Abstract

With intense laser pulses it is possible to generate x-ray radiation up to 3–4 keV [1] via high harmonic generation (HHG). The cut-off can be further extended by HHG from ions and solving the phase matching problem. Quasi phase matching [2, 3] or non-adiabatic phase matching [1] has been successfully demonstrated for HHG from neutral atoms. Here we describe a new approach for HHG with extended cut-off based on the formation of an electron free channel containing dominantly He⁺ ions. Irradiating atoms with high intensity laser pulses generates free electrons, which are pushed away from the central region of the laser beam. The formed ion channel plays an important role in several experiments such as laser electron acceleration, where the channel is formed by laser pulses with relativistic intensity. For the formation of the channel, relativistic intensities are not necessary. For HHG from ions in a channel, we have applied a double pulse scheme to control the ionization and phase matching conditions independently by the pulse to pulse separation. The first pulse fully ionizes the He atoms in the gas jet leaving behind a nearly electron free ion channel. With the second pulse we generate HHG from the He⁺ ions in the channel. The yield is substantially higher due to the favorable phase matching conditions, i.e. the reduced electron density. Due to the strong dependence of the HHG yield from the coherence length, our method allows also a precise monitoring of the evolution of the on-axis free electron density in the formed channel.

© 2009 IEEE