High-order Harmonic Generation

Abstract

X-rays were observed for the first time by Wilhelm Conrad Rontgen in 1895 (Rontgen, 1895). During the first century since that great event X-rays were benefiting mostly from their spatial resolution capability. However, recently it was possible to take advantage also from their temporal resolution due to novel sources providing ultrashort bursts of shortwavelength radiation (i.e. wavelength λ < 100 nm) and to get an inside view of physical processes in molecules and atoms. One possibility of how to obtain ultrashort bursts of coherent extreme ultraviolet (abbreviated XUV or EUV; wavelength spectral range between 10-100 nm), soft X-ray (1-10 nm) and/or X-ray radiation (< 1 nm) is by high-order harmonic generation (HHG) process. It involves interaction of laser light at a given frequency during which it is being converted into integer multiples of the fundamental frequency through a highly nonlinear interaction with a conversion medium (typically a noble gas; Brabec & Krausz, 2000). Laser-driven HHG uses acceleration of electrons on time-scales that are of the order of an optical cycle of the laser field. Currently this technique gives rise to the shortest flashes of light ever generated in a laboratory which are typically of the order of a few hundreds of attoseconds (1 as = 10-18 s; Paul et al., 2001; Kienberger et al., 2004; Schultze et al., 2007). When laser field of intensity of about 1014 1015 W/cm2 and time durations in