The ultrafast modification of molecular electronic state populations by picosecond duration laser pulses is discussed. The photodynamic interactions operative during the dual pulse irradiation of a saturable absorbing dye are treated by a rate equation approach which allows the temporal changes in the gain and attenuation conditions of the absorber to be predicted. The treatment relies on a knowledge of the molecular absorption and stimulated emission profiles and the existence of homogeneously broadened electronic states of the molecule. The approach is applied to a description of the passive modehyphen;locking capability of certain saturable absorbing dyes at wavelengths substantially different from their absorption maxima.
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