Enhancement of optical nonlinearity due to breathers in finite polyenes

We systematically investigate the effect of strongly nonlinear breather (multiphonon bound) states on static polarizability (α) and hyperpolarizability (γ) of finite polyenes ${(\mathrm{C}}_{24}{\mathrm{H}}_{26},{\mathrm{C}}_{40}{\mathrm{H}}_{42})$ within the time-dependent Hartree-Fock approximation. Electronic structure, molecular-dynamics, and geometry optimization calculations are performed using the PM3 parametrization of the modified neglect of differential overlap Hamiltonian as implemented in the MOPAC 93 program. All degrees of freedom are allowed to evolve, including those coupling to the C-C bond distances. Breathers are obtained as coherent nonlinear excitations from appropriate initial conditions, and a strong increase in α and a very significant enhancement in γ in the presence of breathers is found.