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Soviet Journal of Quantum Electronics

Investigation of coherent emission in time-separated fields

L. S. Vasilenko, Н. Н. Рубцова

Published Nov 30, 1982• 0 citations
Publisher

Abstract

An investigation was made of signals of coherent radiation generated by time-separated fields in an SF6 gas at moments t =T+nT (n =1,2,3,4) under the action of two pulses of standing-wave CO2 laser radiation. The amplitudes of such signals were investigated as a function of the detuning of the laser frequency relative to the center of the transition line of SF6, as a function of the amplitude of the exciting radiation, and also as as a function of T. The rates of relaxation of the polarization, ?12= (46?5)?106 sec?1?Torr ?1, and of the populations, ? = (42?4)?106 sec?1?Torr ?1, were found at pressures of 1?5 (mTorr). A comparison of ?12 and ? gave information on the nature of collisions in the SF6 gas.

DOI
10.1070/qe1982v012n11abeh006143

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Sep 26, 1997•0 citations
An investigation was made of signals of coherent radiation generated by time-separated fields in an SF6 gas at moments t =T+nT (n =1,2,3,4) under the action of two pulses of standing-wave CO2 laser radiation. The amplitudes of such signals were investigated as a function of the detuning of the laser frequency relative to the center of the transition line of SF6, as a function of the amplitude of the exciting radiation, and also as as a function of T. The rates of relaxation of the polarization, ?12= (46?5)?106 sec?1?Torr ?1, and of the populations, ? = (42?4)?106 sec?1?Torr ?1, were found at pressures of 1?5 (mTorr). A comparison of ?12 and ? gave information on the nature of collisions in the SF6 gas.
Experimental observations are reported of coherent emission from an absorbing rarefied gas following the interaction with two standing-wave pulses separated by a time T. The emission occurred at a time 2T after the first pulse and was associated with the spatial transport of polarization and the formation of spatial polarization harmonics because of a phase jump following two-photon interaction with the field of the second pulse. The formation of spatial polarization harmonics was responsible for the appearance of Ramsey resonances in the optical region. Intensity oscillations were observed at the center of the absorption line on tuning the frequency of the exciting laser and varying the delay time between the pulses.
Coherent transient phenomena can be generated by the abrupt changes of parameters either of the electromagnetic field or of the matter. Comparative analysis of these two approaches, made on the base of experimental research in molecular gases SF6, 15NH3, HFCO and 13CH3F, excited by the radiation of CW CO2 and 13CO2 lasers, is presented. The first approach of photon echo (and its modifications) generation, based on electro-optic pulses formation from the resonant CW exciting radiation, has proven to be successful for intense infrared molecular transitions. The coherent responses are sufficiently powerful to be applied in high resolution spectroscopy, the spectral markers creation ,and molecular collisions investigations. However, this technique is not very effective for the free polarization decay investigations. Another approach based on the Stark switching molecular levels of polar gas, irradiated by CW laser, was demonstrated for free polarization decay and photon echo generation in 13CH3F. Free polarization decay has shown the pronounced frequency shift because of dynamic Stark effect in the CW radiation field. This specific feature of coherent transients, created by Stark switching, limits the high resolution spectroscopy by the low intensity range of exciting radiation. From other viewpoint, it gives unique possibility to investigate coherent transients generated at the dressed molecular levels.
Method of stimulated photon echo with specially chosen linear polarizations of exciting pulses of coherent resonant radiation is applied for investigation of collisional decay of orientation and alignment in molecular gas SF6 and its mixtures with buffers He and Xe. These relaxation rates are measured as a function of longitudinal velocities. Within experiment accuracy, no remarkable velocity dependence of depolarizing collisions rates was detected. This result confirms conventional approach of depolarizing collisions theoretical treatment.
A solution is obtained to the problem of a resonant interaction of a gas of atoms with the field of three strong standing-wave pulses separated in time. A calculation is reported of a correction to the power absorbed by the gas, which oscillates near the center of an atomic line as a function of the field frequency with a characteristic scale of the order of the reciprocal of the time delay between the pulses. It is shown that a signal then appears for all rational values of the ratio of the delays between the pulses and it is related to the transfer—along the time axis—of higher spatial harmonics of the polarization of a medium and of the level populations.
The method of a stimulated photon echo with specially selected linear polarizations of pumping pulses of a coherent resonant radiation is applied to a research of depolarizing collisions in molecular gas SF6 and its mixtures with buffer gases He and Xe. For the first time in one experiment the rates of a collisional decay of three lowest moments of polarization (population, orientation and alignment) are defined in an ensemble of gas particles. These rates of a relaxation are measured as functions on longitudinal velocities of resonant particles. In the frames of an experimental accuracy any noticeable dependence on velocities of particles was not detected both for orientation and alignment neither in pure resonant gas SF6 nor in mixtures. This result confirms the standard theoretical approach to depolarizing collisions. In pure SF6 decay rates of orientation and alignment have proved lesser than relaxation rate constant due to velocity changing (elastic) collisions. It means, that only the part of elastic collisions participates in destruction of multipole polarization moments of resonant levels. In comparison with our previous work the evidence is obtained that the relaxation of multipole moments seems to be j-dependent one in a medium of molecular gas SF6.