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arXiv:1803.07182 · Physical Review A

High helicity vortex conversion in a rubidium vapor

Aurélien Chopinaud, Marion Jacquey, B. Viaris de Lesegno, Laurence Pruvost

Published Jun 6, 2018• 49 citations
arXivHTMLPDF

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.

DOI
10.1103/physreva.97.063806

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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\ensuremath{-}30\ensuremath−30 to +30+30+30. The conversion is realized in a rubidium vapor, via the 5S1/2\ensuremath−5D5/285Rb5{S}_{1/2}\ensuremath{-}5{D}_{5/2}\phantom{\rule{0.16em}{0ex}}^{85}\mathrm{Rb}5S1/2​\ensuremath−5D5/2​85Rb two-photon transition done with a Gaussian beam at 780nm780\phantom{\rule{4pt}{0ex}}\mathrm{nm}780nm plus a Laguerre-Gaussian beam at 776nm776\phantom{\rule{4pt}{0ex}}\mathrm{nm}776nm with the OAM \ensuremathℓ\ensuremath{\ell}\ensuremathℓ, producing a radiation at 420nm420\phantom{\rule{4pt}{0ex}}\mathrm{nm}420nm. With copropagating input beams, we demonstrate a conversion from red to blue for high-\ensuremathℓ\ensuremath{\ell}\ensuremathℓ 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 \ensuremathℓ\ensuremath{\ell}\ensuremathℓ, 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.
Abstract The manipulation of the orbital angular momentum (OAM) contributes to understand the OAM multiplexing, is significant in free-space optical communication and information processing. We theoretically simulate and experimentally demonstrate the regularity of the OAM transfer, including the angular and radial modes, of Laguerre–Gaussian beam via four-wave mixing process in 85Rb vapor. The 420 nm coherent blue light output field inherits the phase characteristic of 780 nm and 776 nm beams with different OAM modes. The output field OAM modes show the transfer as a typical arithmetic operation of the input field OAM modes with equal-handed angular indice l, while, the conversion between angular and radial modes occurs with the opposite angular indice l. Such rules of the OAM transfer and manipulation have implications on the research of high-capacity information transfer and quantum communication.
Abstract Up-conversion in an atomic vapor is known to produce photon pairs. If addressed by vortex beams, it produces pairs of orbital angular momenta (OAM), that are valuable bricks for quantum processes with OAM. The perfect vortex (PV) beam is a vortex class very pertinent in this context, because its ring radius and its Gouy phase are —in principle— independent of the OAM, giving a huge spiral bandwidth compared to the usual Laguerre-Gaussian (LG) beam. Our experimental study clarifies this point. The PV ring radius and thickness are found to vary hyperbolic-like vs . the OAM, which will limit the spiral bandwidth, but, unlike LG, the number of generated OAM pairs still remains quite comfortable for entanglement applications.
Optical vortex light engendered with integer units of orbital angular momentum (OAM) may be involved in frequency upconversion. Second harmonic generation is usually forbidden in isotropic media due to parity constraints, but it becomes allowed by six-wave mixing. Here, we present a rigorous quantum analysis for the case of a Laguerre-Gaussian input beam comprising photons endowed with a single unit of OAM. Such a process gives rise to the novel entanglement of orbital momentum in two emergent photons; it transpires that the mechanism delivers a harmonic output whose polarization is essentially parallel to the incident radiation. This investigation ascertains the character of the emission, both under forward propagation and back-reflection geometries, and identifies in detail the form of distribution in the entangled total orbital momentum. A distinctive conical spread, originating from the entangled distribution in the emission pair, affords a potential means to determine the individual angular momenta.
In 1992, Les Allen and coworkers within Han Woerdman's group realized that light beams with an azimuthal phase dependence described by an additional exp(iℓθ) term, where ℓ can take any integer value and θ is the angular position within the beam, carry an orbital angular momentum. This orbital angular momentum is additional to the spin angular momentum associated with circular polarization. These azimuthally phased beams comprise ℓ intertwined helical phase fronts, skewed with respect to the beam axis, meaning that at all positions within the beam cross section, the wavefront normal and associated momentum density has a well-defined azimuthal component. The insight of Allen and coworkers was that for all beams described in this way, when integrated over the beam cross section, the orbital angular momentum (OAM) is ℓℏ per photon. Laguerre-Gaussian and high-order Bessel beams are both examples of mode families having these properties. A further feature of these beams is that the phase singularity on the beam axis precludes a nonzero on-axis intensity and therefore all such beams with ℓ ≠0 have an annular intensity cross section (see Fig. 4.1). The circulation of the optical momentum and energy around the singularity leads them also to be called optical vortices. The further recognition that beams carrying defined amounts of OAM could be created within the laboratory means that over the last 15 years they have formed the basis of many investigations, ranging from the transfer of OAM to microscopic particles (in optical spanners or optical wrenches) to studies of the fundamental properties of light in both the classical and quantum regimes. However, although OAM is an immensely useful concept, one must emphasize that all its properties are simply those predicted by standard electromagnetic theory. The purpose of this chapter is to consider how sets of helical laser modes carrying OAM can be used for information transfer.
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Orbital angular momentum (OAM) of single photons represents a new optical degree of freedom for the entanglement of photons [1,2]. One physical realization of OAM carrying light beams are the so called Laguerre-Gaussian modes of light with the required helical phase structure. One big advantage over the well known polarization degree of freedom is the possibility of realizing entanglement between two photons with very high quantum numbers. However, the creation of photonic OAM entanglement using the widely used spontaneous parametric downconversion process is limited by the strongly reduced efficiency for higher momenta [3]. We have realized a novel method to create entanglement between two photons with a very high difference in their OAM quantum number and momentum respectively. Furthermore we have explored hybrid entanglement of photons in the OAM and polarization degree of freedom.