DUAL-WAVELENGTH OPTICAL-SAMPLING TECHNIQUE FOR ULTRAFAST TRANSIENT BLEACHING SPECTROSCOPY

The ultrafast optical sampling technique has been successfully applied to the non-degenerate subpicosecond pump-probe spectroscopy by using two independently oscillating tunable femtosecond lasers with slightly different pulse repetition rates (f1, f2). The choice of the pump and probe wavelengths is fully arbitrary within the tunability range of the two lasers with time resolution (approximately 300 fs) limited only by the width of the cross correlation. In our method the difference frequency f1-f2 is locked to the frequency of an external reference for better temporal accuracy and introduction of the homodyne detection revealed a great improvement of sensitivity. The transient absorption bleaching measurements have been performed for organic dye molecules in solution to demonstrate the abilities of this technique. Signals of the time evolution of the photoexcited molecules ranging from subpicosecond intramolecular relaxation to nanosecond ground-state recovery, as well as picosecond solvation dynamics and subnanosecond orientational relaxation, are simultaneously detected in such a simple manner as varying the sweep time of the oscilloscope.