Achromatic orbital angular momentum generator

TL;DR

This paper introduces a broadband, total internal reflection-based device that efficiently converts linearly polarized light into orbital angular momentum beams, useful for advanced imaging applications.

Contribution

A novel, broadband, isotropic medium-based device using glued hollow axicons for efficient spin-to-orbital angular momentum conversion.

Findings

Achieves 95% conversion efficiency across RGB wavelengths.

Operates via total internal reflection, wavelength-independent.

Capable of generating white vortex beams for imaging applications.

Abstract

We describe a novel approach for generating light beams that carry orbital angular momentum (OAM) by means of total internal reflection in an isotropic medium. A continuous space-varying cylindrically symmetric reflector, in the form of \textit{two glued hollow axicons}, is used to introduce a nonuniform rotation of polarisation into a linearly polarised input beam. This device acts as a full spin-to-orbital angular momentum convertor. It functions by switching the helicity of the incoming beam's polarisation, and by conservation of total angular momentum thereby generates a well-defined value of OAM. Our device is broadband, since the phase shift due to total internal reflection is nearly independent of wavelength. We verify the broad-band behaviour by measuring the conversion efficiency of the device for three different wavelengths corresponding to the RGB colours, red, green and blue. An average conversion efficiency of $95\%$ for these three different wavelengths is observed. %, which confirms its wavelength independence. This device may be used to generate a white vector vortex beam, as we show, and may find applications in imaging from micro- to astronomical systems where a white vortex beam is needed.