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Journal of the Optical Society of America B

Population inversion by nonlinear excitation of atomic inner shells

Misha Ivanov, M. G. Malakhov

Published Apr 1, 1990• 2 citations
Publisher

Abstract

We present a detailed analysis of the creation of population inversion and coherent UV radiation with a new scheme, i.e., nonlinear excitation of an inner atomic electron in a strong laser field, accompanied by filling the resulting inner-shell vacancy with an external-shell electron. Our analysis is given for the experimental results obtained by Boyer et al. in 1984 [ J. Opt. Soc. Am. B1, 3 ( 1984)]. The principal new features of the scheme under consideration are the following: (1) the two-electron character of atomic excitation with subsequent emission of a high-energy photon, (2) the initial absence of multiphoton resonance between the ground state and the level to be excited, and (3) strong-field-induced level crossing and excitation of the upper working level by means of Landau–Zener-type transitions.

DOI
10.1364/josab.7.000467

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We present a detailed analysis of the creation of population inversion and coherent UV radiation with a new scheme, i.e., nonlinear excitation of an inner atomic electron in a strong laser field, accompanied by filling the resulting inner-shell vacancy with an external-shell electron. Our analysis is given for the experimental results obtained by Boyer et al. in 1984 [ J. Opt. Soc. Am. B1, 3 ( 1984)]. The principal new features of the scheme under consideration are the following: (1) the two-electron character of atomic excitation with subsequent emission of a high-energy photon, (2) the initial absence of multiphoton resonance between the ground state and the level to be excited, and (3) strong-field-induced level crossing and excitation of the upper working level by means of Landau–Zener-type transitions.
It is shown that the photoionization of the inner-shell electrons can yield metastable ions. Therefore, the accumulation of excited particles becomes possible at practically realizable optical pumping powers. This method of population inversion may be effective because a large part of the emission spectrum of an optically transparent plasma in a radiation source is in the form of "hard" photons, which are emitted when electrons are captured and atoms or ions go over to their ground state.
A theoretical study of the nonlinearity reduction in a four-level system of a potassium atom in the presence of a strong nanosecond laser field, which excites the transition |4S 1/2 ⟩↔|6S 1/2 ⟩ with two photons, is presented. It is shown that the destructive quantum interference between the laser field and the internally generated radiations results in a linear response of the atomic path-1 (|4S 1/2 ⟩↔|6S 1/2 ⟩↔|5P 3/2 ⟩↔|4S 1/2 ⟩) emitted parametric fields. For sufficiently high laser intensities and/or atomic densities, the path-1 emitted fields are driven into saturation, a regime accompanied by a substantial population redistribution among the states. Reasonable agreement between earlier experimental results and the theoretical ones is obtained. It is also shown that upon saturation of path-1, for low atomic density, the path-2 (|4S 1/2 ⟩↔|6S 1/2 ⟩↔|4P 3/2 ⟩↔|4S 1/2 ⟩) is activated. A subtle interplay between laser intensity and atomic density may determine the activation of path-2. Also, it is shown that the path-2 emission |6S 1/2 ⟩↔|4P 3/2 ⟩ is an amplified spontaneous emission process which induces a |4P 3/2 ⟩↔|4S 1/2 ⟩ emission, without population inversion, in a cascade scheme.
Strong-field excitation provides new methods and means for coherent population of Rydberg states. In this study, we numerically solve the time-dependent Schrödinger equation to investigate the strong-field excitation and radiation dynamics under the cooperative interaction of terahertz (THz) and femtosecond strong laser pulses. In the radiation spectrum, the Rydberg states excited by the strong field are manifested as resonance peaks with energy below the ionization threshold. By using THz fields to control the electron motion on sub-optical cycles, we achieve modulation of Rydberg state population and high-order harmonic radiation under the influence of a strong laser field. We analyze the variation of the excited state population with THz intensity, frequency, and the angle between the infrared field polarization and the THz field. Our results show that, with the assistance of the THz field, population inversion occurs between the Rydberg states. This study provides a feasible scheme for THz amplification based on Rydberg atoms and holds significant implications for THz amplification, high-sensitivity detection, and quantum information processing.
Abstract We extend the 4-step analytical model of resonant enhancement of high harmonic generation to the systems possessing resonant transitions of inner-shell electrons. Resonant enhancement is explained by lasing without inversion in a three-level system of ground, excited and shifted resonant states, which are coupled to the fundamental field and its high harmonics. The role of inelastic scattering is studied by simulation of an excited state’s population dynamics. It is shown that maximal gain is achieved when the energy shift between the excited state and resonant state is close to the energy of the fundamental photon. To prove the concept we demonstrate the enhancement of harmonics in the In plasma using different pumps.
We present experimental and theoretical data on the electron energy spectra from resonance-enhanced multiphoton ionization of atoms. In experiments performed with 594-nm wavelength, linearly polarized laser pulses of 300-fs and 600-fs durations, it is found that excitation of the intermediate resonances during the rising edge of the laser pulse can dominate the electron spectrum. Good agreement is found between the experimental results and a model based on the Landau-Zener energy level crossing theory. Theoretical results obtained with direct integration of the Schrödinger equation show that intermediate resonances can be excited appreciably off resonance if the coupling between the ground state and the excited state is highly nonlinear. The excitation of the intermediate state is not localized at the intensity which makes it six-photon resonant with the ground state. A multilevel model used to simulate the experimental spectra, based on direct integration of the Schrödinger equation, shows excellent agreement with the data.
Abstract On the basis of our theoretical model of the population transfer between two levels interacting suddenly with an intense laser pulse (Zhang et al 2017 J. Phys. B: At. Mol. Opt. Phys. 50 185603), we investigate the mechanism of the population inversion between the electronically excited B 2 Σ u + state and the electronic ground X 2 Σ g + state of N 2 + achieved by the irradiation of a near-IR (800 nm) intense laser pulse, leading to the generation of the coherent emission at 391 nm. We show that one of the Floquet states generated at the moment when N 2 + starts interacting with an intense laser pulse is adiabatically connected to the ground vibrational state of the electronic B 2 Σ u + state and keeps a constant population throughout the period of the interaction with the light field, indicating that the efficient off-resonance B 2 Σ u + – X 2 Σ g + transition proceeds.