Fluorescence decay at subnatural times in a magneto optical trap
Most of the studies on resonance radiation imprisonment (RI) in atomic vapors deal with linear equations of radiative energy transfer. However, the RI problem becomes strongly non-linear if the excitation of atomic vapor is produced by a laser pulse sufficiently strong to saturate the resonance transition. Under such conditions, the optical thickness of the medium becomes dependent on the density of excited atoms n*(). Analytical approach for the treatment of non-linear and non-uniform problems was elaborated in . In particular, that study describes fluorescence decay at subnatural time: in early stages of after-glow regime the escaping radiation decays with an effective constant Geff, which is larger than the Einstein coefficient G of the resonance level of isolated atom. This intriguing phenomenon occurs at a total saturation of gas volume by a laser pulse with sharp fronts. In the beginning of the decay process the medium is transparent (zero opacity) and photons escape from it without reabsorption. After termination of the saturating pulse the density of the absorbing atoms increases rapidly. Consequently, the RI process sets in, delaying the photons from escaping and thus acting as an optical shutter causing a strong decrease in the fluorescence signal.
Fig.1. Fluorescence decay in a MOT.
We have evaluated the subnatural fluorescence as a function of time t under the conditions of the Magneto Optical Trap (MOT) described in . Fig. 1 shows the calculated MOT fluorescence signals (solid curves) for different values of the opacity k. The dotted line shows the natural decay of the fluorescence with time constant 1/G. The subnatural behavior of the fluorescence decay at times t < 1/2G is clearly seen even for opacities as small as k = 2. Importantly, the initial slopes of the curves are sensitive to the opacity, which allows their exploitation for MOT diagnostics. Experimental verification of the above phenomena is planned with an alkali MOT.
 N. N. Bezuglov, A. N. Klucharev, A. F. Molisch, M. Allegrini, F. Fuso, and T. Stacewicz, Phys. Rev. E. 55, 3333 (1997)
 N.N. Bezuglov, A. F. Molisch, A.Fioretti, C.Gabbanini, F.Fuso and M.Allegrini, Phys.Rev.A. 68, 063415 (2003)