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European Quantum Electronics Conference (EQEC)

Spontaneous generation of a density grating in atomic vapours interacting with a strong electromagnetic field

M. Pen-in, J. Javaloyes, Gian Luca Lippi, Antonio Politi

Published Jan 1, 2003• 0 citations
Publisher

Abstract

This study shows that, for appropriate parameter values, a stable density grating can spontaneously emerge and be sustained over arbitrarily long times in a generalized model. The onset of a non-zero bunching is accompanied by a nonzero value of the coherence parameter. This regime, occurring at sufficiently low temperatures, is therefore characterized by both coherence and bunching-values different from zero. In order to shed some light on the role played by the two order parameters, the authors have reconstructed a detailed phase diagram by exploring their dependence on the pump intensity and on the temperature.

DOI
10.1109/eqec.2003.1313907

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This study shows that, for appropriate parameter values, a stable density grating can spontaneously emerge and be sustained over arbitrarily long times in a generalized model. The onset of a non-zero bunching is accompanied by a nonzero value of the coherence parameter. This regime, occurring at sufficiently low temperatures, is therefore characterized by both coherence and bunching-values different from zero. In order to shed some light on the role played by the two order parameters, the authors have reconstructed a detailed phase diagram by exploring their dependence on the pump intensity and on the temperature.
The interaction of an atomic gas confined inside a cavity containing a strong electromagnetic field is numerically and theoretically investigated in a regime where recoil effects are not negligible. The spontaneous appearance of a density grating (atomic bunching) accompanied by the onset of a coherent, back-propagating electromagnetic wave is found to be ruled by a continuous phase transition. Numerical tests allow us to convincingly prove that the transition is steered by the appearence of a periodic atomic density modulation. Consideration of different experimental relaxation mechanisms induces us to analyze the problem in nearly analytic form, in the large detuning limit, using both a Vlasov approach and a Fokker-Planck description. The application of our predictions to recent experimental findings, reported by Kruse et al. [Phys. Rev. Lett., 91 183601 (2003)], yields a semiquantitative agreement with the observations.
The interaction of an atomic gas confined inside a cavity with a strong electromagnetic field is numerically and theoretically investigated in a regime where recoil effects are not negligible. The spontaneous appearance of a density grating (atomic bunching) accompanied by the onset of coherent backpropagation is found to be ruled by a continuous phase-transition. Numerical tests allow us to convincingly prove that the transition is steered by the appearence of a periodic atomic density modulation. Consideration of different experimental relaxation mechanisms induces us to analyze the problem in nearly analytic form, in the large detuning limit, using both a Vlasov approach and a Fokker-Planck description. The application of our predictions to recent experimental findings, reported in [Phys. Rev. Lett. {\bf 91}, 183601 (2003)], yields a semi-quantitative agreement with the observations.
We present measurements of probe beam amplification in Na vapor at intermediate density values, in both the counterpropagating and copropagating directions (with respect to the pump beam) in the case of degenerate pump and probe frequencies. Strong gain is observed only in the counterpropagating direction for parameters that preclude the possibility of Raman or Rayleigh gain. We show that the experimental results can be interpreted in terms of the spontaneous formation of a density grating due to the nonresonant interaction between the electromagnetic field and the atoms.
Abstract Far-from equilibrium dynamics that lead to self-organization are highly relevant to complex dynamical systems not only in physics, but also in life-, earth-, and social sciences. It is challenging however to find systems with sufficiently controllable parameters that allow quantitatively modelling of emergent properties. Here, we study a non-equilibrium phase transition and observe signatures of self-organized criticality in a dilute thermal vapour of atoms optically excited to strongly interacting Rydberg states. Electromagnetically induced transparency (EIT) provides excellent control over the population dynamics and enables high-resolution probing of the driven-dissipative dynamics, which also exhibits phase bistability. Increased sensitivity compared to previous work allows us to reconstruct the complete phase diagram including in the vicinity of the critical point. We observe that interaction-induced energy shifts and enhanced decay only occur in one of the phases above a critical Rydberg population. This limits the application of generic mean-field models, however a modified, threshold-dependent approach is in qualitative agreement with experimental data. Near threshold, we observe self-organized dynamics in the form of population jumps that return the density to a critical value.
Summary form only given. We describe in particular a density grating obtained from the interaction between a strong e.m. pump field and an atomic vapor, which allows for the amplification of a probe field. Experimental results and an interpretation are offered in this case, together with suggestions for possible applications resulting from these systems. In particular we present a recent experiment that shows the combined effects of the two fundamental aspects of quantum optics, in the spontaneous generation, through momentum transfer between light and atoms, of a long range atomic density grating. The experiment is performed in a vapor of atomic sodium at temperatures between 460 and 800K.
Collective behavior in many-body systems, where the dynamics of an individual element depend on the state of the entire ensemble, play an important role in both basic science research and applied technologies. Over the last twenty years, studies of such effects in cold atomic vapors have lead to breakthroughs in areas such as quantum information science and atomic and condensed matter physics. Nevertheless, in order to generate photon-mediated atom-atom coupling strengths that are large enough to produce collective behavior, these studies employ techniques that intrinsically limit their applicability. In this thesis, I describe a novel nonlinear optical process that enables me to overcome these limitations and realize a new regime of collective light-matter interaction. My experiment involves an anisotropic cloud of cold rubidium atoms illuminated by a pair of counterpropagating optical (pump) fields propagating at an angle to the trap's long axis. When the pump beam intensities exceed a threshold value, a collective instability occurs in which new beams of light are generated spontaneously and counterpropagate along the trap's long axis. In order to understand the physical mechanism responsible for this behavior, I study first the system's nonlinear optical response when driven below the instability threshold. I find that the incident optical fields produce an optical lattice that causes the atoms to become spatially organized on the sub-wavelength length scale. This organization corresponds to the formation of an atomic density grating, which effectively couples the involved fields to one another and enables the transfer of energy between them. The loading of atoms into this grating is enhanced by my choice of field polarizations, which simultaneously results in cooling of the atoms from T~30 μK to T~3 μK via the Sisyphus effect. As a result, I observe a fifth-order nonlinear susceptibility χ^{(5)}=1.9x10^-12 (m/V)^4 that is 7 orders of magnitude larger than previously observed. In addition, because of the unique scaling of the resulting nonlinear response with material parameters, the magnitude of the nonlinearity can be large for small pump intensities (\\ie, below the resonant electronic saturation intensity 1.6 mW/cm^2) while simultaneously suffering little linear absorption. I confirm my interpretation of the nonlinearity by developing a theoretical model that agrees quantitatively with my experimental observations with no free parameters. The collective instability therefore corresponds to the situation where the cold vapor transitions spontaneously from a spatially-homogeneous state to an ordered one. This emergent organization leads to the simultaneous emission of new optical fields in a process that one can interpret either in terms of mirrorless parametric self-oscillation or superradiance. By mapping out the phase diagram for this transition, I find that the instability can occur for pump intensities as low as 1 mW/cm^2, which is approximately 50 times smaller than previous observations of similar phenomena. The intensity of the emitted light can be up to 20% of the pump beam intensity and depends superlinearly on the number of atoms, which is a clear signature of collective behavior. In addition, the generated light demonstrates temporal correlations between the counterpropagating modes of up to 0.987 and is nearly coherent over several hundred μs. The most significant attributes of the light, though, are that it consists of multiple transverse spatial modes and persists in steady-state. This result represents the first observation of such dynamics, which have been shown theoretically to lead to a rich array of new phenomena and possible applications.