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arXiv:1311.0071 · Optics Letters

Directional infrared emission resulting from cascade population inversion and four-wave mixing in Rb vapor

A. M. Akulshin, Dmitry Budker, Russell McLean

Published Feb 4, 2014• 58 citations
arXivHTMLPDF

Abstract

Directional infrared emission at 1.37 and 5.23 μm is generated in Rb vapors that are stepwise excited by low-power cw resonant light. The radiation at 5.23 μm originating from amplified spontaneous emission on the 5D(5/2)→6P(3/2) transition and wave mixing consists of forward- and backward-directed components with distinctive spectral and spatial properties. Diffraction-limited light at 1.37 μm generated in the copropagating direction only is a product of parametric wave mixing around the 5P(3/2)→5D(5/2)→6P(3/2)→6S(1/2)→5P(3/2) transition loop. This highly nondegenerate mixing process involves one externally applied and two internally generated optical fields. Similarities between wave mixing generated blue and 1.37 μm light are demonstrated.

DOI
10.1364/ol.39.000845

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arXiv:1701.06015•Journal of the Optical Society of America B•Apr 25, 2017•33 citations
Directional infrared emission at 1.37 and 5.23 μm is generated in Rb vapors that are stepwise excited by low-power cw resonant light. The radiation at 5.23 μm originating from amplified spontaneous emission on the 5D(5/2)→6P(3/2) transition and wave mixing consists of forward- and backward-directed components with distinctive spectral and spatial properties. Diffraction-limited light at 1.37 μm generated in the copropagating direction only is a product of parametric wave mixing around the 5P(3/2)→5D(5/2)→6P(3/2)→6S(1/2)→5P(3/2) transition loop. This highly nondegenerate mixing process involves one externally applied and two internally generated optical fields. Similarities between wave mixing generated blue and 1.37 μm light are demonstrated.
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 We demonstrate 1.37 µ m coherent near-infrared light generation via self-seeded parametric four-wave mixing (FWM) process in Rb atomic vapor. Two lasers (780 nm and 776 nm) in the co-propagating configuration, drive the Rb atoms from the 5S 1/2 ground state to the 5D 5/2 excited state. The parametric FWM around the 5P 3/2 –5D 5/2 –6P 3/2 –6S 1/2 –5P 3/2 transition loop is effectively established under phase matching conditions. The generated coherent light wavelength is confirmed using an optical spectrum analyzer, and the output power is investigated with the input laser frequency, power and atomic density. Consequently, a 1.37 µ m coherent light with the power of 27 µ W and the beam quality of M x 2 = 1.22 and M y 2 = 1.25 is obtained. An additional 795 nm laser is introduced to enhance the output power by 1.5 times through velocity-selective optical hyperfine repumping.
Population inversion on the 5D5/2−6P3/2 transition in rubidium atoms produced by cw laser excitation at different wavelengths has been analyzed by comparing the generated mid-infrared radiation at 5.23 μm originating from amplified spontaneous emission (ASE) and isotropic blue fluorescence at 420 nm. Using a novel method for detection of two-photon excitation via ASE, we have observed directional co- and counter-propagating emission at 5.23 μm. Evidence of a threshold-like characteristic is found in the ASE dependences on laser detuning and Rb number density. The power dependences of the backward- and forward-directed emission can be very similar; however, their spectral dependences are not identical. A new type of temporally and spatially coherent radiation is suggested.
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.
Utilizing two-photon excitation in hot Rb vapor we demonstrate the generation of collimated optical fields at 420 and 1324 nm. Input laser beams at 780 and 776 nm enter a heated Rb vapor cell collinear and circularly polarized, driving Rb atoms to the 5D(5/2) state. Under phase-matching conditions coherence among the 5S(1/2)→5P(3/2)→5D(5/2)→6P(3/2) transitions produces a blue (420 nm) beam by four-wave mixing. We also observe a forward and backward propagating IR (1324 nm) beam, due to cascading decays through the 6S(1/2)→5(1/2) states. Power saturation of the generated beams is investigated by scaling the input powers to greater than 200 mW, resulting in a coherent blue beam of 9.1 mW power, almost an order of magnitude larger than previously achieved. We measure the dependences of both beams in relation to the Rb density, the frequency detuning between Rb ground-state hyperfine levels, and the input laser intensities.
We demonstrate how the orientation of the Rb cell can significantly affect the intensity and spectral characteristics of both the frequency up- and downconverted fields generated by nonlinear processes in Rb vapor. The efficiency of parametric wave mixing in Rb vapor excited to the 5D5/2 level by two-color resonant laser light can be significantly increased by seeding the excitation region with coherent and directional radiation at 5.23 μm, which is resonant with the population inverted 6P3/2–5D5/2 transition. It is shown that the process of velocity-insensitive two-photon excitation is central to understanding the observed coherent blue and mid-IR light enhancements, and that velocity-insensitive and velocity-selective two-photon excitations could produce two co-existing but spectrally and spatially distinguishable mid-IR fields at 5.23 μm.