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Photonics

A Multichannel Vortex Beam Generator via Spatially Structured Bidirectional Two-Color-Pump Four-Wave Mixing in a Single 133Cs Vapor Cell

Dan Wang, Meng-Yu Bian, Zi-Yi Gao, Liang-Hui Huang, Haitao Zhou, Jun-Xiang Zhang

Published Mar 3, 2026• 0 citations
Publisher

Abstract

Multichannel vortex beams serve as an essential physical source for enabling multi-spot laser processing and high-dimensional spatial multiplexing communications. We demonstrate a compact, flexibly tunable multichannel vortex beam generator using spatially structured bidirectional two-color pump vortex four-wave mixing in a single 133Cs vapor cell. To enhance spatial multiplexing, both sides of the cell are utilized. By engineering the propagation directions and frequencies of five input beams, we establish a nonlinear interaction region that supports 16 concurrent phase-matching conditions, thereby enabling the parallel generation of up to eight vortex channels. The orbital angular momentum of the output beams follows deterministic algebraic rules, allowing for programmable control via tailored input orbital angular momentum combinations. Moreover, the channel count can be linearly tuned by selectively deactivating pumps—each switched-off pump reduces the number of output channels by two. This flexible control over orbital angular momentum states, together with channel count and spatial arrangement, establishes a highly integrated platform for on-demand vortex generation. This work highlights the potential of spatially bidirectional structured pumping in atomic vapor to expand optical dimensionality and enhance multiplexing capacity, paving the way toward multidimensional communications, quantum networks, and integrated photonics.

DOI
10.3390/photonics13030247

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Multichannel vortex beams serve as an essential physical source for enabling multi-spot laser processing and high-dimensional spatial multiplexing communications. We demonstrate a compact, flexibly tunable multichannel vortex beam generator using spatially structured bidirectional two-color pump vortex four-wave mixing in a single 133Cs vapor cell. To enhance spatial multiplexing, both sides of the cell are utilized. By engineering the propagation directions and frequencies of five input beams, we establish a nonlinear interaction region that supports 16 concurrent phase-matching conditions, thereby enabling the parallel generation of up to eight vortex channels. The orbital angular momentum of the output beams follows deterministic algebraic rules, allowing for programmable control via tailored input orbital angular momentum combinations. Moreover, the channel count can be linearly tuned by selectively deactivating pumps—each switched-off pump reduces the number of output channels by two. This flexible control over orbital angular momentum states, together with channel count and spatial arrangement, establishes a highly integrated platform for on-demand vortex generation. This work highlights the potential of spatially bidirectional structured pumping in atomic vapor to expand optical dimensionality and enhance multiplexing capacity, paving the way toward multidimensional communications, quantum networks, and integrated photonics.
We demonstrate a seeded resonant multi-four-wave-mixing process in a single hot 133Cs vapor cell. These coexisting four-wave mixing interactions work on three different transition circles and give rise to the generation of four bright resonant output beams. We examine the pump detuning, pump power, and atom temperature dependence of the gain of output modes, and give a theoretical prediction on quadripartite entanglement. This provides significant insight into multimode entanglement of light very close to atomic resonance. Additionally, the orbital angular momentum of the pump beam is transferred as expected into the four generated beams. It’s proven to be a feasible method for designing a 2 × 2 vortex beam array.
Airy vortex beams, which integrate orbital angular momentum with diffraction-resistant and self-accelerating characteristics of Airy beams, offer unique capabilities for high-capacity optical communication and high-resolution imaging. Here, we experimentally demonstrate the generation and manipulation of frequency-converted Airy vortex beams through four-wave mixing in hot Rb vapor. A 780 nm vortex beam and a 776 nm two-dimensional Airy beam coherently participate in the four-wave mixing process, generating a 420 nm output beam that combines the orbital angular momentum of the vortex beam with the spatial structure of the Airy beam. The generated beam exhibits pronounced quasi-nondiffracting propagation and self-acceleration, with an intensity decay rate reduced by a factor of ∼1.88 relative to the input vortex beam and a transverse acceleration ∼1.84 times that of the input Airy beam. Moreover, both the intensity and the spatial profile of the generated beam are flexibly manipulated by varying the Rb vapor temperature and the pump beam power. This work provides a versatile approach for generating and controlling structured light at what we believe are new wavelengths, opening opportunities for short-wavelength optical manipulation and optical information processing.
Detuning Tunable OAM Generation via Double-ΛΛΛ Four-Wave Mixing in Hot Rubidium Vapor
We demonstrate detuning-tunable generation of orbital-angular-momentum (OAM) light using a double Lambda four-wave-mixing (FWM) process in Doppler broadened rubidium vapor. Two near-resonant pumps on the D1 line drive non degenerate FWM that produces bright probe conjugate beams whose transverse modes evolve with pump detuning. A paraxial density-matrix model coupled to split-step propagation predicts detuning-dependent spatial gain shaping that sets the OAM content; experiments with a mode cleaned pump laser and a 12-mm AR-coated vapor cell validate these predictions. We quantify mode formation by imaging, spectroscopy, and power measurements, and verify OAM conservation between the generated beams. The results establish resonant atomic vapor as a compact, tunable platform for structured-light generation with applications to high-dimensional quantum communications and imaging.
Abstract We have proposed a double scheme for the efficient generation and control of spatial four-wave mixing (FWM)-induced ultraviolet light via optical angular momentum transfer from a vortex-field (Laguerre–Gaussian (LG) beam) applied once to the excited transition of the first two-photon ladder and then in the uppermost transition of the three-photon ladder in the FWM-loop configuration of a five-level atomic excitation mechanism. In both the LG-cases, at the conditions of resonant and off-resonant detunings of the two coherent fields with high Rabi frequencies guiding the multi-photon excitation route in the system, it has been shown that the vortex FWM light can propagate either in the superluminal or in the subluminal regime depending upon ‘turned-off’ and ‘turned-on’ conditions of a Raman field outside the FWM-loop, respectively. Such switching phenomenon is the most interesting outcome of the present study along with the significant phase-twisting effects as manifested in the helical phase patterns in both the LG-cases. Thus, the practical implementation of the present model under a cold atomic environment paves up a novel way of obtaining structured high-dimensional light transfer process based on the mechanism of nonlinear photonics, which might have potential applications in quantum information processing and technology in the near future.
Optical vortex beams (OVBs) carrying orbital angular momentum (OAM) offer access to an unbounded set of orthogonal states, enabling a dramatic increase in the information capacity for optical communication systems. However, practical OAM detection in telecommunication platforms remains challenging due to background noise, detector limitations at infrared communication wavelengths, and the destructive nature of conventional sampling methods. Here, we introduce an all-optical, nearly nondestructive OAM detection method based on sum-frequency generation (SFG) that overcomes these limitations by transferring the information on the OAM into a distinct frequency domain. In our scheme, two OVBs─one carrying OAM-encoded information and the other serving as a reference─are coupled within a beta-barium borate (BBO) crystal to generate an SFG signal. Our theoretical and experimental studies demonstrate that SFG efficiency is governed by the spatial overlap of the interacting vortex profiles, providing mode-selective and OAM-resolved detection. Crucially, as only a small fraction of the input photons undergo conversion into the nonlinear signal, the original beams remain functionally intact. Since detection is performed exclusively on the upconverted signal in a spectrally distinct visible wavelength region, the signal-to-noise ratio can be improved. Moreover, the SFG signal is generated only when specific OAM states and temporal overlap conditions are simultaneously satisfied, enabling ultrafast and conditionally gated access to the OAM information. This nonlinear selectivity offers enhanced physical-layer security and high-throughput capabilities. Together, our approach provides a robust and versatile platform for advanced OAM-based optical communications and high-capacity photonics applications.
Nonlinear holography is a powerful technique for shaping beams at new optical frequencies. Selectively reconstructing target wavefront information from multichannel nonlinear holography is challenging due to the critical phase-matching condition. Herein, we propose a scheme for all-optical controlled multichannel nonlinear holography using four-wave mixing (FWM) in Rb vapor. A dispersion-controlled wavevector was dynamically tuned to achieve a full phase-matching condition for efficient target wavefront reconstruction. We adjusted the frequency and intensity of the control beam to selectively reconstruct annular vortex arrays in the FWM signal. Four switchable holographic channels were established for beam shaping via experimental parameter optimization. Our study demonstrates that the atomic system is ideal for studying nonlinear control of structured light and aids in constructing all-optical multichannel devices for classical and quantum information processing.