High-order harmonic source for x-ray laser seeding driven by two-color laser field

Abstract

Seeding x-ray lasers with high-order harmonic (HH) radiation has been successfully demonstrated a few years ago [1] and it is under extensive investigation nowadays [2,3]. With high-order harmonic generation spatially and temporally coherent radiation in a wide spectrum is generated. It consists of discrete spectral lines at the odd multiples of the frequency of the driving laser pulses. Unfortunately, most of the harmonic lines do not coincide exactly with the wavelength of potential x-ray laser transitions. Using a Nd:glass laser system such as PHELIX at GSI for pumping an x-ray laser, the wavelength of the fundamental laser beam can not be tuned. However, harmonic lines can be shifted due to the blue shift of the driving laser wavelength caused by the ionization of the gas media used for HH generation [4]. The magnitude of the shift can be controlled e.g. with the gas backing pressure. However, the obtainable shift is not sufficient to cover the whole wavelength range. The number of possible x-ray laser transitions can be increased by using the two-color scheme for HH generation. Using pulses at the fundamental laser frequency mixed with pulses at the second harmonic, it is possible to generate odd and even harmonics [5,6]. With this schema we generated strong even and odd harmonics in a wide spectral range in Ar gas suitable for seeding Ni-like x-ray lasers.