Abstract

Production of plasmas with intense femtosecond laser pulses has permitted experimental access to the physics of highly ionized matter at unprecedented density and has created the prospect of novel x-ray sources. We study strong-field ionization of atmospheric-density gases by using a new diagnostic: spectral blue shifting of transmitted femtosecond pump or probe pulses caused by asymmetric phase modulation induced by the rapidly growing plasma density. In simple cases, observed shifts are well explained by an ab initio theory of plasma growth based on a Keldysh theory of collisionless ionization. However, anomalously large shifts occur in selected heavier gases (argon and nitrogen) at sufficiently high light intensity and pressure that electron-quiver amplitude exceeds interatomic spacing, inducing electron-impact ionization. Time-resolved experiments reveal further dynamics, such as a small time-delayed red shift of the probe spectrum, which is believed attributable to modulation of the collisional damping rate in the strong light field and implies a low final plasma temperature.

© 1990 Optical Society of America