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Optics and Spectroscopy

On the inversionless amplification in the proximity of transitions from autoionization levels

А. А. Черненко

Published Apr 1, 2002• 1 citation
Publisher

Abstract

The results of theoretical investigations of the line shape for the absorption (amplification) of a weak optical signal in the proximity of the system of autoionization atomic levels are reported. The best conditions and regions of existence of the inversionless amplification in the proximity of transitions from these atomic levels are determined depending on the energy defect between them, the Fano parameters, and the generalized Rabi frequencies, as well as on the incoherent pumping power. The problem of dynamics of atomic level occupancy for the pulse excitation is solved. It is demonstrated that the effect of the inversionless amplification for the transient mode can considerably exceed its own steady-state value.

DOI
10.1134/1.1473594

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The results of theoretical investigations of the line shape for the absorption (amplification) of a weak optical signal in the proximity of the system of autoionization atomic levels are reported. The best conditions and regions of existence of the inversionless amplification in the proximity of transitions from these atomic levels are determined depending on the energy defect between them, the Fano parameters, and the generalized Rabi frequencies, as well as on the incoherent pumping power. The problem of dynamics of atomic level occupancy for the pulse excitation is solved. It is demonstrated that the effect of the inversionless amplification for the transient mode can considerably exceed its own steady-state value.
One consider the amplification of pulses of coherent light in the resonant three-level active media that is excited by short power pulse of coherent light according to the common-upper- level double resonance scheme. The radiation with major frequency (pump) is supposed to be stronger than one with minor frequency (signal). The self-induced transparency is supposed to be established in the pump channel. The exponential growth of stationary shape short signal pulses is studied mainly. One shows that when any described here conditions for initial signal pulse shape are meeting the stationary gained signal pulse of complicated form can be formed. The gains of these pulses are less than ones for signal pulses of quasi-hyperbolic-secant form. Therefore in the case of incomplete condition meeting the competition between the exponentially growing signal pulses with different gains and shapes leads finally to the overwhelming of signal pulse with quasi-hyperbolic-secant-shape form. The effect of inhomogeneous broadening of spectral transition lines on the parameters of signal pulses was studied too. Evaluations that were carried out for any atomic gases show that described in this paper phenomena can be produced at realistic levels of power and duration of pump and signal pulses.
Photoelectron ionization spectra of an autoionization system with one discrete level interacting with a neighbor two-level atom are discussed. The formula for long-time ionization spectra is derived. According to this formula, the spectra can be composed of up to eight peaks. Moreover, the Fano-like zeros for weak optical pumping are identified in these spectra. The conditional ionization spectra depending on the state of the neighbor atom exhibit oscillations at the Rabi frequency. Dynamical spectral zeros occurring once per the Rabi period are revealed in these spectra.
The rate of absorption of energy from a weak signal field by an atom driven by a strong pump field is evaluated. The pump field and the signal field are assumed to induce transitions between the same pair of states, and their frequencies are both assumed to lie near the atomic resonance frequency for the transition in question. We find that the signal-field absorption line-shape function takes on negative values, representing stimulated emission rather than absorption, even though population inversion does not occur. This amplification of the signal field, which is most pronounced at high pump intensities, is shown to occur primarily at the expense of the pump field, which suffers an increased rate of attenuation. The results are discussed in the context of a theorem which expresses the absorption line-shape function for general atomic systems in terms of a suitable atomic correlation function.
The absorption and stimulated emission of coherent electromagnetic radiation tuned close to an autoionizing state is investigated with the aim of clarifying some of the aspects behind the proposals of inversionless gain involving such atomic configurations. This is carried out within the framework of individual atomic evolution rather than by the use of density matrix equations, thus providing better insight into the fundamental processes of absorption and emission. In particular, we find that stimulated emission does not overcome absorption in spite of quantum interference of the Fano type being present. It is also shown that gain, in this configuration, although it can take place without population inversion between the involved levels, requires in any case a strong departure from thermal equilibrium conditions. Thereby a strong pumping mechanism, or equivalently a strong excitation power, is needed to reach the lasing threshold. This raises some questions on the relevance of the population inversion indicator of efficient gain implicitly assumed in similar configurations and in many related works.
Amplification of probe radiation by an inversionless two-level system in the 'red' wing of its spectral line is studied theoretically during the resonance absorption of intense cw laser radiation. The effect is caused by the inequality of the spectral densities of the Einstein coefficients for absorption and stimulated emission outside the absorption line under conditions when homogeneous broadening, due to the interaction of particles with the buffer gas, significantly exceeds the natural broadening (at high pressures of the buffer gas). It is found that the larger the inversionless gain, the higher the buffer gas pressure and the pump radiation intensity. It is established that at high enough pump intensity, the probe field is amplified along the entire region of the 'red' wing of the line (at any negative detunings of the probe field frequency).
The dynamics of the pulse shape and spectra of superfluorescence, stimulated superradiance, and coherent amplification of ultrashort light pulses is studied for a system of four-level atoms under incoherent cw pumping taking into account homogeneous and inhomogeneous broadening of the luminescence lines. It is found that incoherent pumping at a high level of excitation may significantly change the shape of the emission pulse in cooperative coherent processes. Also, it is found that, depending on the ratio of parameters of the pump, nonradiative transitions, and relaxation, different regimes of superfluorescence and stimulated superradiance may be realized.