Fixed blueshift of high intensity short pulse lasers propagating in gas chambers

An explanation is given for the fixed blueshift observed of an intense shor t pulse laser propagating lin a gas chamber filled with helium. The observe d shift is found to be independent of the laser power and gas pressure abov e some critical power. The results are explained by the formation of filame nts, which occur due to nonlinear gas polarization effects. The blueshift i s in qualitative agreement with a one dimensional particle-in-cell simulati on including ionization of the gas. (C) 2000 American Institute of Physics. [S1070-664X(00)03212-2].