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arXiv:2012.15449 · Optics Letters

Directional THz generation in hot Rb vapor excited to a Rydberg state

Mark Lam, Sambit Bikas Pal, Thibault Vogt, Martin Kiffner, Wenhui Li

Published Feb 1, 2021• 16 citations
arXivHTMLPDF

Abstract

We optically excite 85 R b atoms in a heated vapor cell to a low-lying Rydberg state 10 D 5 / 2 and observe directional terahertz (THz) beams at 3.3 THz and 7.8 THz. These THz fields are generated by amplified spontaneous emission from the 10 D 5 / 2 state to the 11 P 3 / 2 and 8 F 7 / 2 states, respectively. In addition, we observe ultraviolet (UV) light produced by four-wave mixing of optical pump lasers and the 3.3 THz field. We characterize the generated THz power over the detuning and power of pump lasers, and identify experimental conditions favoring THz and UV generation, respectively. Our scheme paves a new pathway towards generating high-power narrowband THz radiation.

DOI
10.1364/ol.418618

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We optically excite 85 R b atoms in a heated vapor cell to a low-lying Rydberg state 10 D 5 / 2 and observe directional terahertz (THz) beams at 3.3 THz and 7.8 THz. These THz fields are generated by amplified spontaneous emission from the 10 D 5 / 2 state to the 11 P 3 / 2 and 8 F 7 / 2 states, respectively. In addition, we observe ultraviolet (UV) light produced by four-wave mixing of optical pump lasers and the 3.3 THz field. We characterize the generated THz power over the detuning and power of pump lasers, and identify experimental conditions favoring THz and UV generation, respectively. Our scheme paves a new pathway towards generating high-power narrowband THz radiation.
We use two continuous-wave laser beams of 780 nm and 515 nm to optically drive Rb85 atoms in a heated vapor cell to a low-lying Rydberg state 10D5/2. We observe a collimated ultraviolet (UV) beam at 311 nm, corresponding to the transition frequency from the 11P3/2 state to the 5S1/2 state. This indicates the presence of a coherent four-wave mixing process, built up by two input laser fields as well as terahertz (THz) radiation of 3.28 THz, which is generated by amplified spontaneous emission between the 10D5/2 and 11P3/2 states. We characterize the 311 nm UV light generation and its dependence on various physical parameters. This scheme could open up a new possibility for generating narrow-band THz waves as well as deep UV radiation.
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.
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 5D5/2 state. Under phase-matching conditions coherence among the 5S1/2 → 5P3/2 → 5D5/2 → 6P3/2 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 6S1/2 → 5P1/2 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 mWpower, 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. (C) 2014 Optical Society of America
We introduce a unique technique for generating directional coherent emissions that could be utilized to create coherent sources in a wide range of frequencies from the extreme ultraviolet (XUV) to the deep infrared. This is accomplished without population inversion by pumping a two-level system with a far-detuned strong optical field that induces the splitting of the two-level system. A nonlinear process of four-wave mixing then occurs across the split system, driving coherent emission at sidebands both red- and blue-detuned from the pump frequency, and propagates both forward and backward along the pump beam path. We observed this phenomenon in dense rubidium vapor along both the D1 and D2 transitions. The sideband emission exhibits a short pulse duration (<1 ns) with threshold-like behavior dependent on both the pump intensity and Rb vapor density. This technique offers a new capability for manipulating the emission frequency simply through intensity-induced atomic modulation that can be scaled to most frequency regimes using various atomic/molecular ensembles and pump energies. A scheme for generating coherent directional emission without the need for population inversion could be a valuable alternative to the laser. Developed by scientists at Texas A&M University in the USA, the approach can in principle be applied anywhere in the electromagnetic spectrum—from the extreme ultraviolet to the far infrared. It involves pumping a two-level system with a strong detuned optical signal, which induces transient AC Stark splitting of the energy levels and four-wave mixing. Initial tests with a dense rubidium vapour indicated that the near-infrared sidebands generated in the process are coherent pulses that are less than 1 nanosecond in duration. The approach should work with other atomic and molecular systems at other wavelengths and may be an attractive way for generating coherent extreme ultraviolet or X-ray radiation as well as a novel approach for remote sensing.
Rydberg atom-based sensors have emerged as highly sensitive tools for terahertz (THz) metrology, yet most current imaging techniques discard crucial phase information. In this Letter, we present a coherent THz-to-optical conversion scheme in warm Rb vapor that enables complex-amplitude field imaging. By manipulating the phase-matching conditions via an adjustable interference pattern of optical probe beams, we demonstrate the ability to perform tomographic reconstruction of the THz field distribution. We experimentally validate the spatial resolution and phase-sensitivity of the system by resolving sub-centimeter features and identifying incident angles of arrival. Our results establish a robust framework for phase-resolved THz imaging and holography using atomic vapors at room temperature.
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.