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EPJ Web of Conferences

Fluorescence dynamics of an ensemble of cold atoms

Sergey V. Bozhokin, Alexey Kuraptsev

Published Jan 1, 2019• 0 citations
Publisher

Abstract

We have calculated the fluorescence dynamics of cold atoms excited by pulsed radiation. We have considered both Radon-Nikodym spectral method and multi-stage relaxation model describing collective effects in cold atomic gases. Relaxation times and amplitudes both at short times (superradiance) and long times (subradiance) have seen obtained.

DOI
10.1051/epjconf/201922003005

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We have calculated the fluorescence dynamics of cold atoms excited by pulsed radiation. We have considered both Radon-Nikodym spectral method and multi-stage relaxation model describing collective effects in cold atomic gases. Relaxation times and amplitudes both at short times (superradiance) and long times (subradiance) have seen obtained.
Abstract The fluorescence of cold atoms, cooled to ultra-low temperatures and excited by pulsed laser radiation, is calculated. The fluorescence of cold atoms is calculated based on the solution of the non-steady-state Schrödinger equation for a combined system of atoms and an electromagnetic field. A universal function is proposed in the work showing the deviation of the fluorescence decay at the end of the pulse from the single-exponential model. To analyze the dynamics of fluorescence, a Radon–Nikodym method has been used based on the studies on the eigenvalues of a matrix constructed on the basis of the fluorescence signal. It is shown that a correlation exists between the parameter of distribution asymmetry inherent in signal matrix eigenvalues, and the temporal law of fluorescence decay. As an alternative method of fluorescence analysis, a multistage relaxation model is used, caused by collective multiatomic effects. The values of characteristic amplitude and relaxation time both at short times (“superradiance”) and at long times (“subradiance”) have been found.
Abstract The fluorescence of cold atoms, cooled to ultra-low temperatures and excited by pulsed laser radiation, is calculated. The fluorescence of cold atoms is calculated based on the solution of the non-steady-state Schrödinger equation for a combined system of atoms and an electromagnetic field. A universal function is proposed in the work showing the deviation of the fluorescence decay at the end of the pulse from the single-exponential model. To analyze the dynamics of fluorescence, a Radon–Nikodym method has been used based on the studies on the eigenvalues of a matrix constructed on the basis of the fluorescence signal. It is shown that a correlation exists between the parameter of distribution asymmetry inherent in signal matrix eigenvalues, and the temporal law of fluorescence decay. As an alternative method of fluorescence analysis, a multistage relaxation model is used, caused by collective multiatomic effects. The values of characteristic amplitude and relaxation time both at short times (“superradiance”) and at long times (“subradiance”) have been found.
Abstract A fluorescence dynamics of a cold atomic cloud, laser–cooled to a very low temperature, optically excited by a short laser impulse is calculated by applying non–stationary Schrödinger equation to the system of atoms and electromagnetic field. The solution gives fluorescence time–relaxation dependence, with relaxation time distribution depending on the system itself and external electromagnetic field applied. The distribution of the relaxation rates changes, when an external electric field affecting the atomic ensemble is applied. To analyze the relaxation process a novel Radon–Nikodym approach is applied: the relaxation curve is not piece–wise interpolated, but instead is converted to a matrix, the eigenvalues of which provide relaxation rate distribution. In contrasts to the random matrix theory, where the weights, corresponding to an eigenvalue, are considered equal, in this work the weight, corresponding to an individual eigenvalue, is determined by the Lebesgue integral quadrature weights.
Abstract The fluorescence dynamics of optically dense but dilute atomic ensembles cooled to sub-Doppler temperatures and excited by resonant pulsed radiation is studied. Apart from total radiation intensity, the temporal variations of the spectral composition of fluorescence and its polarization, as well as the variation of the spatial distribution of excitation in the atomic ensemble, are analyzed. Based on this analysis, it is shown that the role of radiation trapping becomes smaller with time, and subradiance quantum states formed as a result of the resonant dipole–dipole interatomic interaction become the main factor determining the slow atomic excitation decay. The dependence of the subradiance form on the ensemble size is analyzed.
The dynamics of the fluorescence of dilute atomic ensembles excited by resonant pulsed radiation has been analyzed. It has been found that the motion of atoms in ensembles cooled to sub-Doppler temperatures in certain time intervals noticeably enhances subradiance rather than suppressing it. It has also been shown that the time dependence of the spontaneous decay rate can be nonmonotonic. The revealed features occur because the character of the slow decay of the atomic excitation is determined after a certain time mainly by the dynamics of subradiant two-atom quantum states, which are due to the resonant dipole–dipole interaction between atoms.
We report the investigation of the influence of atomic motion on the fluorescence dynamics of a dilute atomic ensemble driven by resonance pulse radiation. We show that even for sub-Doppler temperatures, the motion of atoms can significantly affect the nature of both superradiation and subradiation. We also demonstrate that, in the case of an ensemble of moving scatterers, it is possible to observe the nonmonotonic time dependence of the fluorescence rate. This leads to the fact that, in certain time intervals, increasing temperature causes not an decrease but an increase of the fluorescence intensity in the cone of coherent scattering. We analyze the role of the frequency diffusion of secondary radiation as a result of multiple light scattering in an optically dense medium. It is shown that spectrum broadening is the main factor which determines radiation trapping upon resonant excitation. Along with broadening, we found a shift and distortion of the shape of the spectrum over time. At later time, after the trapping stage, the dynamics is dominated by close pairs of atoms (dimers). The dynamics of the excited states of these dimers has been studied in detail. It is shown that the change in the lifetime of the given adiabatic term of the diatomic quasimolecule induced by the change in the interatomic distance as well as possible nonadiabatic transitions between sub- and superradiant states caused by atomic motion can lead not to the anticipated weakening of subradiation effect, but to its enhancement.