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Control of nonlinear processes in two-photon excited rubidium vapour by resonant laser light

Nafia Rahaman, Russell McLean, A. M. Akulshin

Published Dec 30, 2019• 0 citations
Publisher

Abstract

Frequency up- and down-conversion in warm alkali vapours over a range of applied cw laser intensities has received much attention in recent years due to promising applications in the fields of remote detection, quantum information and computing, specifically in the implementation of correlated photon pairs, photon storage and quantum memory interface to name a few. The generation of directional laser-like blue and infrared light in alkali vapours has been extensively studied. However, important details of the interplay of parametric and nonparametric nonlinear processes, such as the four-wave mixing (FWM) and amplified spontaneous emission (ASE) responsible for the new field generation, are not entirely clear.

DOI
10.1117/12.2539921

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Frequency up- and down-conversion in warm alkali vapours over a range of applied cw laser intensities has received much attention in recent years due to promising applications in the fields of remote detection, quantum information and computing, specifically in the implementation of correlated photon pairs, photon storage and quantum memory interface to name a few. The generation of directional laser-like blue and infrared light in alkali vapours has been extensively studied. However, important details of the interplay of parametric and nonparametric nonlinear processes, such as the four-wave mixing (FWM) and amplified spontaneous emission (ASE) responsible for the new field generation, are not entirely clear.
The parametric four-wave mixing (4WM) process responsible for the generation of coherent blue light in alkali vapors is a self-seeded process which starts by population inversion and amplified spontaneous emission (ASE). Lately, attention has been turned toward frequency up- and down-conversion in alkali vapors, using relatively low pump powers with CW diode lasers. In this Letter, we investigate the interplay between ASE and 4WM in rubidium (Rb) vapors by studying the mid-infrared (mid-IR) radiation emitted in the forward direction at 5.23 μm. We show that the ASE can be suppressed by 4WM in the CW regime. Thus, we demonstrate the generation of coherent mid-IR light at 5.23 μm in hot Rb vapors via parametric 4WM.
Abstract Warm rubidium vapours are known to be a versatile medium for a variety of experiments in atomic physics and quantum optics. Here we present experimental results on producing the frequency converted light for quantum applications based on spontaneous and stimulated processes in rubidium vapours. In particular, we study the efficiency of spontaneously initiated stimulated Raman scattering in the Λ-level configuration and conditions of generating the coherent blue light assisted by multi-photon transitions in the diamond-level configuration. Our results will be helpful in search for new types of interfaces between light and atomic quantum memories.
The parametric and nonparametric nonlinear processes responsible for generation of directional infrared radiation in sodium vapors, excited to the 4D 5/2 energy level by resonant laser radiation, are identified by the transfer of orbital angular momentum from the pump radiation to the generated fields. We show that the optical fields generated by amplified spontaneous emission (ASE) simultaneously participate in several loops of four-wave mixing (FWM) generating new directional emission. The measured spectral linewidth of the FWM emission at 819.7 nm sets an upper limit to the linewidth of two fields resulting from ASE at 2338.6 and 9093.0 nm, assuming their spectra are uncorrelated. Understanding details of the new-field generation is central to applications such as directional laser guide stars, stand-off magnetometry, and entangled field generation.
The nonlinear response of sodium vapor with a laser tuned near the 4D two-photon resonance is investigated over a large range of sodium concentrations and pump intensities with emphasis on the influence of coherent cancellation effects on the induced polarizations and the primary mechanism for the generation of the new frequencies. Over a wide range of circumstances, the response of the medium is dominated by parametric four-wave mixing as compared to amplified spontaneous emission or stimulated electronic hyper-Raman scattering. Secondary four-wave-mixing fields are generated by stimulated Raman scattering of one parametric four-wave-mixing field and a coupling of these fields with the driving laser field. A sharp onset of saturation of the parametric four-wave-mixing signals with sodium number density and a transition from a quadratic to a linear power dependence is observed. Both effects are explained in terms of the two-photon cancellation effect.