Conjugated Organic Polymer Nonlinear Optical Polarizability Theory

The nonlinear optical (NLO) properties of conjugated organic materials attract much attention due to their potential exploitation in optoelectronic or photonic devices. There are elementary excitations such as soliton, polaron, and bipolaron, etc., which are closely related to their nonlinear optical susceptibility. In this paper, based on the tight-binding Su-Schrieffer-Heeger (SSH) model and the Extended Hubbard model, we investigate the electronic properties and nonlinear optical susceptibility of the elementary excitations under the level of unrestricted Hartree-Fock mean field approximation. Simultaneously, we employ local atomic charge derivatives with the aim to study the contributions of individual parts. Additionally, we investigate the effects of electron-electron interactions (U, V) and symmetry break term (te) on nonlinear optical susceptibility.The growth of polarizability and hyberpolarizability for regular polyenes shows saturation behavior at long length. We also find that the elementary excitations such as soliton, polaron, and bipolaron possess different effects on nonlinear optical susceptibility due to the different structures induced by elementary excitations. Additionally, electron-electron interactions, which change the lattice structures of polyenes, possess different effects on nonlinear optical susceptibility. The on-site coulomb repulsion (U) strengthens polarizability and hyberpolarizability of soliton but decreases that of polaron and its excited state. While the nearest-neighbor interaction (V) decreases polarizability and hyberpolarizability of soliton and polaron but strengthens that of excited polaron. Symmetry break term also possess different effects on nonlinear optical susceptibility.