Spectral blue shifting of a femtosecond laser pulse propagating through a high-pressure gas

Plasma-induced spectral blue shifting in rare gases has been investigated with a subpicosecond KrF excimer laser focused to a peak intensity in the region of 1014 W/cm2 (adiabaticity parameter in the range 8 < γ < 10). The quiver energy of a free electron under these conditions is sufficiently small to ensure that ionization occurs solely by optical-field-induced processes. Blue shifts as large as 2 nm have been observed, and the blue-shifted spectrum shows an interferencelike oscillatory structure. Experimental results are compared with numerical simulations to show that the blue-shifted spectra are the result of plasma-induced self-phase modulation and can be modeled qualitatively by assuming tunneling ionization and plane-wave pulse propagation. The structure in the spectrum is closely related to that observed in earlier experiments on self-phase modulation in quite different systems.