High Helicity Vortex Conversion in a Rubidium Vapor

TL;DR

This paper demonstrates the conversion of optical vortices carrying orbital angular momentum from red to blue light in rubidium vapor, maintaining mode purity and phase matching, with potential applications in quantum communication.

Contribution

It introduces a novel method for high-helicity vortex conversion in rubidium vapor, preserving OAM and mode quality for quantum tech applications.

Findings

Successful red-to-blue vortex conversion for high OAM values.

Output blue vortex maintains azimuthal phase matching and mode purity.

Conversion efficiency decreases with increasing OAM, consistent with overlap calculations.

Abstract

The orbital angular momentum (OAM) of light is a quantity explored for communication and quantum technology, its key strength being a wide set of values offering a large basis for q-working. In this context we have studied the vortex conversion from a red optical vortex to a blue one, for OAMs ranging -30 to +30. The conversion is realized in a rubidium vapor, via the $5S_{1/2}-5D_{5/2}$ $^{85} Rb$ two-photon transition done with a Gaussian beam at 780 nm plus a Laguerre-Gaussian beam at 776 nm with the OAM $\ell$, producing a radiation at 420 nm. With co-propagating input beams, we demonstrate a conversion from red to blue for high-$\ell$ input vortices. We show that the output blue vortex respects the azimuthal phase matching, has a size determined by the product of the input beams and a power decreasing with $\ell$ in agreement with their overlap. Its propagation indicates that the generated blue wave is a nearly pure Laguerre-Gaussian mode. The vortex converter thus permits a correct OAM transmission.