We report the transfer of phase structure, and in particular of orbital angular momentum (OAM), from near-infrared pump light to blue light generated in a four-wave-mixing process in85Rb vapour. The intensity and phase profile of the two pump lasers at 780nm and 776nm, shaped by a spatial light modulator (SLM), influences the phase and intensity profile of light at 420nm which is generated in a subsequent coherent cascade. In particular we observe that the phase profile associated with orbital angular momentum is transferred entirely from the pump light to the blue. Pumping with more complicated light profiles results in the excitation of spatial modes in the blue that depend strongly on phase matching, thus demonstrating the parametric nature of the mode transfer. These results have implications on the inscription and storage of phase information in atomic gases.
We report the transfer of phase structure, and in particular of orbital angular momentum (OAM), from near-infrared pump light to blue light generated in a four-wave-mixing process in85Rb vapour. The intensity and phase profile of the two pump lasers at 780nm and 776nm, shaped by a spatial light modulator (SLM), influences the phase and intensity profile of light at 420nm which is generated in a subsequent coherent cascade. In particular we observe that the phase profile associated with orbital angular momentum is transferred entirely from the pump light to the blue. Pumping with more complicated light profiles results in the excitation of spatial modes in the blue that depend strongly on phase matching, thus demonstrating the parametric nature of the mode transfer. These results have implications on the inscription and storage of phase information in atomic gases.
We report the transfer of phase structure and, in particular, of orbital angular momentum from near-infrared pump light to blue light generated in a four-wave-mixing process in 85Rb vapor. The intensity and phase profile of the two pump lasers at 780 and 776 nm, shaped by a spatial light modulator, influences the phase and intensity profile of light at 420 nm, which is generated in a subsequent coherent cascade. In particular, we observe that the phase profile associated with orbital angular momentum is transferred entirely from the pump light to the blue. Pumping with more complicated light profiles results in the excitation of spatial modes in the blue that depend strongly on phase matching, thus demonstrating the parametric nature of the mode transfer. These results have implications on the inscription and storage of phase information in atomic gases.
In this work we present a series of results on four-wave mixing (FWM) processes induced by structured light in atomic media. We start with a theoretical study focusing on a cascade FWM process in rubidium vapor, generating a blue light field at 420 nm. In this setting, we show that in the extended-medium regime the Hermite-Gaussian basis presents a unique property of optical structure transfer. We then exploit this result to show that, by carefully tailoring the structure of the pump beams, one can obtain desired spatial modes with cylindrical and elliptical symmetries with high fidelity at the blue light output. Following that, we consider an experimental configuration where two degenerate FWM signals are generated in different directions of space by the same pump fields. In this setup, we present results in heated rubidium vapor evidencing the simultaneous conservation of orbital angular momentum (OAM) in the two FWM processes. We show that the two-channel setting allows to encode the OAM content of the input beams onto the OAM carried by the two FWM signals. Next, we explore the transverse mode dynamics in the context of optical modes contained in the so-called orbital angular momentum Poincaré sphere (PS). Defined in analogy with a general polarization state, this family of modes is parametrized in terms of polar and azimuthal angles. We show that the two FWM signals can also be described as belonging to Poincaré spheres, and that the angles on the output PS are related to those on the input PS by well-defined symmetries. The predicted FWM intensity profiles, as well as the consequences of the symmetry properties, are in good agreement with our experimental results. We also explore interesting scenarios, such as combinations of different PS modes and the fulfillment of one of the symmetry relations in each sphere independently, and restrictions that arise in the extended-medium regime. In the last part, we study the correlations between the light fields participating in the two-channel FWM configuration. Following our recent work on cold atoms, we investigate the correlations originating from the conversion of phase-noise to amplitude-noise as a result of the light-atom interaction, and discuss our attempts to discriminate transverse spatial dependencies on these correlations. Finally, we outline the quantum theory of FWM, exploring the multi-spatial-mode nature of the generated light state and the associated spatial correlations.
Experimental results on the phase transfer from structured beams to parametric light, generated by inducing four-wave mixing in an atomic gas, are reported. Near-infrared, Mathieu, and Gaussian pump beams yield a blue coherent Mathieu electromagnetic field and an infrared beam. Two complementary techniques enable full characterization of the up-converted modes. The unambiguous and efficient phase heritage of these complex quasipropagation invariant modes to up-converted photons is demonstrated. The presented data enable a quantitative study of dynamical variables related to Mathieu beams. These include elliptic angular momentum, a generalization of the orbital angular momentum of light, which is also up-converted through the nonlinear process.
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.
We report on experimentally observed addition, subtraction, and cancellation of orbital angular momentum (OAM) in the process of parametric four-wave mixing that results in frequency up- and down-converted emission in Rb vapor. Specific features of OAM transfer from resonant laser fields with different optical topological charges to the spatially and temporally coherent blue light (CBL) have been considered. We have observed the conservation of OAM in nonlinear wave mixing in a wide range of experimental conditions, including a noncollinear geometry of the applied laser beams, and furthermore, that the CBL accumulates the total OAM of the applied laser light. Spectral and power dependences of vortex and plane wavefront blue light beams have been compared.
In recent years there has been great interest in the applications of the interaction of the phase and intensity of laser light in atomic vapours. The generation of light beams with arbitrary phase and intensity patterns can now be easily achieved by using Spatial Light Modulators (SLMs). The transformation of quantized units of phase information to atomic vapours has implications in the fields of quantum optics for the realisation of sources of entangled photons, optical switching, and quantum information storage. Spatially shaped beams with non-trivial intensity geometries have found use in single atom quantum well traps and for the enhancement of density of standard Magneto Optical Traps. The work in this thesis is focussed around two main pro jects, involving the interaction of holographically shaped light beams with cold trapped atoms and with a hot atomic vapour respectively. An enhancement to the previously investigated technique of a SPontaneous force Optical Trap SPOT of 87 Rb is presented here which aims to solve various issues which naturally arise from compressing cold atoms in a Magneto Optical Trap (MOT) such as unavoidable heating during the compression. High density/high atom number traps are highly sought after in many experiments for more efficient transfer of atoms to Bose Einstein Condensates and for improved quantum storage capabilities in cold atom traps. The highest density achieved in our SPOT was 2.5 × 10^12 atoms cm−3 for 2 × 10^8 atoms at a temperature of approximately 100µK. This represents almost 2 orders of magnitude increase in density from the standard MOT setup with no adverse heating of the trap while maintaining 75% of the atoms. In the second part of this work hot atomic vapours are utilized for the efficient transfer of orbital angular momentum information from near infra-red pump fields, driving from 5S_1/2 to 5D_5/2 on a two-photon transition, to a cascade from 5D_5/2 to 6P_3/2 to 5S_1/2 generating 5230nm light and a coherent blue, 420nm, beam respecitively. This generation is performed using four wave mixing in 85 Rb. We observe the complete conversion of all input quantum information, the Orbital Angular Momentum (OAM) from the pump fields to the blue. In addition we show the additional phase coherence effects of this experiment through the use of simple superpositions of Laguerre-Gaussian (LG) modes showing that the process is indeed quantum in nature. A theoretical basis for the transfer of all OAM information to only the 420nm beam is also discussed here.