Effect of aggregation on nonlinear optical properties of a naphthalocyanine

The magnitude and dynamic response of the third-order optical nonlinearity of silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) (SiNc) were measured by femtosecond degenerate four-wave mixing (DFWM) technique under resonant conditions. The temporal profiles of the DFWM signal were obtained with a time resolution of 0.3 ps (FWHM), and were found to consist of at least two components, i.e., the coherent instantaneous nonlinear response (electronic response) and the slow response due to the excited state population grating. The present SiNc in neat film shows small red-shift compared to monomer absorption because of the large distance between adjacent naphthalocyanine rings, which results in weak delocalization of Frenkel exciton in the aggregated state. However, time-resolved DFWM measurements indicate that the SiNc films show fast nonlinear optical response and large third-order nonlinear susceptibility. The effective χe(3) value of the neat SiNc film was evaluated to be as high as 4 × 10−7 esu, and the figure of merit of third-order nonlinearity F (F = χ(3)/α), was calculated to be about 4.1 × 10−13 esu cm.