Ultrafast response of resonant third-order optical nonlinearity of organic J-aggregates

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Although one-photon resonance enhances the third-order optical nonlinearity, their response could be very slow due to production of excited states. In addition, optical materials could be easily damaged.In the present study, thin films containing organic dyes, e.g., cyanine dyes, squarilium dyes, croconium dyes, naphthalocyanine dyes, and their J-like aggregates, showed high third-order optical nonlinearity and very fast response at the same time. The optical nonlinearity was measured using femtosecond degenerate four-wave mixing technique.First, we measured resonant molecular hyperpolarizabilities of dye monomers, and obtained values of 10^<-30> - 2 × 10^<-28> esu. These values contain only spontaneous electronic response and do not include slower responses such as population gratings. Then, we measured electronic values of third-order optical nonlinear susceptibility of polymer films, sol-gel silica films, and neat films including these dyes and aggregates.The thin films which gave J-like aggregates, electronic χ^<(3)>, and laser wavelength were as follows : Cyanine dye NK-2612 / PVA film (20 wt%) 1.1 × 10^<-7> esu (850 nm), cyanine dye DTDC / PMMA film (20wt%) 2.1 × 10^<-8> esu (850 nm), cyanine dye NK-3261 / neat film 1.1 × 10^<-7> esu (850 nm), cyanine dye NK-1969 / sol-gel silica film (30 wt%) 2 × 10^<-8> esu (850 nm), Silicon 2, 3-naphthalocyanine bis(trihexylsilyloxide) neat films 4 × 10^<-7> esu (800 nm). All J-aggregates we studied show very fast optical nonlinear response under resonant conditions.