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Acta Optica Sinica

Vector-model theory of coherence effects in saturated spectrocopy

Q Wang

Published Jan 1, 1986• 0 citations
OpenAlex

Abstract

In this paper, the optical Bloch equation with relaxation term in saturated spectroscopy is solved directly with the help of the perturbation theory and Laplace inversion, giving approximate results of any order when coherence effects in the strong field are taken into account. Peculiarities of the line shape of weak-wave absorption can be explained by the vector model.

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Saturated absorption spectroscopy linewidths down to half the natural linewidth have been obtained for the sodium ${\mathit{D}}_{1}$ transition in a strongly absorbing gas cell. The strong-line-narrowing effect is achieved by using an experimental ``tandem'' approach with two absorbing cells in series, where the saturated absorption output from the first cell is used as an input beam for a saturated absorption measurement in the second cell. A physical interpretation of the line-narrowing mechanism in highly absorbing media is presented, which, among other things, explains why the narrowing effects are different for crossover transitions. It is further argued that in these experiments the line-narrowing effect is caused by optical pumping and can be appropriately described by rate equations. This is different from several other recent papers on high-contrast transmission spectroscopy, where full Bloch vector models were necessary to fully describe the line shapes. Here a rate equation model, which includes two spatial dimensions, is developed, and reasonable agreement is obtained between experimental and theoretical data. \textcopyright{} 1996 The American Physical Society.

Physical Review A•Mar 1, 1996•6 citations
In this paper, the optical Bloch equation with relaxation term in saturated spectroscopy is solved directly with the help of the perturbation theory and Laplace inversion, giving approximate results of any order when coherence effects in the strong field are taken into account. Peculiarities of the line shape of weak-wave absorption can be explained by the vector model.
The purpose of this work was to investigate coherence effects in the interaction of two counterpropagating electromagnetic waves with the atoms of a gas, in the case in which one of the waves is weak and frequency modulated, while the other is sufficiently intense to cause saturation. Solving the equations for the density matrix in first-order perturbation theory, the authors obtained in analytic form the real part of the susceptibility integrated over the atom's velocity, expressed in terms of the rate equation model (REM) predictions plus the coherence corrections. The relaxation constants of the levels involved in the transition were kept different in order to allow a wider range of applications of the results. Calculated line shapes related either to the frequency-modulated (FM) sideband or to the ordinary dispersion techniques are presented. The REM approximation and the coherence corrections are shown in detail for an optical line which is of interest for frequency standard investigations.
A general theory of the saturated-absorption phenomenon in a two-level atomic system is developed, in which the phase relationships between the oscillating atomic dipoles as well as the population differences between energy levels are taken into account. The absorbing gaseous medium considered in this theory is subjected to irradiation by a quasi-running-wave composed of a strong pump field and a weak probe field propagating in opposite directions. Some new interesting results, not predicted by the so-called hole-burning or rate-equation model, are obtained. In particular, the probe-field-transmission peak line shape is found to be markedly different from what is expected according to the rate equations. For finite Doppler widths and a strong saturating pump field, absorption of the probe field can even change sign, and amplification of this field can actually occur. All these features are explained by a close inspection of the evolution of each atomic ensemble of given velocity. Detailed comparison with similar phenomena already observed in rf experiments is presented and permits us to clarify the new predicted effects.
Results of theoretical researches of the saturated absorption resonance shape in a method of the probing field on V-type of transitions are represented. It is shown that in case of opposite circulary polarized optical fields the resonance is shown in the form of cross, and its form strongly depends on relaxation constants of levels and it can be represented as in the form of a dip, and absorption peak. Thus the peak form has exclusively coherent character. Atomic transitions are offered, on which observation of the given effect is possible.
Saturated absorption spectroscopy linewidths down to half the natural linewidth have been obtained for the sodium D1{\mathit{D}}_{1}D1​ transition in a strongly absorbing gas cell. The strong-line-narrowing effect is achieved by using an experimental ``tandem'' approach with two absorbing cells in series, where the saturated absorption output from the first cell is used as an input beam for a saturated absorption measurement in the second cell. A physical interpretation of the line-narrowing mechanism in highly absorbing media is presented, which, among other things, explains why the narrowing effects are different for crossover transitions. It is further argued that in these experiments the line-narrowing effect is caused by optical pumping and can be appropriately described by rate equations. This is different from several other recent papers on high-contrast transmission spectroscopy, where full Bloch vector models were necessary to fully describe the line shapes. Here a rate equation model, which includes two spatial dimensions, is developed, and reasonable agreement is obtained between experimental and theoretical data. \textcopyright{} 1996 The American Physical Society.