and $p=0$. Furthermore, we comparatively show that the purity of the generated vortex beams outperform those generated with other well-established phase-only metasurface approaches. In addition, we highlight the formation of 'ghost' orbital angular momentum orders from azimuthal gratings (analogous to ghost orders in ruled gratings), which have not been widely studied to date. Our work brings higher-order vortex beams and their unlimited potential within reach of wide adoption.