Spectral broadening of short radiation pulses in a thin resonant medium

А. Ч. Измайлов

The method of sub-Doppler spectroscopy is theoretically elaborated, which is based on the specific dynamics of a number of optically excited atomic particles (atoms or molecules) of a rarefied gas medium in a thin cell after the action of the resonance pulse of the monochromatic radiation. Corresponding calculations are carried out on the basis of density matrix equations for the resonance optical transition between Zeeman degenerate ground and excited quantum levels of particles in case of the linear polarization of the laser pulse at its normal incidence on the cell. The situation is considered when the radiative lifetime of the excited level is much more than the characteristic transit time of particles between nearest plane-parallel walls of the cell. Then the distribution of a number of excited particles versus the pulse frequency detuning narrows in the process of particles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in comparison with the Doppler broadening of the spectral line of the resonance transition) may be more than the ratio of the characteristic transverse size of the thin gas cell to its inner thickness. We discuss possible use of given sub-Doppler resonances (of the number of excited particles) in the high-resolution spectroscopy and also in high-selective processes of photo-ionization and photo-dissociation, especially, for isotope (or isomer) separation and detection of rare (in particular single) atoms or molecules of a gas medium.

R. F. Boikova, Evald E. Fradkin

H. Javid

The method of sub-Doppler spectroscopy is theoretically elaborated, which is based on the specific dynamics of a number of optically excited atomic particles (atoms or molecules) of a rarefied gas medium in a thin cell after the action of the resonance pulse of the monochromatic radiation. Corresponding calculations are carried out on the basis of density matrix equations for the resonance optical transition between Zeeman degen- erate ground and excited quantum levels of particles in case of the linear polarization of the laser pulse at its normal incidence on the cell. The situation is considered when the radiative lifetime of the excited level is much more than the characteristic transit time of particles between nearest plane-parallel walls of the cell. Then the distribution of a number of excited particles versus the pulse frequency detuning narrows in the process of par- ticles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in compar- ison with the Doppler broadening of the spectral line of the resonance transition) may be more than the ratio of the characteristic transverse size of the thin gas cell to its inner thickness. We discuss possible use of given sub- Doppler resonances (of the number of excited particles) in the high-resolution spectroscopy and also in high- selective processes of photo-ionization and photo-dissociation, especially, for isotope (or isomer) separation and detection of rare (in particular single) atoms or molecules of a gas medium.

Yu. A. Vdovin, V. M. Ermachenko

Svetlana A. Varentsova, Tatiana M. Lysak, Vyacheslav A. Trofimov

The response of a medium at the triple frequency under the action of few-period laser pulses is considered within the framework of the thin-optical-layer approximation for the case where the triple frequency is close to the natural frequency of the linear oscillator. It is shown that a bistable dependence of the polarization amplitudes on the external action amplitude simultaneously appears at both the natural frequency and the frequency of the external action. This allows one, in particular, to reveal the presence of bistability of the regime of third harmonic generation in a physical experiment by using the transmitted radiation of the acting pulse. A decrease in the duration of the pulse incident on the medium leads to an increase in the hysteresis loops. The effect of the absolute phase of a short pulse on the spectral composition of the response of the medium is studied. For pulses of medium duration, in addition to the resonant response of the medium, the presence of the dynamic response of the medium at the triple frequency was revealed, despite a detuning of the latter from the resonance. The presence of this frequency in the spectrum of the response of the medium results in the dependence of the resonant response of the medium on the absolute phase even for a sufficiently long pulse containing tens oscillations of the electric field strength under the pulse envelope. To obtain the dynamics of the spectral lines from the results of computer simulation, the Fourier-Gabor method is used, the applicability of which is demonstrated by the comparison of the results obtained on its basis with the corresponding analytical dependences.

R. I. Sokolovskiǐ