Anomalous excitation density dependence of nonlinear optical responses in pseudoisocyanine J aggregates

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Abstract
Anomalous excitation density dependence of nonlinear optical (NLO) responses in pseudoisocyanine bromide J aggregates have been observed at 77 K. The degenerate four‐wave mixing signals decrease with the increase of the excitation density between 1 and 3 μJ cm−2. This anomalous excitation density dependence shows that the J aggregates in themselves have a switching property and can be used as an optical logic element with an exclusive OR function. This is the first observation of the inverted region in which the NLO properties decrease with the increase of the excitation density. The anomaly is ascribed to the coherent exciton‐exciton interaction in a one‐dimensional molecular aggregate.