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Physical Review Letters

Resonantly enhanced degenerate four-wave mixing of millimeter-wave radiation in gas

Nikolay A. Bogatov, Mikhail S. Gitlin, Aleksandr G. Litvak, Alexander G. Luchinin, Gregory S. Nusinovich

Published Dec 21, 1992• 8 citations
Publisher

Abstract

Millimeter-wave phase conjugation was realized under resonantly enhanced degenerate four-wave mixing in gaseous carbonyl sulfide. The nonlinear wave interaction was caused by saturation of a transition between rotational levels. For radiation with \ensuremath{\lambda}=1.64 mm, phase-conjugate reflectivity of about 0.4% has been observed at a gas temperature of 200 K, and under optimal pressure.

DOI
10.1103/physrevlett.69.3635

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Optics Letters•Sep 17, 1990•0 citations
Millimeter-wave phase conjugation was realized under resonantly enhanced degenerate four-wave mixing in gaseous carbonyl sulfide. The nonlinear wave interaction was caused by saturation of a transition between rotational levels. For radiation with \ensuremath{\lambda}=1.64 mm, phase-conjugate reflectivity of about 0.4% has been observed at a gas temperature of 200 K, and under optimal pressure.
Continuous wave (cw) phase conjugation of a HF laser beam has been observed using resonant degenerate four-wave mixing in a HF gas absorption cell. A HF laser was operated in the P1(8) transition to provide total (forward plus backward) pump beam and probe beam intensities of about 400 and 30 W/cm2, respectively, in the cell. Conjugate reflectivities were measured as a function of HF pressure and frequency detuning from line center. Reflectivities of the order of 10−4 were measured at HF gas pressures of the order of 2 Torr when the laser frequency was tuned near line center.
The use of resonantly enhanced four-wave mixing to generate high reflectivity phase-conjugate waves is discussed. A physical understanding of the mechanisms underlying the nonlinear response for a simple two-level system is presented followed by a discussion of the complicating effects of pump absorption, pump depletion, unbalanced pumps, and thermal motion. Modifications of the theory to describe four-level behavior are presented and compared with experiments. The application of nearly degenerate four-wave mixing to narrow-band filtering complicated by the A. C. Stark effect is dis-cussed along with examples of experimental observations.
A comprehensive theoretical analysis is developed for the vectorial phase conjugation using resonant four-wave mixing (FWM) in a highly degenerate rotational vibrational molecular system. The dynamic Stark shifts, saturation, and Doppler broadening are included for a realistic analysis. It is shown that the electromagnetically induced multilevel coherence controls the nonlinear wave mixing yielding interesting results for the phase conjugate (PC) reflectivity. It turns out that the efficiency of the PC reflectivity is decided by the relative phase of the Zeeman coherence and the population grating. When these two contributions are aligned in phase by a small detuning of the pump frequency, a large PC reflectivity ($~2020%) is obtained with moderate pump intensity (20~500mW/cm500 mW/cm500mW/cm{}^{2}$).
We report what is to our knowledge the first observation of phase-conjugate waves by degenerate four-wave mixing in Ba vapor near its 6s(2)(1)S(0) ? 6s6p(1)P(1) resonance transition. A maximum reflectivity of 36% at 10 mbars of Ar buffer gas pressure is observed together with a temporal pulse shortening of 50%. Conical emission in the two pump waves after their passage through the nonlinear medium is also observed. The dependence of the phase-conjugate signal on the pressure of the Ar buffer gas shows a small state-changing collisional decay rate.