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arXiv:2505.19930 · Physical Review A

Temporal dynamics in the Bragg reflection of light by cold atoms: Flash effect and superradiant decay

Stephan Asselie, J.-M. Nazon, Romain Caldani, C. Roux-Spitz, William Guerin

Published Nov 14, 2025• 4 citations
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Abstract

The authors study the transient optical response of a Bragg mirror made of cold atoms trapped in a one-dimensional lattice. They find that after the switch-off of the incoming beam, depending on its frequency, the reflected intensity can temporarily increase, or, on the contrary, can undergo a decay much faster than the natural timescale of the response of individual atoms.

DOI
10.1103/lcx8-k7b7

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The authors study the transient optical response of a Bragg mirror made of cold atoms trapped in a one-dimensional lattice. They find that after the switch-off of the incoming beam, depending on its frequency, the reflected intensity can temporarily increase, or, on the contrary, can undergo a decay much faster than the natural timescale of the response of individual atoms.
We present a theoretical and experimental analysis of transient optical properties of a dense cold atomic gas. After the rapid extinction of a weak coherent driving field (mean photon number ∼1.5\sim 1.5∼1.5), a transient `flash' is observed. Surprisingly the decay of the `flash' is faster than the decay of the fastest superradiant mode of the system. We show that this `faster than superradiance decay' is expected due to the interference between collective eigenmodes that exhibit a range of frequency shifts away from the bare atomic transition. Experimental results confirm that the initial decay rate of the superradiant flash increases with optical depth, in agreement with the numerical simulations for the experimental conditions.
We experimentally and numerically study the temporal dynamics of light scattered by large clouds of cold atoms after the exciting laser is switched off, in the low intensity (linear-optics) regime. Radiation trapping due to multiple scattering as well as subradiance lead to decay much slower than the single atom fluorescence decay. These two effects have already been observed separately, but the interplay between them remained to be understood. Here, we show that with well chosen parameters of the driving field, the two effects can occur at the same time, but follow different scaling behaviors. The subradiant decay is observed at late time and its rate is independent of the detuning, while the radiation trapping decay is observed at intermediate time and depends on the detuning through the optical depth of the sample. Numerical simulations based on random walk process and coupled-dipole equations support our interpretations. Our study clarifies the different interpretations and physical mechanisms at the origin of slow temporal dynamics of light in cold atoms.
Since Dicke's seminal paper on coherence in spontaneous radiation by atomic ensembles, superradiance has been extensively studied. Subradiance, on the contrary, has remained elusive, mainly because subradiant states are weakly coupled to the environment and are very sensitive to nonradiative decoherence processes. Here, we report the experimental observation of subradiance in an extended and dilute cold-atom sample containing a large number of particles. We use a far detuned laser to avoid multiple scattering and observe the temporal decay after a sudden switch-off of the laser beam. After the fast decay of most of the fluorescence, we detect a very slow decay, with time constants as long as 100 times the natural lifetime of the excited state of individual atoms. This subradiant time constant scales linearly with the cooperativity parameter, corresponding to the on-resonance optical depth of the sample, and is independent of the laser detuning, as expected from a coupled-dipole model.