Recent Results in High-Order Harmonic Generation

Abstract

High order harmonic generation by atoms in intense laser fields has drawn considerable attention in the last few years. Experiments^1–5 have demonstrated the possibility to reach wavelengths shorter than 8 nm with reasonable conversion efficiencies, thus opening the way to the development of high brightness coherent sources in the vuv and soft x-ray ranges. The understanding of these processes involves both the single atom response to a strong electromagnetic field (strongly non-perturbative) and in particular its emission spectrum^6–8, and the generation and propagation of the harmonic fields in dense, partly ionized, gaseous media⁹. A quite fascinating aspect of this field is this interplay between atomic physics in a strong field regime and nonlinear optics in a rather unusual situation. Since the early results¹⁰, progress has been made in three directions. Shorter wavelengths and much higher nonlinear orders have been generated by using light atoms (or ions) and strong (low frequency) radiation fields^1–5,11. Detailed studies have been undertaken for the moderately high harmonics and have led to a better understanding of some aspects of these processes, in partieular of phase matching effects^12,13. Finally, some first attempts at characterizing the harmonic emission have been made^14–16.