Monomolecular layer of squarylium dye J aggregates exhibiting a femtosecond optical response of delocalized excitons

We report on the demonstration of the femtosecond nonlinear optical response from a two-dimensional monomolecular layer of squarylium dye J aggregate at 5 °C. The formation of a monomolecular layer Langmuir film was achieved by spreading squarylium dye modified by two propyl and two hexyl groups at the air–water interface, which resulted in a very strong J band (o.d.=0.3) at 777 nm. The transient absorption spectra in a resonant pump-probe measurement showed a low absorption saturation power (9.7×106 W/cm2) and an ultrafast response (300 fs), which are indicative of exciton delocalization over 18 molecules in this J aggregate, even at 5 °C.