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要旨:
The absorption spectrum of a bound state with a finite lifetime gives rise to the well-known Lorentzian line shape. This fundamental mechanism breaks down at high intensities. Here we demonstrate how even a single intense multicycle excitation pulse can result in Fano absorption profiles in the frequency domain. The intensity-dependent line shapes indicate the dipole responses are phase modulated by the single excitation pulse itself. Further, as the atomic density increases, we observe broader Fano-like shapes and the appearance of novel substructures in absorption lines. These are clear signatures of propagation effects in which collective interactions need to be taken into account. An analytical model is applied, which describes both effects observed in the experiment. Understanding the transition from optically dilute to dense medium is crucial to distinguish between single-atom and macroscopic effects.
