Enhanced optical nonlinearity by photoinduced molecular orientation in absorbing liquids

We present a theoretical and experimental study of the molecular orientation induced in a light-absorbing liquid of rodlike molecules by a laser pulse that explains the large enhancement of anisotropic optical Kerr nonlinearity observed in liquids in which a small amount of dye has been dissolved. Our model is bas;cd on the assumption that a molecule changes significantly its rotational kinetic and equilibrium behavior when it is electronically excited by the absorption of a photon. The anisotropy of the absorption probability then results in a corresponding orientational anisotropy of the molecules involved in electronic transitions, which in turn orient the whole liquid via intermolecular interactions. To test our model, we performed nanosecond pump-and-probe measurements of nonlinear birefringence at varying pump pulse energy and temperature, In our experiments the host was a liquid-crystal mixture in its isotropic phase and the dye was an anthraquinone derivative. Both the decrease in the nonlinearity enhancement observed for increasing pulse energies and the pretransitional behavior occurring for temperatures approaching the isotropic-nematic transition point are in good agreement with the predictions of our model. The contribution of secondary effects related to light absorption and light-induced heating has been also taken into account in order to properly compare the nonlinearity of an absorbing liquid with that of a transparent one. [S1050-2947(98)03612-9].