Electroabsorption investigation of optical nonlinearities in polymeric materials

Organic Nonlinear Optical (NLO) materials have shown significant enhancement of nonlinearities over conventional inorganic materials. In order to understand the behavior of these materials for their practical applications, it is necessary to investigate the NLO chromophore orientation, the relationships between different order of NLO susceptibilities, and to investigate their high-order NLO properties. In this dissertation research, Electroabsorption Spectroscopy was used to investigate the nonlinear optical properties of these materials.; Electroabsorption Spectroscopy is an experimental method for measuring the change of light intensity due to the applied electric field after traveling through the material. Conventionally, normal incidence of Electroabsorption is used to investigate the second and the third order nonlinear optical properties, which is not enough to perform a thorough investigation of the desired nonlinear optical properties. In this dissertation research, new experimental techniques were developed, which leads to significant advancements in Electroabsorption Spectroscopy. Further understandings of the mechanisms of NLO chromophores based on these recently observed phenomena were also established.; One of these contributions is the experimental confirmation on a relationship between the adjacent order of NLO susceptibilities, which states that a particular order of hyperpolarizability of a chromophore corresponds to the derivative of its next lower order of hyperpolarizability with respect to the internal field of the material. This is an important relationship in nonlinear optical materials, and it is the first time this relationship is observed in experiment.; Through this dissertation research, the method of oblique incidence was developed in Electroabsorption Spectroscopy with sandwiched geometry, which leads to experimental determination of the normal component of the third-order nonlinear optical susceptibility $c33\; 333$. The ratio of the two tensor components $c33\; 333$ to $c31\; 133$ ($c31\; 133$ is obtained by normal incidence) was further used to predict the nonlinear optical chromophore orientation, which is a key feature for application of organic nonlinear optical materials. Furthermore, the in-plane tensor components $c31\; 111$ and $c32\; 211$ were measured independently from the experiment with interdigital electrodes, and the ratio of these two tensor components $c31\; 111$ to $c32\; 211$ was used similarly to predict the in-plane orientation of NLO chromophores.; The most exciting part of this dissertation is the investigation of the fifth-order nonlinear optical properties. This is the first time a dispersion of the fifth-order nonlinear optical susceptibility is obtained experimentally by means of electro-optics. A theoretical framework is also established for nonlinear optical spectra. The model is in excellent agreement with the experimental data. This is a significant breakthrough in nonlinear optical materials and will help us to understand the mechanism of nonlinear optical polymers.