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Soviet Journal of Quantum Electronics

Double resonances of parametric susceptibilities in the case of vibrational-rotational transitions in molecules

A. V. Karov, Mikhail Yu. Ryabikin, Gennadii I Freidman

Published Oct 31, 1984• 0 citations
Publisher

Abstract

An analysis is made of the frequency addition processes in the case of a two-frequency double resonance involving vibrational-rotational transitions and their potential applications in nonlinear spectroscopy. A study is made of the influence of the saturation effect and of the frequency detuning from one- and two-photon resonances on the conversion processes and estimates are obtained for a specific molecular gas. It is shown that in the case of two-frequency interaction under double resonance conditions the efficiency of conversion of a probe wave into a wave at the signal frequency increases at least several hundred times, compared with the case of the one-frequency interaction.

DOI
10.1070/qe1984v014n10abeh006415

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An analysis is made of the frequency addition processes in the case of a two-frequency double resonance involving vibrational-rotational transitions and their potential applications in nonlinear spectroscopy. A study is made of the influence of the saturation effect and of the frequency detuning from one- and two-photon resonances on the conversion processes and estimates are obtained for a specific molecular gas. It is shown that in the case of two-frequency interaction under double resonance conditions the efficiency of conversion of a probe wave into a wave at the signal frequency increases at least several hundred times, compared with the case of the one-frequency interaction.
An expression is obtained for the nonlinear susceptibility governing the addition of frequencies under resonance conditions. Allowance is made for the Stark shift, changes in populations, and transient response of the medium. Analysis is made of the stimulated emission of the third harmonic as a result of two-photon resonance in parallel and focused beams. It is shown that the saturation effect limits seriously the conversion coefficient of a nonlinear medium of fixed length. Optimal intensities of the fundamental-frequency radiation are found. It is shown that when a beam is focused sharply in a nonlinear medium, the saturation effect and Stark shift make it possible to add frequencies in media with normal dispersion. The relationship between four-photon parametric effects and coherent resonant processes such as self-induced transparency is established for ultrashort pulses.
It is shown that the resonance nonlinear susceptibility of four-photon parametric processes in gaseous media can be increased by several orders of magnitude. The increase in the susceptibility is due to suppression of the Doppler broadening of two-photon transitions in strong pump fields when the frequencies of the fields interacting with these transitions can be very different. Under these conditions there is an additional opportunity for increasing the susceptibility by reducing the frequency detuning relative to intermediate levels participating in two-photon transitions.
A theoretical analysis is made of the influence of the pump intensity on the nonlinear susceptibility of the third harmonic excited under conditions of two-photon resonance between the pump wave and the Doppler-broadened transition. Pumping by both traveling and standing waves is considered. For traveling wave pumping, an expression for the nonlinear harmonic susceptibility in an arbitrary field is derived and analyzed. The optimal detunings of twice the pump frequency relative to the two-photon transition frequency are determined. For standing wave pumping an expression for the nonlinear harmonic susceptibility is obtained from perturbation theory up to and including the third order. It is shown that an important factor in the reduction of the harmonic susceptibility is the saturation of the transition due to Doppler-free two-photon absorption of opposite traveling waves.
A theoretical analysis is made of the process of parametric up-conversion of the frequency of an infrared signal involving two-photon resonance of a pump field whose spectrum is considerably wider than the widths of the atomic transition lines. It is shown that the maximum steady-state efficiency of conversion in a wideband field is only slightly different from the maximum efficiency of conversion in a regular and monochromatic field. A dynamic improvement in the conversion efficiency is possible under transient interaction conditions.
We report electromagnetically induced gain in a highly degenerate two-level rotational vibrational molecular system. Using two photon (Raman-type) interaction with right and left circularly polarized pump and probe waves, the Zeeman coherence is established within the manifold of degenerate sublevels belonging to a rotational vibrational eigenstate. We analytically and numerically calculate the third-order nonlinear optical susceptibility for a Doppler-broadened molecular transition for an arbitrary high rotational angular momentum $(J\ensuremath{\ge}20)$. It is shown that for a QQQ-type open transition, a weak probe will experience an electromagnetically induced gain in presence of a strong copropagating pump wave. The inversionless gain originates due to cancellation of absorption from the interference of the coupled $\ensuremath{\Lambda}$- and V-type excitation channels in an NNN-type configuration. A detailed analysis of the optical susceptibility as a function of Doppler detuning explains how the gain bands are generated in a narrow transparency window from the overlapping contributions of different velocity groups. It is shown that the orientation dependent coherent interaction in presence of a strong pump induces narrow resonances for the probe susceptibility. The locations, intensity, and sign (positive or negative susceptibility) of these resonances are decided by the frequency detuning of the Doppler group and the strength of the coupling field. The availability of high power tunable quantum cascade lasers covering a spectral region from about 4 to $12\text{ }\ensuremath{\mu}\text{m}$ opens up the possibility of investigating the molecular vibrational rotational transitions for a variety of coherent effects.