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Journal of Experimental and Theoretical Physics

Spontaneous emission of atoms in a strong laser field

Anna V. Bogatskaya, Ekaterina A. Volkova, A. M. Popov

Published Oct 1, 2017• 18 citations
Publisher

Abstract

The spontaneous emission of an atomic system in the field of a high-intensity femtosecond laser pulse is considered within the framework of a consistent quantum-mechanical approach based on an examination of the interaction of a quantum system with a set of quantized field modes in a vacuum state. Both even and odd harmonics of the driving field are shown to appear in the atomic emission spectrum, and the mechanisms of their generation are elucidated. A comparison with the semiclassical theory of laser harmonic generation is made. The semiclassical approach in describing the spontaneous emission in strong laser fields, especially under conditions of significant depletion of the ground (initial) state in a laser field, is shown to be limited.

DOI
10.1134/s1063776117090114

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Journal of Physics B Atomic Molecular and Optical Physics•Oct 14, 1994•20 citations
The spontaneous emission of an atomic system in the field of a high-intensity femtosecond laser pulse is considered within the framework of a consistent quantum-mechanical approach based on an examination of the interaction of a quantum system with a set of quantized field modes in a vacuum state. Both even and odd harmonics of the driving field are shown to appear in the atomic emission spectrum, and the mechanisms of their generation are elucidated. A comparison with the semiclassical theory of laser harmonic generation is made. The semiclassical approach in describing the spontaneous emission in strong laser fields, especially under conditions of significant depletion of the ground (initial) state in a laser field, is shown to be limited.
Abstract A new approach for studying the spontaneous emission of an atomic system in the presence of a high-intensity laser field is used to study the process of harmonic generation. The analysis is based on consideration of quantum system interaction, with the quantized field modes being in the vacuum state, while the intense laser field is considered to be classically beyond perturbation theory. The numerical analysis of the emission from the single one-electron 1D atom irradiated by the femtosecond laser pulse of a Ti:Sa laser is discussed. It is demonstrated that not only odd, but also even harmonics can be emitted if the laser field is strong enough. The origin of the appearance of even harmonics is studied. The obtained results are compared with those found in the framework of the semiclassical approach that is widely used to study harmonic generation. It is found that the semiclassical approach is inapplicable in the strong-field limit.
A new approach to the study of the spontaneous emission of the quantum system driven by a high-intensity laser field is developed. This approach is based on the accurate consideration of quantum system interaction with vacuum quantized field modes in the first order of perturbation theory, while the intense laser field is considered classically beyond the perturbation theory which allows to observe any-order stimulated processes governed by classical field. The proposed approach is applied to the study of a number of quantum systems in intense laser field. The obtained data are compared with those obtained in the frames of semiclassical approximation typically used for analyzing of the strong-field dynamic. It is found that the applicability of the semiclassical approach is strictly limited. It is valid for calculation of transitions to the initially populated state only if the population of this state is close to unity during the pulse and in the after-pulse regime. If its population is depleted, the semiclassical approach fails.
Abstract The spontaneous emission of a quantum system driven by a high-intensity, high-frequency classical laser field is analyzed. The study is based on the accurate consideration of the quantum system interacting with vacuum-quantized field modes in the first order of perturbation theory, while the intense laser field is considered classically beyond this theory. It is demonstrated that the spectrum of the spontaneous emission can be used for analyzing the strong-field dynamics and the structure of the energy spectrum of an atomic system. In particular, it is found that in high-frequency fields (where the energy of the laser quanta is greater than the ionization potential) atoms manifest the features of the Kramers–Henneberger atom. It is also found that in the stabilization regime, the atom emits both odd and even laser radiation harmonics.
Spontaneous emission of the quantum system driven by a high intensity classical laser field is analyzed. The study is based on the accurate consideration of quantum system interaction with vacuum quantized field modes in the first order of perturbation theory, while the intense laser field is considered classically beyond the perturbation theory. It is demonstrated that the spectrum of the spontaneous emission can be used for analyzing of the strong-field dynamics and structure of the energy spectrum of the atomic system. The obtained data are compared with that obtained in the frames of semiclassical approximation typically used for analyzing of the strong-field dynamics. It is found that the applicability of the semiclassical approach is strictly limited.
We study the general problem of matter-laser interaction within the framework of nonrelativistic quantum electrodynamics, with particular emphasis on strong laser field effects. Consequently, we formulate a well-defined approximation leading in a straightforward manner toward the conventional semiclassical mode of description. Namely, we arrive naturally to two coupled equations of motion: (i) the Schr\"odinger equation which governs the quantum dynamics of an atomic system driven by classically described radiation field (composed of an incoming laser pulse plus radiation emitted from the atom), and (ii) the classical Maxwell wave equation which describes the emission of radiation from the mentioned atomic source. Employing the formalism of adiabatic Floquet theory, we derive a simple criterion of validity of the just described semiclassical approach. It shows that the semiclassical treatment is justified in most situations. On the other hand, it turns out that the semiclassical approximation breaks down completely in certain special but realistic cases, regardless of the fact that the incoming laser pulse contains a huge number of photons. Under such special circumstances, we anticipate new effects arising due to the quantized nature of the radiation field, to be observable, for example, in harmonic generation spectra. Our considerations are illustrated more explicitly using a simple model of a two-level atom strongly driven by a laser. The quantum dynamics of this model problem is resolved within the framework of quantum electrodynamics while adopting well-defined and physically justifiable approximations. As an outcome, analytic formulas are found serving as a quantitative criterion of (non)applicability of the semiclassical approach and demonstrating the breakdown of semiclassical theory under well-defined conditions. An illustrative numerical calculation is provided.
We present a theory of harmonic generation by a strongly driven atom within the framework of non-relativistic QED. The atom is assumed to possess a finite number N of levels and ionization is neglected. This atom is dressed by the driving field using a unitary time-dependent transformation. Within the quasistatic approximation this transformation is shown to lead from the bare to the Floquet atom. The interaction of the Floquet atom with the vacuum modes is derived and it is shown to give rise both to the high-order harmonic and to the hyper-Raman spectrum. In the case N=2 the spectrum of the light scattered into the vacuum modes can be evaluated analytically and it is shown to consist of two parts. Neglecting interference between these two parts, the first of them is the high-order harmonic spectrum displaying peaks at the odd harmonics of the driving field. The second part is the hyper-Raman spectrum which has finite bandwidth and which displays peaks at the even harmonics of the laser frequency, centred at the Stark-shifted natural atomic frequency. These two spectra partially overlap and they are discussed in various ranges of intensity of the driving field, both in the minimal coupling and in the multipolar scheme. The results support recent suggestions that the presence of the plateau is related to interference between the two spectra. The dependence of the high-order harmonic spectrum from the driving field intensity is found to be related to that of the hyper-Raman spectrum in such a way that when the former increases the latter decreases and vice versa. It is argued that this might help to explain the non-visibility of the hyper-Raman lines when the high-order harmonics are visible.