WAVELENGTH-DEPENDENT NONLINEAR-OPTICAL PROPERTIES OF PSEUDOISOCYANINE J-AGGREGATES

Degenerate four wave mixing of tunable picosecond pulses were carried out in pseudoisocyanine J-aggregates near resonance. The conversion efficiency was very high even at very low intensities of interacting beams. The power of intensity dependence of the converted signal was shown to change with wavelength within the J-band. The range of the power dependence was from approximate to 1.5 (in the center of the J-band) to three (at the wings of the J-band). This effect is related to the saturation of population density, to singlet-singlet annihilation and secondary processes of local vibrational excitation or heating. The different extent of saturation and heating causes the deviation from third order power dependence on intensity within the J-band. The dependence of the timing of the three pulses supports the interpretation of such a behaviour by an annihilation and secondary processes. The cubic susceptibility was estimated at the wings of the J-band to a value of 10(-8) esu.