Resonant third-order optical nonlinearity of molecular aggregates with low-dimensional excitons

The observation is reported of large resonant third-order optical nonlinearity from an organic system with low-dimensional excitons, the pseudoisocyanine J aggregates. The nonlinearity, expressed as α2/α0, is determined to be (2.6 ± 0.5) × 10−7 cm2/W, which compares favorably with those for CdSxSe1−x-doped glasses (1 × 10−7 cm2/W for Corning 3-69 glass) and polydiacetylene (7.4 × 10−8 cm2/W). The large nonlinearity originates from the efficient bleaching of the sharp exciton absorption (~180 cm−1 at room temperature). Also, induced absorption is observed at the high-energy side of the exciton peak, which contributes to the saturation of the nonlinearity at higher power. The mechanism of the exciton bleaching and the nature of the induced absorption are discussed. It is demonstrated that organic systems with low-dimensional excitons represent an interesting class of nonlinear optical material.