Enhancing second-order nonlinear optical properties by controlling the wave function in one-dimensional conjugated molecules

Abstract

We have developed a way to control the wave function to improve the second-order optical nonlinearity in organic materials. We simulated second-order hyperpolarizability \ensuremath{\beta} of four types of molecules with a polydiacetylene structure with donor and acceptor substitutions with the Austin Model 1 method of Dewar et al. [J. Am. Chem. Soc. 107, 3902 (1985)]. To improve \ensuremath{\beta} by wave-function control, the transition dipole moment ${r}_{\mathrm{gn}}$ and dipole-moment difference \ensuremath{\Delta}r between the ground and excited states must be optimized simultaneously by controlling the charge separation of the wave function by adjusting conjugated lengths and donor- and acceptor-substitution sites. We expect a maximum second-order hyperpolarizability per unit molecular length of about 3000\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}30}$ esu/(10 A\r{}) at a detuning energy of 0.2 eV, which corresponds to an electro-optic coefficient >100${r}_{33}$(${\mathrm{LiNbO}}_{3}$). The sharpening absorption band should induce an exceptionally large electro-optic coefficient >10 000${r}_{33}$(${\mathrm{LiNbO}}_{3}$).