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We study the time-resolved fluorescence spectrum in two-level systemsinteracting with an incident coherent field, both in the weak and intermediatecoupling regimes. For a single two-level system in the intermediate couplingcase, as time flows, the spectrum develops distinct features, that are notcaptured by a semi-classical treatment of the incident field. Specifically, fora field on resonance with the atomic transition energy, the usual Mollowspectrum is replaced by a four peak structure, and for a frequency that is halfof the atomic transition energy, the time-dependent spectrum develops a secondharmonic peak with a superimposed Mollow triplet. In the long-time limit, ourdescription recovers results previously found in the literature. Afteranalyzing why a different behavior is observed in the quantum and classicaldynamics, the reason for the occurrence of a second harmonic signal in atwo-level system is explained via a symmetry analysis of the total (electronand photon) system, and in terms of a three level system operating in limitingregimes. We find an increased second harmonic signal in an array of two-levelsystems, suggesting a superradiance-like enhancement for multiple two-levelsystems in cavity setups. Finally, initial explorative results are presentedfor two-level model atoms entering and exiting a cavity, which hint at aninteresting interplay between cavity-photon screening and atomic dynamicseffects.

Time resolved multi-photon effects in the fluorescence spectra of two-level systems at rest and in motion