Nonlinear Optical Properties In Random Composite Of Core-shell Structured Particles

Effective nonlinear optical properties of core-shell structure composites with different microstructures are investigated in this thesis. With the aid of the theoretical formulas and numerical simulations, we find that the effective nonlinear optical response can be enhanced by tuning the material properties of coated particles, the microstructure (the shape of the particle or the core-shell structure) and the volume fraction of particles and so on. Our results give the theoretical instructions to prepare the composites with enhanced dielectric response. The thesis is organized as follows:In Chapter 1, we give a general introduction and summation of the background of the nonlinear optical properties of random composites that currently interested. In addition, our methods, models and important results are presented.In Chapter 2, we study the properties of a core-shell particle with a shell of spherically anisotropic material. Such a material has a matrix dielectric function that is diagonal in the spherical coordinates. The electric field in the core region is found to be enhanced when compared to an isotropic shell of comparable dielectric constant for large anisotropy. The enhanced field can drive an enhanced nonlinear response if the core region carries an intrinsic nonlinear susceptibility. For a metallic core, the surface plasmon resonance frequency is found to depend on the degree of anisotropy or the thickness of the shell.In Chapter 3, we study the enhancement of the second-harmonic generation (SHG) coefficient in a random composite consisting of ellipsoidal particles with a core-shell structure in a linear dielectric host. The material making up the ellipsoidal core is assumed to be dielectric, but with a nonlinear susceptibility for SHG. The coating material is assumed to be metallic with a linear susceptibility. The effective SHG coefficient is derived and its expression is related to various local field factors. The numerical calculations of the effective SHG response per unit volume of nonlinear material can be greatly enhanced at certain frequencies. For coated ellipsoidal particles, the core-shell structure and the particle shape allow for tuning of the resonance through the choice of material parameters and/or the ratio of the core to shell volume fraction and the depolarization factor of the particles.In Chapter 4, the effects of geometric structures of coated particles on the dielectric properties of composites are derived. For a dilute suspension of coated ellipsoids with dielectric core and metallic shell embedded in a linear dielectric host, we obtain the linear condition of partial resonance for coated ellipsoids which is relative to the particle shape. Under the partial resonance condition, the property of the inner core can be extended to the outer shell. When the inner core is a weakly nonlinear material, the nonlinearity of the composite can be greatly enhanced at the linear partial resonance. To achieve the condition of partial resonance, a metallic shell may be suitable to reach the case if the frequency is far away from the plasmon frequency. It indicates that the nonlinearity can also be enhanced in terms of the geometric structure of materials as well as the properties of themselves.In Chapter 5, we summarize the main work of this thesis, and suggest some further studies in future.