Abstract
We study the time-resolved fluorescence spectrum in two-level systems interacting with an incident coherent field, both in the weak and intermediate coupling regimes. For a single two-level system in the intermediate coupling case, as time flows, the spectrum develops distinct features, that are not captured by a semi-classical treatment of the incident field. Specifically, for a field on resonance with the atomic transition energy, the usual Mollow spectrum is replaced by a four peak structure, and for a frequency that is half of the atomic transition energy, the time-dependent spectrum develops a second harmonic peak with a superimposed Mollow triplet. In the long-time limit, our description recovers results previously found in the literature. After analyzing why a different behavior is observed in the quantum and classical dynamics, the reason for the occurrence of a second harmonic signal in a two-level system is explained via a symmetry analysis of the total (electron and photon) system, and in terms of a three level system operating in limiting regimes. We find an increased second harmonic signal in an array of two-level systems, suggesting a superradiance-like enhancement for multiple two-level systems in cavity setups. Finally, initial explorative results are presented for two-level model atoms entering and exiting a cavity, which hint at an interesting interplay between cavity-photon screening and atomic dynamics effects.
Highlights
Fluorescence, a type of luminescence [1,2,3], is a hallmark of quantum mechanics at work: A system that has absorbed electromagnetic radiation re-emits it at a later time, while the spins of the electrons involved in the deexcitation process conform to specific selection rules.In addition to being an operational mechanism in several biological systems [4], fluorescence serves in many technologies of different complexity, ranging from simple indoor lighting to in-depth spectroscopic characterization at the atomic scale
After analyzing why a different behavior is observed in the quantum and classical dynamics, the reason for the occurrence of a secondharmonic signal in a two-level system is explained via a symmetry analysis of the total system and in terms of a three-level system operating in limiting regimes
To understand how second-harmonic generation (SHG) can happen in a two-level system, we look at the parity of the eigenstates of H
Summary
Fluorescence, a type of luminescence [1,2,3], is a hallmark of quantum mechanics at work: A system that has absorbed electromagnetic radiation re-emits it at a later time, while the spins of the electrons involved in the deexcitation process conform to specific selection rules. We address the (so far largely unexplored) multiphoton effects in fluorescence spectra, which depend separately on field intensity and light-matter coupling strength. This will be done in situations of progressive complexity: a single two-level atom at rest, an array of two-level atoms at rest, and a single two-level atom moving through an optical cavity. Current experimental capabilities provide practical and closeto-ideal realizations of these premises with optical cavity setups and make time-resolved studies possible In this way, it is possible to investigate the actual development of the fluorescence signal before the steady-state signal sets in.
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