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Quantum Electronics

Changes in the refractive index of a molecular gas in the field of resonant radiation when spectral lines overlap

Alexander M. Starik, O. V. Taranov

Published Jun 30, 1997• 4 citations
Publisher

Abstract

An analysis is made of the characteristic features of changes in the refractive index of a molecular gas in the field of resonant radiation when spectral lines overlap. It is shown that, in contrast to absorption in an isolated line, an increase in the gas pressure can alter not only the magnitude but also the sign of the changes in the refractive index.

DOI
10.1070/qe1997v027n06abeh000969

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An analysis is made of the characteristic features of changes in the refractive index of a molecular gas in the field of resonant radiation when spectral lines overlap. It is shown that, in contrast to absorption in an isolated line, an increase in the gas pressure can alter not only the magnitude but also the sign of the changes in the refractive index.
An approximate analog of the Kramers-Kronig relationships for an isolated spectral line is proposed. Expressions are obtained for the real part of the dielectric susceptibility of a substance near a spectral line for collisional, Doppler, and time-of-flight spreading of the spectral line. It is shown that, generally, the imaginary part of the complex form-factor of the spectral line attenuates very slowly, namely, according to the hyperbolic law, as the deviation from the center frequency grows. Therefore, even in the case when practically there is no radiation absorption beyond the spectral line, the influence of this spectral line on the refractive index of the substance manifests itself rather far from it.
The features of resonance fluorescence and absorption of resonance laser radiation in atomic vapors in a superthin cell with thickness L of the order of a wavelength lambda are investigated theoretically. Taking into account the Fabry-Perot interference effect, it is found that the dependence of the fluorescence and absorption spectra on the ratio L/lambda is substantially different. Coherent spectral narrowing - the Dicke effect - when L=(2n+1)lambda/2 and broadening of the spectral line when L = n lambda (n is a whole number) are observed in the absorption spectra. This effect is absent in the spector of resonance fluorescence in extended media. It is also shown that the processes of saturation and optical pumping significantly alter the spectral line shape as the laser intensity increases; in particular, additional sub-Doppler dips appear at the frequency of the atomic transitions. Calculated curves are given for the D2 lines of ^85Rb atoms. © 2004 Optical Society of America
A slightly refined perturbation theory is used to obtain a unitary transformation which approximately diagonalizes the total Hamiltonian for a system made up of a radiation field in interaction with gas molecules. The dependence of the energy of the total system on the number of photons present gives the usual formula for the refractive index of a gas. The transformation is also applied to the field operators and it is shown that, within the approximations made, the phenomenological Maxwell equations are obeyed in the new representation as equations between certain matrix elements of operators. The transformed representation is also used to derive the Frank-Tamm formula for \ifmmode \check{C}\else \v{C}\fi{}erenkov radiation.
Specific features of the propagation of light pulses with frequency lying in the range of autoionization resonances of the medium are analyzed. It is shown that the interference nature of the autoionization spectra makes it possible to combine a strong frequency dispersion of the refractive index with small absorption when the radiation frequency approaches the spectral range of the "transparency window" of an isolated resonance or of a series of overlapped autoionization resonances. This allows one to control the time delay of the pulses and their group velocity. Depending on the parameters of the medium, the pulse group velocity may be maximally reduced by a factor of 10 4 –10 5 .
An investigation is made of the change in the magnetic properties of a diamagnetic molecule (atom) induced by a resonance optical radiation field. Calculations are made for the 0→1 molecular transition in the presence of an external magnetostatic field. Expressions are derived for the magnetization and the magnetic susceptibility of the molecule, which depend nonlinearly on the light wave intensity, in the case of longitudinal and transverse directions of the external magnetic field with respect to the direction of propagation of the radiation. It is shown that the field-induced nonlinear magnetic susceptibility of a diamagnetic substance can change sign and increase considerably (by several orders of magnitude compared with the linear diamagnetic susceptibility). The field-induced nonlinear magnetization of the substance does not depend on its magnetic properties and is the same for diamagnetic and paramagnetic substances.