Phase conjugation has been obtained with pump powers as low as 0.5 mW on the 546.1-nm line of atomic mercury. Collisional processes that oppose the effects of optical pumping sustain the signal. Line splittings observed in the Doppler regime are compared with theory. Reflectivities agree with those obtained from a model that includes pump absorption.
Phase conjugation has been obtained with pump powers as low as 0.5 mW on the 546.1-nm line of atomic mercury. Collisional processes that oppose the effects of optical pumping sustain the signal. Line splittings observed in the Doppler regime are compared with theory. Reflectivities agree with those obtained from a model that includes pump absorption.
The phase-conjugate signal observed in resonant degenerate four-wave mixing on the 6/sup 3/P/sub 2/ to 7/sup 3/S/sub 1/ transition of atomic mercury in a Hg-Ar discharge is investigated. At a fixed argon pressure the variation of the signal with pump powers is explained by a model that includes the effects of velocity-changing collisions (VCCs). As the argon pressure was varied from 0 to 1 torr an increase in the phase conjugate signal was observed and is ascribed to a change in the discharge dynamics with argon pressure and to the influence of VCCs. To further clarify the role of collisions and optical pumping, degenerate four-wave mixing spectra are examined as a function of pump power. Line shapes are briefly discussed.>
The 73S1 → 63P2 (λ = 546.1-nm) transition of atomic mercury has received considerable spectroscopic attention. In this paper we examine phase conjugation reflectivities and line shapes for various pump powers on this line. The geometry is that of backward D4WM: counterpropagating pump beams and a weak probe intersect at an angle of 0.750° in a quartz mercury discharge cell. The inductively coupled rf discharge provides a density difference of 109–1011 atoms/cm3 between the levels of interest, which yields a small signal absorption α in the 0.3–3-cm−1 range. The best fit to the measured α(l) on line center is obtained using α(l) = α0/[1 + (l/l s )] with l s = 30 mW/cm2. This form of α(l) is different from what one would expect for a low pressure gas with Doppler broadening. Phase conjugation has been observed with pump powers as low as 3 mW. Spectra obtained by observing the conjugate beam as the laser frequency is scanned through resonance exhibit line broadening of 64–490 MHz depending on pump power. Additionally, line splitting is observed with increasing pump power. These splittings were 72 MHz at 134 mW of pump power. With an estimated Rabi frequency (Ω) of ~60 MHz, the predicted splitting is 3/2 Ω = 90 MHz.
We analyze the properties of phase-conjugate (PC) emission via resonant, Doppler-broadened, backward four-wave mixing in the saturation regime, when both counterpropagating pump waves are arbitrarily intense. One considers two atomic situations, either a four-level system modeled as a pair of uncoupled transitions, or a three-level system. The first case leads to an analytical expression of the PC field, for arbitrary pump intensities, in the Doppler limit. The second case, in general, necessitates a numerical solution, except in the asymptotic regime when one pump is very intense compared to the other one. One finds that both cases lead to quite similar predictions: (i) two-peaked or four-peaked emission line shape, depending on the forward to backward pump intensity ratio; (ii) the maximum of PC reflectivity is obtained for infinite backward saturation, but for a fi-p nite value of the forward pump intensity; (iii) for backward saturation, the emission line shape is practically independent of the forward pump intensity. Finally, the theoretical analysis is extended to the case of a saturating probe and its effect on emission line shape and PC reflectivity, as well as its application to the properties of emission processes involving higher-order optical susceptibilities (degenerate multiwave mixing).
In this paper, we present a detailed analysis of saturation effects in resonant degenerate four-wave mixing (phase conjugation) or multiwave mixing (high-order diffraction on an optically induced grating). The experiments are performed in a low-pressure Ne discharge in the Doppler-broadened regime, both on an "ideal" two- or three-level system (1s_{4}(J = 1) - 2p_{3}(J = 0)) or on a degenerate multi-level system (1s_{5}(J = 2) - 2p_{9}(J = 3)). The saturation effects appearing when both pump beams are intense are studied, and a very good agreement is found with the previously developed theory [8]. It is demonstrated that there is an absolute maximum to the phase-conjugate reflectivity, and various effects on the lineshape (splitting, linewidth, etc.) are investigated. The effects produced by the Gaussian nature of the incident beams are analyzed in order to interpret some of the discrepancies between theory and experiment. Several new theoretical results for high-order diffraction are also presented, and compared with the experiments. The possibility of dealing with the saturation effects in four-wave mixing when all three incident fields are saturating appears as a consequence of these studies on multiwave mixing. Notably discussed are the case of an intense diffracting field, and of intense grating fields, whose intensity may be unbalanced. Finally, the polarization selection rules for high-order diffraction experiments are established.
Nondegenerate four-wave mixing on the j=0\ensuremath{\rightarrow}j=1 transition, in a ``V '' system, using orthogonal, circularly polarized pump beams is used to study theoretically the generation of the phase-conjugate signals. The efficiency of generation of the conjugate wave is studied as a function of the various system parameters such as pump-probe detuning, Zeeman splitting, pump-atom detuning, collisional parameters, and the intensities of the pump beams. The origin of the various resonances in the reflectivity is explained. For weak pump fields, some resonances are shown to have subnatural linewidths. In the degenerate case, the reflectivity as a function of pump-atom detuning has, in contrast to a two-level system, a doublet structure for moderate absorption-length parameters and field strengths. The reflectivity is shown to have additional resonances due to pump imbalance. The phase-conjugate signal is also shown to exhibit the pressure-induced extra resonances.
We have derived an analytic expression for the intensity distribution of two counterpropagating waves of arbitrary input intensity in a saturable absorber. We use this distribution to extend the theory of degenerate four-wave mixing to include the effects of pump wave absorption. In particular, the spatial variation of the nonlinear absorption and coupling constants that appear in the coupled-amplitude equations for the probe and the signal (i.e., conjugate) waves are determined. These coupled-amplitude equations are solved numerically in a noniterative manner, leading to a prediction for the phase-conjugate reflectivity. The nature of the predictions differs qualitatively from those1,2 derived from theories that ignore pumpwave absorption, especially for the case of off-line-center excitation. At large values of the input pump intensities, the phase conjugate reflectivity can be larger than that predicted by theories that ignore pump-wave absorption.