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Journal of the Optical Society of America B

Two-photon resonant forward four-wave mixing in rubidium vapor involving Rydberg states

Natalia R. de Melo, S. S. Vianna

Published Jun 27, 2014• 18 citations
Publisher

Abstract

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.

DOI
10.1364/josab.31.001735

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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 report on experimental results of four-wave mixing processes in rubidium vapor where coherence is induced on the three-photon resonant transition from 5s5s5s to 6p6p6p states via intermediate Rydberg levels. It is shown that the use of two beams in a noncollinear configuration, i.e., $\ensuremath{\theta}\ensuremath{\ne}0$, and high atomic density unveil new features. First, the $\ensuremath{\theta}=0$ (collinear configuration) odd-photon destructive interference between the incident and generated fields is strongly inhibited for $\ensuremath{\theta}\ensuremath{\ne}0$. Second, most importantly, the observed cooperative frequency shift of the three-photon transition is strongly enhanced for small, but nonzero, values of $\ensuremath{\theta}$ due to the factor ${(1\ensuremath{-}cos\ensuremath{\theta})}^{\ensuremath{-}1}$, which is not present if the generated radiation field is not considered self-consistently in the Maxwell-Bloch equations.
We present new results on the coherent control of a four-wave mixing signal in rubidium vapor involving Rydberg levels. By varying the relative polarizations of the laser beams responsible for the nonlinear process, we control atomic excitation routes and thus quantum interference in the medium. We observed interference fringes as a function of the relative polarizations and further developed a theoretical model based on the density-matrix formalism that is in very good agreement with our experimental results.
In this work, we present a set of experiments involving four-wave mixing processes in rubidium atoms. We employ several experimental configurations exploring two- and three-level systems in cascade or Λ configurations, all of which are in a copropagating geometry, to investigate different physical phenomena present in each scenario. In the first experiment, we combine a cw diode laser and a 1 GHz femtosecond pulse train to generate coherent blue light through the four-wave mixing process in hot rubidium vapor. The generated coherent signal is analyzed by scanning either the diode frequency or the repetition rate of the frequency comb. In both cases, the cw laser induces an Autler- Townes splitting with a wide separation in the doublet structure due to the configuration of copropagating fields. To model these results, we employ the density matrix formalism to write the Bloch equations and solve them numerically. The solutions allow us to explore other cases beyond those involving our experimental setup, comparing the response of the upper-level population and the generated signal coherence for homogeneously and non-homogeneously broadened systems. Moreover, in this same experiment, we also observed an unexpected interference effect between excitation routes of the four-wave mixing process when the repetition rate of the pulse train is scanned. The theoretical model demonstrates that the interfering effect appears only when the repetition rate is carefully chosen to produce a pair of modes of the frequency comb resonant with both one- and two-photon transitions for the same atomic velocity group. In the second experiment, still with a hot rubidium vapor, we explore two symmetrical degenerate four- wave mixing signals induced by two almost copropagating laser beams, with~ka and~kb wave-vectors, and detected simultaneously in the 2~ka −~kb and 2~kb −~ka directions. Each spectrum has a single peak with a short frequency separation between them, which our model indicates is due to the nonlinear behavior of the refractive indexes, a consequence of the high intensity of both input fields. Finally, we use the same experimental setup for the two symmetrical signals to study the field intensity fluctuations of the degenerate four-wave mixing signals and the transmission signals. In this case, the sample is an ensemble of cold rubidium atoms in a magneto-optical trap. There is an intensity- intensity cross-correlation between the signals due to the resonant phase-noise to amplitude-noise conversion. In this type of cold system, we can neglect the Doppler- broadening, allowing a straightforward study of the dependency of the correlations on the laser detuning. In these measurements, we observe a type of oscillation in the correlation curves that the theoretical model indicates to be connected with Rabi oscillations, revealed by the fluctuations long after the transient period.
We report on time-resolved pulsed four-wave mixing (FWM) signals in a thermal Rubidium vapor involving a Rydberg state. We observe FWM signals with dephasing times up to 7 ns, strongly dependent on the excitation bandwidth to the Rydberg state. The excitation to the Rydberg state is driven by a pulsed two-photon transition on ns timescales. Combined with a cw de-excitation laser, a strongly directional and collective emission is generated according to a combination of the phase matching effect and averaging over Doppler classes. In contrast to a previous report (Huber et al. in Phys Rev A 90: 053806, 2014) using off-resonant FWM, at a resonant FWM scheme we observe additional revivals of the signal shortly after the incident pulse has ended. We infer that this is a revival of motion-induced constructive interference between the coherent emissions of the thermal atoms. The resonant FWM scheme reveals a richer temporal structure of the signals, compared to similar, but off-resonant excitation schemes. A simple explanation lies in the selectivity of Doppler classes. Our numerical simulations based on a four-level model including a whole Doppler ensemble can qualitatively describe the data.
We have observed an interference in four-wave mixing involving two Rydberg states that play the key role of near two-photon resonant intermediate levels. This interference, due to the coherence of the nonlinear process, can be explained either classically or quantum mechanically and it depends on the lasers' polarizations and detunings. We have performed calculations that are in very good agreement with the experimental data. Our results provide new information about the system, such as ionization rates of the Rydberg levels and the ratio of reduced matrix elements involved.