High helicity vortex conversion in a rubidium vapor

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 quantum working. In this context we have studied the vortex conversion from a red optical vortex to a blue one, for OAMs ranging $\ensuremath{-}30$ to $+30$. The conversion is realized in a rubidium vapor, via the $5{S}_{1/2}\ensuremath{-}5{D}_{5/2}\phantom{\rule{0.16em}{0ex}}^{85}\mathrm{Rb}$ two-photon transition done with a Gaussian beam at $780\phantom{\rule{4pt}{0ex}}\mathrm{nm}$ plus a Laguerre-Gaussian beam at $776\phantom{\rule{4pt}{0ex}}\mathrm{nm}$ with the OAM $\ensuremath{\ell}$, producing a radiation at $420\phantom{\rule{4pt}{0ex}}\mathrm{nm}$. With copropagating input beams, we demonstrate a conversion from red to blue for high-$\ensuremath{\ell}$ 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 $\ensuremath{\ell}$, 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.