Hong-Jun Zhang

Self-focusing and self-trapping in new types of Kerr media with large nonlinearities

Self-focusing and self-trapping of light in common liquid media such as Chinese tea, Chinese herbal medicine, and solutions of chlorophyll have been observed. These materials are found to be new types of natural Kerr media, and their nonlinear coefficients have been determined. Laser-heating-induced self-phase modulation in these materials has also been observed.

Phase control of switching from positive to negative index material in a four-level atomic system

Electric and magnetic responses of the medium to the probe field are analysed in a four-level loop atomic system by taking into account the relative phase of the applied fields. An interesting phenomenon is found: under suitable conditions, a change of the refractive index from positive to negative can occur by modulating the relative phase of the applied fields. Then the medium can be switched from a positive index material to a negative index material in our scheme. In addition, a negative index material can be realized in different frequency regions by adjusting the relative phase. It may give us a convenient way to obtain the desired material with positive or negative index.

Vectorial equations and polarization characteristics of pulsed lasers

Incorporating the vector feature of the laser field, the longitudinal spatial length, and the orientation distribution of the dipole moment in the gain medium, we develop a set of equations that describe the polarization dynamics of a class B laser with a modulated pump, a self-mode-locking laser, and an actively mode-locked laser with a modulated loss. Numerical results show that the first two types of laser operate in a linearly polarized state which changes direction while the third type of laser operates in an elliptical polarization state. In class B lasers with a modulated pump, because the coupling between the light and the medium depends on the dipole moment orientation of the population inversion, and the dominant gain appears alternately in different directions, the changing linear polarization state is induced by the cross-saturation effect of the two orthogonal linearly polarized laser fields. On the other hand, in the self-mode-locking laser the linear polarization state is the result of the coincidence of the dominant field polarization and gain preference for any of the two pulses within the cavity. For the actively mode-locked laser, the gain preference and the dominant polarization of the only pulse in the cavity are always perpendicular to each other, which makes the laser run in an elliptical polarization state.

Frequency shifts and dynamic instabilities in photorefractive self-pumped and mutually pumped phase conjugation

Frequency shifts and dynamic instabilities in photorefractive cat self-pumped phase conjugators and bridge mutually pumped phase conjugators are studied by use of a two-dimensional model. Intrinsic electric fields inside the crystals induce the frequency shifts and dynamic instabilities observed in these experiments. In cat mirrors, for small values of the electric field the phase-conjugate reflectivity and the frequency shift are constant. With a further increase in the electric field the reflectivity and the frequency shift become periodic through Hopf bifurcation. For a large value of the electric field both the reflectivity and the frequency shift fluctuate chaotically. In bridge mirrors, for small values of the electric field the phase-conjugate reflectivity is stable, and no frequency shift exists. With a further increase in the electric field the reflectivity and the frequency shift become periodically oscillating in time. For a large value of the electric field both the reflectivity and the frequency shift fluctuate chaotically. The phase-conjugate outputs of the two beams oscillate in an almost synchronous manner.

Controlling chaos in a hybrid optical bistable system

Using a one-dimension piece-wise function map, we have developed a method for controlling chaos in a hybrid optical bistable system. The method requires only a small time-dependent perturbation of a system parameter according to the error signal, and need not rely on an analytical model. We have successfully controlled the chaos in a hybrid optical bistable system with noise. The experimental results are in good agreement with the numerical simulation.

Vector phase conjugation by degenerate four-wave mixing in a nematic liquid crystal film

Abstract Using scalar phase conjugation, vector phase conjugation is demonstrated by conjugating each orthogonal polarization component of the probe wave via degenerate four-wave mixing in a nematic liquid crystal film. The polarization-preserving property, and the ability to correct polarization distortion, of the vector phase conjugator are verified. Measurements also show that the conjugator does not change the helicity of a circularly polarizef probe wave.

Global stability and oscillation properties of a two-level model for a class-B laser with feedback

The global stability and self-pulsing oscillation properties of a two-level model of a class-B laser with external feedback is analyzed. In the parameter plane of the feedback the boundary between the stable and the unstable region is obtained by linear stability analysis. With numerical simulations, the period and amplitude of the self-pulsing oscillation versus the control parameters are obtained. Chaos is also found inside the unstable region.

Spatial fidelity of image amplification in photorefractive crystals

We numerically studied the spatial fidelity of coherent image amplification by two-wave mixing configurations in photorefractive BaTiO(3):Ce using a three-dimensional analysis. The results are given for the case when the input one-dimensional rectangular amplitude of the image-bearing extraordinary beam is finite in the plane of incidence and when the amplitude is finite in the orthogonal plane. The fidelity and the gain versus the angle between the propagating direction of the image-bearing beam and the crystal c axis, the pump-to-image intensity ratio, and the input beamwidth, are analyzed.

Enhanced Kerr nonlinearity in a negative refractive atomic medium

The Kerr nonlinearity of a left-handed material is analyzed in a four-level atomic system. It is shown that, due to the effect of quantum interference, a large enhanced Kerr nonlinearity accompanied by vanishing absorption can be realized via choosing appropriate parameters in this negative refraction atomic medium. It not only shows the large nonlinearity but also acts as the phase and amplitude compensating effects.

Spatial fidelity of externally pumped phase conjugation in photorefractive crystals

The spatial fidelity and the reflectivity of the externally pumped phase conjugation by four-wave mixing in photorefractive crystals are numerically studied by a three-dimensional analysis. The results are given for both the case that the input one-dimensional rectangular amplitude of the image-bearing beam has a finite extent in the plane of incidence of the four extraordinarily polarized beams and the case that the amplitude has a finite extent in the orthogonal plane. The fidelity and the reflectivity versus the input pump ratio, the moving velocity of the crystal, and the externally applied electric field are analyzed. Although one can enhance the reflectivity by moving the crystal or by applying an electric field, the output phase deviates increasingly from the conjugate phase.