Induced Diffraction And Dispersion In Phase Mismatched Second Harmonic Generation

Second-Harmonic Generation is a typical nonlinear optical process, which occurs as a result of the part of the atomic response that depends quadratically on the strength of the applied optical field. In order to get high power and short wavelength laser to meet application requirement, people mostly focused on the method and technique of highly efficient frequency doubling previously. Based on phase-matching parametric process, various techniques such as intra-cavity second-harmonic generation,extra-cavity resonance and quasi-phase matching. In recent years, with the progress in cascaded nonlinearities(under phase-mismatching condition, two cascaded quadratic nonlinearity processes can be treated as a third-order nonlinearity), research on second-harmonic process regained people's interest, many important nonlinear optical processes such as X-type electromagnetic wave,broad band conical emission and high dimensional optical soliton are discovered, and a new research field has been formed gradually. Cascaded nonlinearity also brings about many new technical applications, which promotes the development of quadratic nonlinear optics, for example, high-power femtosecond pulse compression and Kerr-lens mode locking technique under normal dispersion condition.Among these new nonlinear phenomena and technique researches, phase-mismatching is their common preconditions, and this enriched nonlinear second-harmonic process greatly, therefore it is quite necessary to study second-harmonic process under phase-mismatching condition systematically. From the view of beam transmission, this process deals with nonlinear transmission of light wave,diffraction,dispersion etc. In this paper, we mainly focus on the interaction between linear and nonlinear transmission of light wave, and try to figure out rules that govern diffraction and dispersion in the nonlinear process. Our research work mainly deals with:1. Theoretical research on induced dispersion in the phase-mismatching frequency doubling process.That femtosecond pulse will be broadened through dispersion in materials is the typical linear transmission characteristic, its basic physical process is group velocity dispersion (GVD), which resulted from the intrinsic dispersion of materials as well as the structure of material and boundary condition, for example, mode-dispersion in optical fiber, angle dispersion induced from grating diffraction. These dispersion mechanisms are all linear, and play an irreplaceable role in the development of ultra-fast laser system. Research work on nonlinear effect on dispersion only evolves the formation mechanism of GVD in periodically-poled lithium niobate (PPLN), which essentially roots in the special structure of materials. Through theoretical analysis and numerical computation, we studied the new mechanism of induced dispersion in material. When phase mismatching is relatively large, dispersion of fundamental wave will be transferred to the harmonic wave, hence the equivalent dispersion on the harmonic wave comes from material itself and GVD of fundamental wave. We also studied the conditions under which this new induced dispersion can form, as well as its physical mechanism.2. Experimental validation and Applications of induced dispersionIn order to ascertain the existence of this induced dispersion and its important effect, we studied the second harmonic of femtosecond laser in BBO crystal. From this experiment we confirmed the mechanism of the induced dispersion. As an important example for application, the contribution of the induced dispersion to the form of spatiotemporal soliton is discussed. We found that when certain requirements are met, we could make the equivalent dispersion of second harmonic wave be negative. The feasibility of this scheme has been confirmed by numerical computation. High dimensional soliton is produced by the use of this technique.3. Theoretical research on induced dispersion in the process of phase-mismatching second-harmonic generationThough diffraction and dispersion are qualitatively comparable, they are quantitatively different. The equivalent diffraction is much larger than that of fundamental wave, which lead to a new phenomenon, that is, the spot size of harmonic wave is larger than that of fundamental wave. The above phenomena and conclusions are all confirmed by numerical computation.4. Research on diffraction and dispersion correlated nonlinear optical phenomenaSpace and time instability is the basic characteristic of nonlinear optical system, and Colorful Conical Emission (CCE) is one of its main results. We studied both experimentally and theoretically conical emission, and found that although it seems quite similar to traditional Optical Parametric Generation (OPG), they are two different nonlinear processes after experimental diagnosis. From this, we studied the effect of diffraction and dispersion in detail, which would be of help in understanding the kinetic process of this new conical emission.