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 $(\mathrm{o.d.}\text{=0.3)}$ at 777 nm. The transient absorption spectra in a resonant pump-probe measurement showed a low absorption saturation power ${\text{(9.7×10}}^6\hspace{0.17em}{\mathrm{W/cm}}^{\mathrm{2}})$ and an ultrafast response (300 fs), which are indicative of exciton delocalization over 18 molecules in this $J$ aggregate, even at 5 °C.