Effective Nonlinear Optical Properties Of Granular Composites

Effective nonlinear optical properties of granular composites with different microstructures are investigated in this paper. With the aid of the theoretical formulas and numerical simulations, we find that the effective linear and nonlinear responses are found to be dependent on the distribution exponent, the granular shape, the volume fraction and so on. The thesis is organized as follows: In the first chapter, we give a general introduction and summation of the current study background and states in the nonlinear optical properties of random composites. Our method, model and important results are presented. In the second chapter, we investigate the effective nonlinear response of random composites consisting of nonlinear conductors and linear conductors with an anomalous distribution. The distribution exponent has great impacts on the enhancement of nonlinear response. With Maxwell-Garnett approximation and Bruggeman approximation, we find that the effective nonlinear response can be achieved as the distribution exponent decreases. In the DC case, our theory is found to be in good agreement with numerical simulation, while in the AC case, the theoretical results describe nonlinear responses of our numerical simulations qualitatively. In the third chapter, we present a theoretical description and numerical simulation for the effective linear and nonlinear optical properties of compositionally graded films consisting of nonlinear metal particles and linear dielectric particles. As a result, the effective second-rank dielectric constant tensor and fourth-rank nonlinear optical susceptibility tensor are directly determined by regarding the graded film as a multilayer one. The results show that the compositionally graded film can be served as a suitable candidate material for obtaining large optical nonlinearity. In the fourth chapter, with the aid of the spectral representation and decoupling approximation, we investigate the effective optical susceptibilities for second harmonic generation (SHG) and third harmonic generation (THG) of nonlinear composites media, in which the nonlinear spheroidal particles are embedded in a linear host medium. We emphasize the effect of the granular shape on SHG and THG susceptibilities in two types of microstructures corresponding to those described by the generalized Maxwell-Garnett Theory (MGT) and Effective Medium Approximation (EMA). As the depolarization factor increases, the enhancement of SHG and THG is stronger. For third harmonic generation, we consider the contributions not only from the presence of the intrinsic third harmonic susceptibility of the spheroidal particle, but also from the inducement of the second harmonic susceptibility in the spheroidal particles. Numerical results show that the contribution of the second-order optical susceptibility of spheroidal particles pays more influence on the third harmonic generation than that of the intrinsic third-order optical susceptibility in the spheroidal particles. We also note that there are two enhancement peaks of SHG located at ωand 2ωfrequencies, and three enhancement peaks of induced THG corresponding to ω, 2ωand 3ωfrequencies.