Observation of sub-100-fs optical response from spin-coated films of squarylium dye J aggregates

For spin-coated films of squarylium dye J aggregates, ultrafast nonlinear optical responses were investigated by pump–probe measurements. By using a broadband mode-locked titanium:sapphire laser, we succeeded in observing the optical response with a time resolution of better than 60 fs. Time-resolved transmission data are shown for different excitation wavelengths, resonant to the excitonic absorption band and off-resonant. Relaxation times of the absorption saturation were evaluated to be 140 fs (fast component) and 950 fs (slow component) in the case of resonant excitation and 98 fs in the case of off-resonant excitation.