Ultrafast multiphoton forest fires and fractals in clusters and dielectrics

We describe the interaction of ultrashort infrared laser pulses with clusters and dielectrics. Rapid ionization occurs on a sub-laser wavelength scale below the conventional breakdown threshold. It starts with the formation of nano-droplets of plasma which grow like forest fires, without any need for heating of the electrons promoted to the conduction band. The dimensionality of the damaged area can be fractal and changes during the laser pulse. This mechanism is operative in both rare gas clusters and dielectrics interacting with ultrashort, moderately intense laser pulses which include only several periods of the driving field, so that the traditional avalanche mechanisms have no time to develop.