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Quantum Electronics and Laser Science Conference

Fully resonant four-wave mixing in optically thick Doppler broadened medium

A. K. Popov, S. A. Myslivets, E. Tiemann, B. Wellegehausen, Gennady Tartakovsky

Published Jan 20, 2003• 1 citation
Publisher

Abstract

Summary form only given. Coherent coupling processes in multilevel schemes have recently gained tremendous attention in connection with attempts to manipulate nonlinear-optical response, level populations, and refraction indices of the resonant medium. While single and multiphoton resonances are approached, giant enhancement of nonlinear-optical cross-sections occurs. At the same time, many other accompanying effects raise and interplay.

DOI
10.1109/qels.1999.807531

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Summary form only given. Coherent coupling processes in multilevel schemes have recently gained tremendous attention in connection with attempts to manipulate nonlinear-optical response, level populations, and refraction indices of the resonant medium. While single and multiphoton resonances are approached, giant enhancement of nonlinear-optical cross-sections occurs. At the same time, many other accompanying effects raise and interplay.
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An influence of nonlinear interference processes at quantum transitions in strong resonance electromagnetic fields on absorption, amplification and refractive indices as well as on four-wave mixing processes is investigated. Doppler broadening of the coupled transitions, incoherent excitation, relaxation processes, as well as power saturation processes associated with the coupled levels are taken into account. Both closed (ground state is involved) and open (only excited states are involved) energy level configurations are considered. Common expressions are obtained which allow to analyze the optical characteristics (including gain without inversion and enhanced refractive index at vanishing absorption) for various V, (Lambda) and H configurations of interfering transitions by a simple substitution of parameters. Similar expressions for resonant four-wave mixing (FWM) in Raman configurations are derived too. Crucial role of Doppler broadening is shown. Theory is applied for numerical analysis of some recent and potential experiments.
Nonlinear effects in pulse propagation through a medium consisting of four-level double-$\ensuremath{\Lambda}$-type systems are studied theoretically. We apply three continuous-wave driving fields and a pulsed probe field such that they form a closed interaction loop. Due to the closed loop and the finite frequency width of the probe pulses, the multiphoton resonance condition cannot be fulfilled, such that a time-dependent analysis is required. By identifying the different underlying physical processes we determine the parts of the solution relevant to calculate the linear and nonlinear response of the system. We find that the system can exhibit a strong intensity-dependent refractive index with small absorption over a range of several natural linewidths. For a realistic example we include Doppler and pressure broadening and calculate the nonlinear self-phase modulation in a gas cell with sodium vapor and argon buffer gas. We find that a self-phase modulation of $\ensuremath{\pi}$ is achieved after the propagation of a few centimeters through the medium while the absorption and pulse shape distortion in the corresponding spectral range is small.