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Optics Letters

Angular dependence of phase conjugation in SF_6

Claudio Lenz Cesar, J. W. R. Tabosa, Paulo C. de Oliveira, M. Ducloy, J. R. Rios Leite

Published Dec 1, 1988• 13 citations
Publisher

Abstract

Optical phase conjugation by degenerate four-wave mixing in SF(6) was studied with a cw CO(2) laser. The intensity and angular dependence of reflectivity were measured for the 10.6-microm P(18) line. The results verify the predictions of the inhomogeneously broadened resonant two-level gas model.
DOI
10.1364/ol.13.001108

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We present in this paper a theoretical study of the angular dependence of phase-conjugate emission via resonant degenerate four-wave mixing (FWM) in Doppler-broadened gas media. The nonlinear optical polarization is calculated through an expansion of the atomic density matrix up to the third order in the incident fields. One finds that, when the incident frequency is resonant for a single-photon transition, both strength and lineshape of the phase-conjugate emission strongly depend on the angle 0 between standing pump wave and probe wave. The emission intensity, maximum at grazing incidences, decreases rapidly by several orders of magnitude when 0 increases. Simultaneously the emission linewidth increases from homogeneous to Doppler width. Similar effects are predicted for nearly-degenerate FWM (different pump and probe frequencies). In particular, one shows that the bandwidth of the equivalent optical reflection filter undergoes a Doppler-broadening linearly increasing with sin θ. On the opposite, none of the above effects is predicted in two-photon-resonant FWM which does not exhibit any angular dependence.
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.
CW phase conjugation on the ν3 vibrational bands of SF6 and OsO4 is demonstrated as a means of investigation in high resolution spectroscopy through resonant degenerate four-wave mixing at 10.6 µm by using a stable low power CO2 laser operating on the P(18), P(16), P(12) lines respectively. Peak reflectivities of 3.8 10-3 are measured in SF6. They only reach 0.34 10-3 in OsO4. Phase conjugate spectra with 2 MHz, namely sub-Doppler resolutions, are compared with simultaneous recordings of the saturated-absorption spectra over a 80 MHz bandwidth. The saturation splitting encountered on a one-photon transition in SF6 is also analysed in terms of pressure broadening and pump imbalance.
Degenerate-four-wave mixing (DFWM) is an important technique for the generation of phase-conjugate wave, to which much attention has been paid. In this paper. a tunalile CW CO2 1.aser is used as the pump sourse and gaseous SF6 as the nonlinear medium, the dependence of reflectivity of phase-conjugate wave generated by DFWM on the pump intensity, the pressure in SF6, the angle between the pump and probe beams as welt as the potarization of the optical fields etc, is investigated experimentatty and exptained theoretically. Peak reftectivity of 2.1x101 is abtained at P=4 Torr in the gaseous SF6. In the Doppler-free phase-conjugate spectrum of SF6 five absorption dips are observed and the spectral resolving power is measured to be about 3 MHz.
Phase-conjugate emission by resonant degenerate four-wave mixing with two counterpropagating weak pump beams and a noncollinear strong object beam was calculated for a Doppler-broadened two-level system. Singlepeak Doppler-free lines with small saturation broadening at a collinear configuration became split for a wide angle. The effect is demonstrated with the use of a CO(2) laser and SF(6). Conversely, calculated line shapes with Stark splitting in a collinear arrangement became single peaked for a wide angle.