Abstract
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.
Highlights
Collective effects in light scattering by atomic ensembles have recently been the subject of intense research, both theoretically and experimentally [1, 2]
It has been shown experimentally that the subradiant decay rate depends on the resonant optical depth b0, independently of the detuning ∆ = ω − ω0 from the atomic resonance ω0, which has been confirmed by numerical simulations [20, 21, 22]
At zero temperature and for large enough optical depth, radiation trapping could be slower than subradiance and dominate even at late time, the frequency redistribution due to Doppler broadening strongly reduces the number of scattering events that light can undergo before escaping, and we find that, at T ∼ 100 μK, subradiant decay always dominates at late time
Summary
Collective effects in light scattering by atomic ensembles have recently been the subject of intense research, both theoretically and experimentally [1, 2]. Near resonance, when the actual optical depth b(∆) ∝ b0/(1 + 4∆2/Γ2) is large, light undergoes multiple scattering This leads to a slowed transport velocity inside the diffusive medium [23] and to a slow decay when the incident light is switched off. At zero temperature and for large enough optical depth, radiation trapping could be slower than subradiance and dominate even at late time, the frequency redistribution due to Doppler broadening strongly reduces the number of scattering events that light can undergo before escaping, and we find that, at T ∼ 100 μK, subradiant decay always dominates at late time.
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