Effect of particular nonlinear dispersion in photorefractive four-wave mixing on slow and fast light

The phase shift occurred by optical waves in nonlinear interaction may be considered as phase shift resulted from a nonlinear refractive index. Consequently, a spectrum of the nonlinear phase shift in non-degenerate in frequency interactions represents nonlinear dispersion. Thus the nonlinear interaction with such dispersion may be used to achieve slow and fast light. The phase-conjugate reflectivity in photorefractive four-wave mixing may reveal spectrum with two maxima, which are located symmetrically with respect to zero frequency detuning. A complicated nonlinear dispersion corresponds to such reflectivity spectrum. Qualitative analysis of the nonlinear dispersion suggests unusual behavior when the nonlinear effect, i.e., delay of the light pulse, may decrease with increase of nonlinear coupling strength. The numerical calculations confirm such a nontrivial behavior. The experimental conditions are found, for which the delay of the phase-conjugate pulse decreases when the coupling strength increases. The conclusions of the theoretical analysis are confirmed experimentally for photorefractive four-wave mixing in barium titanate.