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Physical Review A

Spatiotemporal profile of yoked superfluorescence from Rb vapor in the strong-excitation regime

Kenta Kitano, H. Maeda

Published Jun 22, 2018• 5 citations
Publisher

Abstract

We investigated both spatial and temporal profiles of the 420-nm yoked superfluorescence (YSF) from the atomic vapor of rubidium by driving the $5S\ensuremath{-}5D$ two-photon transition with an ultrashort laser pulse. By increasing the pump-pulse power beyond the saturation intensity, the spatial profile of the 420-nm YSF periodically changed between a central bright spot and a ring-shaped radial profile. The temporal profile of the 420-nm YSF was measured with a sampling oscilloscope with a time resolution of 50 ps. The experimental results were successfully reproduced by simulations using the Maxwell-Bloch equations. The simulated results confirmed that, in the strong excitation regime, the 420-nm emission from the lower transition component of the YSF is significantly delayed with respect to the 5.2-$\ensuremath{\mu}\mathrm{m}$ emission from the upper transition, while they are overlapped in the weak excitation regime.

DOI
10.1103/physreva.97.063418

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•Jan 1, 1998•2 citations
We investigated both spatial and temporal profiles of the 420-nm yoked superfluorescence (YSF) from the atomic vapor of rubidium by driving the 5S\ensuremath−5D5S\ensuremath{-}5D5S\ensuremath−5D two-photon transition with an ultrashort laser pulse. By increasing the pump-pulse power beyond the saturation intensity, the spatial profile of the 420-nm YSF periodically changed between a central bright spot and a ring-shaped radial profile. The temporal profile of the 420-nm YSF was measured with a sampling oscilloscope with a time resolution of 50 ps. The experimental results were successfully reproduced by simulations using the Maxwell-Bloch equations. The simulated results confirmed that, in the strong excitation regime, the 420-nm emission from the lower transition component of the YSF is significantly delayed with respect to the 5.2-\ensuremathμm\ensuremath{\mu}\mathrm{m}\ensuremathμm emission from the upper transition, while they are overlapped in the weak excitation regime.
In this study, we investigate 420-nm yoked superfluorescence (YSF) emitted from the atomic vapor of rubidium (Rb) by driving the Rb 5S - 5D two-photon transition with an ultrashort pulsed laser. When the pump pulse is close to its transform limit (~ 100 fs) or down-chirped up to around 200 fs, the 420-nm YSF appears as a low-divergence beam with a ring-shaped radial profile. Although such a beam profile is less sensitive to the vapor pressure of Rb in a cell, its diameter rigorously varies as a function of the pump-pulse power. By numerically solving a time-dependent Schrödinger equation for a single-Rb atom, we well reproduce our experimental observation, indicating that a single-atom Rabi oscillation is responsible for the spatial beam profile of the 420-nm emission.
We investigated the transverse properties of 420-nm yoked superfluorescence (YSF) emitted from the atomic vapor of rubidium by driving the $5S\text{\ensuremath{-}}5D$ two-photon transition with an ultrashort laser pulse. From the observed interferograms of the superfluorescent pulses, we derived both the amplitude and phase properties of their electric fields. By increasing the pump-pulse intensity, the spatial profile of the 420-nm YSF changed from a central bright spot into a ring-shaped radial profile, accompanied by a simultaneous distortion of the wave front. This behavior can be explained by considering the intensity-dependent phase shifts of the $5S\text{\ensuremath{-}}5D$ wave packets during the excitation process, which was further imprinted to the 420-nm YSF wave front. The single-shot experiments enabled us to clarify the locally coherent regions of the 420-nm YSF as well as their stochastic feature.
Summary form only given.We have observed yoked superfluorescence (YSF) on the 5D-6P-5S channel in atomic rubidium after two-photon excitation of the 5D-5S transition by a short (3 ps), 778-nm pulse from a Ti:sapphire laser system. The excitation pulse is formed by a pair of spatially overlapping pulses of wave vectors k/sub 1/ and k/sub 2/ where |k/sub 1/|=|k/sub 2/|=2/spl pi///spl lambda/ and /spl lambda/ is the wavelength of the 5D-5S transition. When the atoms are excited by taking one photon from along k/sub 1/ and one from along k/sub 2/, then phase matching requires that the yoked transition consists of photon pairs along k/sub DP/ and k/sub PS/ where k/sub 1/+k/sub 2/=k/sub DP/+k/sub PS/ and k/sub DP/ and k/sub PS/ are the wave vectors of, respectively, the upper (5D-6P) and the lower (6P-5S) transition components of the YSF. The YSF pulses are emitted in a cone determined by Brownell et al. (1995). This conical emission of YSF was observed on the 420 nm lower transition. The measured behavior of the conical apex angle /spl beta/ as a function of the angle between the excitation pulses /spl alpha/ is given as a series of points and shows good agreement with the solid line based on Brownell et al.
We study the superfluorescence (SF) from a gas of rubidium atoms. The atoms of a dense vapor are excited to the 5D5D5D state from the 5S5S5S state by a two-photon process driven by 100-fs laser pulses. The atoms decay to the 6P6P6P state and then to the 5S5S5S state. The SF emission at 420 nm on the 6P6P6P--5S5S5S transition is recorded by a streak camera with picosecond time resolution. The time duration of the generated SF is tens of picoseconds, which is much shorter than the time scale of the usual relaxation processes, including spontaneous emission and atomic coherence dephasing. The dependence of the time delay between the reference input pulse and SF is measured as a function of laser power. The experimental data are described quantitatively by a simulation based on the semiclassical atom-field interaction theory. The observed change in scaling laws for the peak intensity and delay time can be elucidated by an SF theory in which the sample length is larger than the cooperation length.
A dense sample of atomic rubidium vapor excited by a short (4 ps) transform-limited laser pulse two-photon resonant with the 5S-5D transition in a large Fresnel number geometry emits a delayed omnidirectional infrared superfluorescence pulse (coherent flash) on the 5D-6P transition. This superfluorescence emission involves a coherent population transfer to the 6P level and results in prompt directional yoked UV emission and coherent population transfer back to the ground 5S state. This emission completes the process of parametric four-wave mixing, selectively involving only those atoms on the 6P level whose 5D-6P emission satisfies the phase-matching condition. The unique feature of this four-wave mixing radiation is that it occurs after the termination of the pump pulse, as evidenced by direct measurement of emission timing. The multidirectional character of superfluorescent emission is confirmed by the observation of non-collinear parametric four-wave mixing when exciting the sample by two simultaneous overlapping pulses, for a wide range of beam angles. Also observed and investigated is superfluorescent transient diffraction that appears when the sample is excited by a sequence of three laser pulses.