Three-dimensional Beam Focusing Induced By Cross-phase Modulation Effects In Self-defocusing Media

Self-focusing and self-defocusing have their origin in the intensity dependence of the refractive index of the nonlinear medium. The picture becomes more complicated when two copropagating optical beams affect each other through nonlinear cross-phase modulation. Recently focusing of optical beams and pulse beams by cross-phase modulation has been considered both theoretically and experimentally. During these processes, diffraction, self-phase modulation, cross-phase modulation and physical distance between the beam centers all have important effects on the beams and the pulse beams propagation. In this, we focus on the phenomena of the induced focusing when two beams propagation in self-defocusing nonlinear media in an off-axis geometry. The historical review is given about the research on the induced focusing of optical beams in self-defocusing nonlinear media. The contribution at the dissertation includes:1. We study the evolution of a probe beam profile in the influence of a pump beam in the off-axis geometry and obtain the typical process of probe focusing. We have shown for the first time, which the profile of the probe beam becomes a crescent moon shape when it is focusing.2. We discuss the effect on the focusing of the probe beams for the five initial parameters: the amplitude of the pump beam, the separation between the pump and the probe beam centers, the wavelength ratio of the pump and the probe beams and the ratio of the initial beam widths at the directions of X and Y. Our numerical results show that, more the amplitude of the pump beam, the wavelength ratio of the pump and the probe beams and the ratio of the initial beam widths at the direction of Y, more the probe focus; the separation between the pump and the probe beam centers and the ratio of the initial beam widths at the direction of X have optimum values, respectively. It is interesting that the ratio of the optimum values of the separation between the pump and the probe beam centers and the ratio of the initial beam widths at the direction of X is a constant, which is 0. 4~0. 6.