Second-order nonlinear optical properties and relaxation dynamics of aligned cross-linked polyurethanes with hemicyanine-type chromophores

Three types of different cross-linked polyurethane were prepared by the reaction of poly[(phenylisocyante)-co-formaldehyde] with three different hydroxy-functionalized nonlinear optical chromophores. For poled and cured polymers the χ⁽²⁾ values were between 16.8 and 37.4 pm/V, measured at a wavelength of 1.064 μm. Thermal stability studies performed on a sample having bonding sites of vertical-parllel structure between the chromophore and the liquid polymer matrix indicated, by second-harmonic-generation activity, a minimum decay of the χ⁽²⁾ value owing to lattice hardening of the polyurethane matrix with three bonding sites. The relaxation times of aligned dipoles in vertical-parallel polyurethane were measured by variation of poling and cross-linking conditions. Three decay modes were found to exist: the mode that corresponds to partially bonded or free unbonded chromophores, the mode of thermal relaxation of bonded chromophores, and the mode that is due to the relaxation of an elastically stressed main chain induced by the poling elastic field.