Femtosecond dephasing of coherent excitation and ultrafast switching of resonant optical nonlinearity of molecular aggregates

One-and two-photon-resonant nonlinear-optical interactions of ultrashort laser pulses are employed to measure the dephasing time of coherent excitation and to investigate the ultrafast population dynamics of molecular aggregates. Dephasing of coherent excitation of molecular aggregates with a characteristic phase relaxation time of about 120 fs is visualized. Ultrafast switching of optical nonlinearity in molecular aggregates with a typical switching time of about 450 fs is demonstrated.