Detailed simulations of plasma-induced spectral blueshifting

A one-dimensional model, including pulse propagation and detailed ionization dynamics, is developed in order to calculate the spectrum of an intense femtosecond laser pulse transmitted through an atmospheric-density noble gas. The spectrum is broadened and blueshifted due to self-phase-modulation resulting from a combination of optical-field-induced ionization and collisional ionization. Different features in the spectrum can be used to indicate the relative importance of the two ionization mechanisms. The calculations are in good agreement with the recent experimental results of Wood, Siders, and Downer [Phys. Rev. Lett. 67, 3523 (1991)], but indicate that a significant degree of defocusing occurs as a result of plasma generation. The simulations could possibly be used as a test of various theoretical models for optical-field-induced ionization.