Large third-order optical nonlinearities of centrosymmetric squaraines with heterocyclic donor groups measured by femtosecond degenerate four-wave mixing technique

The magnitude and dynamic response of the third-order optical nonlinearity of four squaraines were measured by femtosecond degenerate four-wave mixing (DFWM) technique at 800 nm. Ultrafast off-resonant optical responses have been observed and the magnitude of the second-order hyperpolarizabilities of the squaraines were measured as large as 10−31 esu. The large optical nonlinearities of squaraines can be attributed to their rigid and intramolecular charge transfer structure, and the instantaneous NLO responses of squaraines are mainly contributed from the electron delocalization.