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Soviet Journal of Quantum Electronics

Coherent emission generated in time-separated fields with the aid of hyperfine structure levels

Alexander V. Evseev, I. V. Evseev, Решетов Владимир Александрович

Published Mar 31, 1985• 0 citations
Publisher

Abstract

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.

DOI
10.1070/qe1985v015n03abeh006287

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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.
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.
Abilities of the method of accumulation of coherent unstable response of a medium for amplification of the rotated component of the probe wave’s polarization in the atomic medium of three-level atoms are studied. Probe wave is examined in frames of the perturbation theory and its frequency is considered near to one photon resonance from the ground to the first excited level. The intense monochromatic pump field connects two excited energy levels and propagates through the medium without a change. The broadening of spectral lines caused by the relaxation processes and the Doppler broadening are included into the calculations. It is shown that by multiple consistent instantaneous change of input probe field’s phase from 0 to π and back, it is possible to obtain, in the tail part of the rotated component of the probe field, a separate narrowed light pulse along with the increase in intensity by more than an order of magnitude with respect to the input field.
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.
We have performed an analysis of the effect of hyperfine structure in an experiment in which an aligned excited state is probed using linearly polarized light. The state is born with a given electronic alignment resulting from a collisional process. It is probed after a time which is short in comparison to its radiative lifetime but much longer than ω−1hfs, where ℏωhfs is the hyperfine splitting. This means that hyperfine structure has ample time to ‘‘develop.’’ It is therefore essential to take into account both the hyperfine structure of the level and the manner in which the alignment is affected by recoupling to the new basis. Ratios of fluorescent intensities for different laser polarization configurations contain information on the alignment of the system. We show how these may be calculated for a system with several isotopes, some of which have hyperfine structure, and where many components are probed with appreciable strength. A comparison is made with our experimental results for the Hg(6 3P1) level probed on the 6 3P1–7 3S1 transition. The presence of hyperfine structure has a considerable influence on the measurements. It is important to understand such effects if one is to obtain information on the nascent electronic alignment by observation of the fluorescent intensities.
An analytic solution is obtained for the self-consistent equations describing propagation of an ultrashort pulse in a resonant two-level medium. The first approximation in respect of the dispersion is used to allow for phase modulation and for the dependence of the group velocity on the intensity and on the running value of the frequency. The conditions for the formation of a shock wave are established. It is shown that asymmetric broadening of the spectrum of a pulse results in coherent absorption, as a result of which the propagation of this pulse in such a medium leaves behind a polarization at the atomic transition frequency responsible for the radiation represented by the pulse.
In this paper the semi-classic theory of coherent excitation of a two-level atom was modified and applied to a two-level atom with a hyperfine structure. The im portant result of calculation is as follows: when the exciting laser power is la rge enough, due to the power broadening effect, a narrow-band laser can excite a two-level atom with a hyperfine structure effectively. So it is not necessary t o use a broad-band laser to guarantee excitation efficiency for an atom with hyp erfine split levels.