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Soviet Journal of Quantum Electronics

Coherent emission in time-separated fields

L. S. Vasilenko, N. M. Dyuba, Mikhail N Skvortsov

Published Aug 31, 1978• 2 citations
Publisher

Abstract

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.

DOI
10.1070/qe1978v008n08abeh010641

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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.
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.
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 amplitude and polarization are found for the first coherent radiation pulse generated in time-separated fields with the aid of resonant levels exhibiting a hyperfine structure. It is shown that such hyperfine splitting of the resonant levels does not alter qualitatively the conclusion that it should be possible to observe narrow structures in the intensity of such a pulse under conditions of scanning of the frequency of the exciting pulses. This feature is important because of possible technical applications of such narrow structures. The polarization properties of the first coherent radiation pulse generated in time-separated fields depend strongly on whether one or all hyperfine components of each of the resonant levels participate in the formation of a pulse.
A calculation is presented to describe the response of an atomic system subjected to two strong standing-wave field pulses separated in time. One finds a sequence of output pulses following input pulses which is reminiscent of classical photon echoes. A physical picture of the processes involved in echo formation is presented, and connection is made with the classical picture of photon echoes. The application of these techniques to collision studies is emphasized. It is shown that studies of echoes produced by standing-wave fields can prove advantageous for exploring the effects of small-angle scattering on both level populations and atomic coherences.
A calculation is presented to describe the response of an atomic system subjected to two strong standing wave field pulses separated in time. One finds a sequence of output pulses following the input ones, reminiscent of classical photon echoes. A physical picture of the processes involved in echo formation is presented and connection is made with the classical picture of photon echoes. The application of these techniques to collision studies is emphasized. It is shown that studies of echoes produced by standing wave fields can prove advantageous for exploring the effects of small angle scattering on both level populations and atomic coherences. (Author)
We have considered the induced processes that occur in a driven two-level atom, where a laser photon is absorbed and emitted by the ground and by the excited states of the atom, respectively. In the low-intensity limit of the laser field, the induced spectra arising when a laser photon is absorbed by the ground state of the atom consist of two peaks describing induced-absorption and stimulated-emission processes, respectively, where the former prevails over the latter. Asymmetry of the spectral lines occurs at off-resonance and its extent depends on the detuning of the laser field. The physical, process where a laser photon is emitted by the excited state is the reverse of that arising from the absorption of a laser photon by the ground state of the atom. The former differs from the latter in that the emission of a laser photon by the excited state occurs in the low-frequency regime and that the stimulated-emission process prevails over that of the induced absorption. In this case, amplification of ultrashort pulses is likely to occur without the need of population inversion between the optical transitions. The computed spectra are graphically presented and discussed.