Wavelength-tunable ultrashort-pulse output of a photonic-crystal fiber designed to resolve ultrafast molecular dynamics

Wavelength-tunable 100 fs pulses generated through the soliton self-frequency shift in a photonic-crystal fiber are employed to visualize femtosecond coherence and population relaxation dynamics in molecular aggregates by means of time-resolved sum-frequency generation. This technique reveals an ultrafast dephasing of coherent molecular excitations with a phase relaxation time of about 120 fs and resolves an ultrafast switching of the nonlinear-optical response of molecular aggregates.