Determining the order parameters of organic nonlinear optical chromophores by Monte Carlo methods.

摘要

Monte Carlo simulations are a useful simulation technique for modeling the ordering of organic nonlinear (ONLO) materials. The collaborative effort of theoretical calculations, experimental synthesis, and device engineering techniques to produce functional electrooptic devices from ONLO materials is driven by the requirement for small, efficient structures with high processability, large bandwidth, and excellent mechanical stability to enhance and eventually replace the materials used to date in the production of electrooptic devices. The latest generation of ONLO materials can satisfy these and other requirements. An important consideration is that of translating the molecular electrooptic properties of a given ONLO chromophore to the response of a bulk material to incident light fields. The strong electrostatic interactions between chromophores in the bulk prevents a simple linear scaling of the acentric ordering with density; statistical mechanical calculations such as the Monte Carlo (MC) calculations presented herein provide the bridge to connect the molecular response to that of bulk films and materials processed in the laboratory. While a rigorous quantum mechanical description of the position of all electrons and nuclei in a bulk system would produce the most quantitatively accurate results this approach is at present computationally unfeasible. Approximations such as that of replacing discrete atomic charges with dipole moments and replacing nuclear/electronic repulsive effects with coarse-grained steric potentials allow computer modeling to predict the ordering of bulk systems while remaining practical and efficient from a computational sense. This dissertation presents results of lattice-based and off-lattice MC calculations of ONLO chromophores at levels of sophistication ranging from that of simple dipole moments on lattice sites to that of multi-ellipsoidal dendrimeric chromophores that are allowed to behave as liquid systems. A picture of how charge- and shape-based effects drive the ordering of a system of ONLO chromophores emerges from which new molecules and materials may be developed.