A. A. Chernenko

On the shape of cross resonances in counterpropagating wave spectroscopy

Physical processes that generate cross resonances are studied. It is revealed that not only populational, but also coherent, effects can make a contribution to formation of cross resonances. The effects of coherent processes, lifetimes of levels, the parameter of radiation branching from the upper level, and light fields’ characteristics are shown to be qualitatively different for the Λ-, V-, and J = 1 − J = 1 types of transition. Conditions for the change in the sign of cross resonances are found and a situation wherein the cross resonance has a purely coherent nature is shown.

Saturation and self-saturation effects in the field of intense laser waves: The J = 1 − J = 1 transition

The shape of the resonance of a saturated absorption is calculated at an arbitrary intensity of a probe field. It is revealed that, in the case of orthogonal polarizations of strong and probe fields, an increase in the intensity of the probe field can invert the shape of the nonlinear resonance of clearing of the medium, changing it to the resonance of absorption. The saturation of optical transitions by intrinsic spontaneous radiation increases the contrast of the inverted resonance of saturation and narrows its width. In the case of parallel polarizations of the optical fields, an absorption peak can arise in the shape of the nonlinear resonance upon splitting of the lower state.

Probe-field spectroscopy under conditions of self-saturation

Nonlinear absorption of a probe field is analyzed taking into account intrinsic spontaneous radiation (the self-saturation effect). Systems of two and three nondegenerate levels, as well as a system of two levels with the angular momenta equal to unity, are considered. Changes produced by the self-saturation in the population, polarization, and nonlinear interference effects, as well as in the field splitting of lines, are determined. In certain situations, incoherent spontaneous radiation enhances the interference phenomena.

Polarization phenomena in the transparency and adsorption effects induced by the field of counterpropagating waves

The physical processes that form the resonances of saturated absorption and magnetic scanning in the field of counterpropagating waves of an arbitrary intensity when their polarizations change are numerically simulated. The atomic transition with level moment J = 1 is used as an example to show that the anomalies in the experimental saturated absorption spectra are determined by the degree of opening of the atomic transition. In the case of magnetic scanning, the anomalies are caused by the magnetic coherence induced by the wave fields at the levels of the lower state rather than by its transfer from the excited states, as was proposed earlier.

Effect of saturation in the field of coherent and intrinsic spontaneous radiation

Saturation effects in gaseous media in the presence of a high-intensity resonant laser field and in the field of incoherent radiation caused by the intrinsic spontaneous emission are considered. A simple nonlinear set of kinetic equations is proposed for describing the saturation effect in the case of optically thin media and nondegenerate levels. The Bennett structure and the band of homogeneous saturation, whose parameters are interdependent, exist in the velocity distribution. The regularities in the behavior of the band and the Bennett structure under variation in the transition parameters and the intensity of laser radiation are elucidated.

Photoionization of the 4d state of the sodium atom in the field of a nanosecond pulse of a Nd:YAG laser

Results of experiments on photoionization of excited sodium atoms in the 4d state in the field of nanosecond pulses of a Nd:YAG laser are reported. The dependence of the photoionization signal for the given atomic state on the ionizing pulse energy was obtained. The experimental results are compared with the model calculations of the saturation curve for the photoionization signal.

Role of spontaneous emission through operating transition in probe-field spectroscopy of two-level systems

Analytical and numerical investigations are carried out of the effect of spontaneous decay through operating transition on the shape of a resonance in the work of a probe field under a strong field applied to the transition. A narrow nonlinear resonance arising on transitions with long-living lower level in the work of a probe field can manifest itself in the form of a traditional minimum and a peak as a function of the first Einstein coefficient for the operating transition. The transformation of the resonance from a minimum to a peak is attributed to the specific character of relaxation of lower-level population beatings on a closed or almost closed transition (the decay of the upper level occurs completely or almost completely through the operating transition).

Effect of own radiation on the linewidth and absorption spectrum of a probe field

The absorption spectrum of a probe field upon self-saturation on the transition between levels with angular momenta equal to 1/2 is considered. A new class of nonlinear spectroscopic phenomena caused by the nonlinearity (with respect to the density matrix) of the radiative relaxation matrix is predicted. The relative role of the own spontaneous radiation in the broadening of the combining levels and the line, as well as the dependence of the absorption on the polarization of the probe field, is elucidated.

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

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.

Effect of the pressure of light on the shape of saturated-absorption resonance in a gas

The effect of a force of resonance pressure of light on the amplitude and frequency properties of saturated-absorption resonance in a gas of two-level atoms was studied. On the basis of numerical solutions of the Liouville and Fokker-Planck equations, dynamics of changes in the velocity-distribution function of an ensemble of atomic particles, depending on the saturation parameter and time of their interaction with a strong field of light, was evaluated. The shape of a resonance line, related to saturated absorption, and its location relative to the frequency of the atomic transition were determined taking into account the dynamics of the distribution function. The results of the calculations are compared with the experimental data.