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arXiv:1812.04465 · Journal of the Optical Society of America B

Saturation and alternate pathways in four-wave mixing in rubidium

Erik Brekke, Noah Swan

Published Jan 23, 2019• 7 citations
arXivHTMLPDF

Abstract

We have examined the frequency spectrum of the blue light generated via four-wave mixing in a rubidium vapor cell inside a ring cavity. At high atomic density and input laser power, two distinct frequency components separated by 116±4 MHz are observed, indicating alternate four-wave mixing channels through the 6p3/2 hyperfine states. The dependence of the generated light on excitation intensity and atomic density are explored, and they indicate that the primary process has saturated. This saturation results when the excitation rate through the 6p state becomes equal to the rate through the 5p state, giving no further gain with atomic density while a quadratic intensity dependence remains.

DOI
10.1364/josab.36.000421

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We have examined the frequency spectrum of the blue light generated via four-wave mixing in a rubidium vapor cell inside a ring cavity. At high atomic density and input laser power, two distinct frequency components separated by 116±4 MHz are observed, indicating alternate four-wave mixing channels through the 6p3/2 hyperfine states. The dependence of the generated light on excitation intensity and atomic density are explored, and they indicate that the primary process has saturated. This saturation results when the excitation rate through the 6p state becomes equal to the rate through the 5p state, giving no further gain with atomic density while a quadratic intensity dependence remains.
We have incorporated a ring cavity to enhance the efficiency of parametric four-wave mixing in rubidium (Rb). Using an input coupler with 95% reflectance, a finesse of 19.6±0.5 is achieved with an Rb cell inside. This increases the circulating intensity by a factor of 5.6±0.5, and through two-photon excitation on the 5s1/2→5d5/2 transition with a single excitation laser, up to 1.9±0.3 mW of power at 420 nm is generated, 50 times what was previously generated with this scheme and comparable to low-power two-step excitation. The dependence of the output on the Rb density and input power has been explored, suggesting the process may be approaching saturation. The blue output of the cavity also shows greatly improved spatial quality, combining to make this a promising source of 420 nm light for future experiments.
This work introduces an approach to generate coherent 420 nm (blue) light utilizing narrowband, CW pump beams at 795 nm, 532 nm and 1320 nm in a rubidium vapor cell. The frequencies giving the strongest signal intensity are found. The method is compared to other transition pathways that generate 420 nm light in rubidium vapor, namely using the 776 nm and 780 nm absorption lines. The approach discussed in this work has the potential to be used in a diagnostic measurement capability as a means to spectrally filter a component of a scattered signal.
We report the first use of a ring cavity to both enhance the output power and dramatically narrow the linewidth (<1 MHz) of blue light generated by four-wave mixing in a rubidium vapor cell. We find that the high output power available in our cavity-free system leads to power broadening of the generated blue light linewidth. Our ring cavity removes this limitation, allowing high output power and narrow linewidth to be achieved concurrently. As the cavity blue light is widely tunable over the Rb855S1/2F=3→6P3/2 transition, this narrow linewidth light would be suitable for near-resonant rubidium studies including, for example, second-stage laser cooling.
Abstract The coherent blue light generated in rubidium vapor due to the combined action of an ultrashort pulse train and a continuous wave diode laser is investigated. Each step of the two-photon transition 5S–5P 3 / 2 –5D is excited by one of the lasers, and the induced coherence between the 5S and 6P 3 / 2 states is responsible for generating the blue beam. Measurements of the excitation spectrum reveal the frequency comb structure and allow us to identify the resonant modes responsible for inducing the nonlinear process. Further, each resonant mode excites a different group of atoms, making the process selective in atomic velocity. The signal dependency on the atomic density is characterized by a sharp growth and a rapid saturation. We also show that for high intensity of the diode laser, the Stark shift at resonance causes the signal suppression observed at low atomic density.
We report on observations of four-wave mixing in rubidium vapor via two-photon resonant excitation with nd and (n+2)s states, for n=14–20. The intensity of the generated beam is studied in a collinear configuration, and the results are consistent with the predictions of a theoretical model based on the semiclassical Maxwell–Bloch equations, considering interference effects associated with multiple excitation pathways and with internally generated fields. We have also observed spectral broadening of the two-photon resonant lines, which is well described by a linear dependence on atomic density, in accordance with collision-induced broadening.
We demonstrate production of continuous coherent blue laser light by using a five-level system in rubidium vapor. Two low-power lasers, at 780 and 776 nm, induce strong atomic coherence in the 5S-5P-5D states. The atoms decay to the 6P excited state, from which stimulated emission produces a coherent blue (420 nm) beam. We have coupled both ground-state hyperfine levels, effecting coherence between four levels. The coherent blue output is enhanced by several mechanisms, including stronger coupling to a larger fraction of the atomic population, operation at a detuning such that the vapor is nominally transparent to the 780 nm pump field, reduced losses owing to optical pumping, and optimal phase matching. We report experimental findings and compare them with results from a semiclassical Maxwell-Bloch model.