Abstract

The availability of intense few-cycle laser pulses has enabled experiments accessing time scales much shorter than the duration of the corresponding optical cycle [1,2]. The mechanism underlying this exceptionally high temporal resolution is ionization due to tunnelling of electrons through the potential barrier induced by the modification of the Coulomb potential by the laser field. In this mechanism, the ionization rate peaks sharply at the maxima of the laser electric field [3]. After ionization, the electrons will be accelerated and decelerated in the electric field, and a small fraction may recollide and even recombine with the parent ion, giving rise to high-order harmonic generation [4]. However, the strong peaks of the ionization rate coinciding with the electric field maxima, give rise to a recollision-free generation of odd harmonics [5]. In this work, we present a detailed study of the properties of 3^rd harmonic emission due to electron tunnelling, and compare it to 3^rd harmonic generation in the perturbative regime.

© 2009 IEEE