Nonlinear Optical Properties In Composite Particles With Dielectric Anisotropy

Effective nonlinear optical properties of granular composites with radial dielectric anisotropy are investigated in this paper. We find that the effective linear and nonlinear responses can be greatly enhanced by tuning dielectric anisotropy, the microstructure (interfacial parameter, 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. In addition, we introduce the dielectric anisotropic materials and present our method, model and important results.In the second chapter, we investigate the nonlinear dielectric response in partially resonant composites with radial dielectric anisotropy. We consider the composite media in which a dilute (or nondilute) suspension of coated cylinders randomly embedded in a linear host. The coated cylindrical inclusions consist of nonlinear core and linear anisotropic dielectric shell. Under partial resonant conditions ( sε^sr0 +ε_h⁰=0 or sε_sr⁰ +ε_c⁰=0), without changing the external potential, the coated cylinders can be replaced by the solid ones with larger nonlinear susceptibility and volume fractions than those of original cylindrical cores. As a result, the effective nonlinear dielectric response of the partially resonant systems may be greatly enhanced. Moreover, analytical results show that increasing dielectric anisotropy results in further enhancement ofχ_e. We conclude that the partially resonant composites with radially dielectric anisotropy can be served as a new kind of nonlinear dielectric materials. In the third chapter, we present our theoretical investigations on the optical nonlinearity enhancement in compositionally graded films of nonspherical nanoparticles. With the aid of the effective media approximation and spectral presentation, we derive the effective second-rank dielectric constant tensor and fourth-rank nonlinear optical susceptibility tensor by regarding the graded film as a multilayer one. For a power-law composition profile, we find that the suitable adjustment of the particle shape results in appreciable enhancement of effective nonlinearity. In consequence, the introduction of nonspherical inclusions in compositionally graded films is helpful to produce large optical nonlinearity and even optical figure of merit.In the fourth chapter, we investigate the second- and third- harmonic generations for a nondilute suspension of coated particles with radial dielectric anisotropy. We consider two types of coated particles. The first is that the core possesses a second order nonlinear susceptibility and the shell is linear and radially anisotropic, while the second is that the core is linear with radial anisotropy and the shell has a second order nonlinear susceptibility. We derive expressions for the effective nonlinear susceptibility tensors for both the second harmonic generation (SHG) and induced third harmonic generation (THG) of nonlinear composite materials, in which nondilute coated particles randomly embedded in the linear host. We find that both the core-shell structure and the dielectric anisotropy play important roles in determining the nonlinear enhancement and the surface resonant frequencies. Moreover, due to the coating material being metallic, there exists two fundamental resonant frequenciesω_c1 andω_c2, according to double enhancement peaks of SHG and THG.In the fifth chapter, the intrinsic optical bistability in nonlinear composites of coated spherical particles with a nonlinear metallic core and an anisotropic shell is investigated. By taking into account the radial dielectric anisotropy in the shell, we establish the relation between the local field in the nonlinear core and the external applied field. We find that the radial anisotropy can optimize the threshold intensity and enhance the bistable response. Moreover, there exists an anisotropy-dependent critical interfacial parameterλ_c, above which the bistable response vanishes.