The pulse width of an electromagnetic wave is determined by the frequency band width of the wave used. Therefore, one femtosecond is the ultimate in pulse width for an “optical” wave. For this reason, several methods have been proposed for the generation of an ultrashort optical pulse. For example, resonance/non-resonance four-wave mixing would be one of the candidates for generating multi-color laser emission in an extremely wide spectral region, thus breaking the 1-fs barrier.

To date, numerous emission lines have been generated from the deep-ultraviolet to the...