Laser produces coherent 1.3 keV X-ray pulse

An Austrian research group recently succeeded in obtaining highly collimated, spatially coherent X-rays, at a wavelength of about 1 nm and at photon energies extending to 1.3 keV, from high-order harmonic generation in an atomic gas ionized by a 720-nm, 5-fs, 0.2-TW laser pulse. The beam divergence was evaluated as 0.2 mrad for the spectral range above 200 eV from a knife-edge scan, indicating perfect coherence of the atomic dipoles within a macroscopic volume of diameters of 13μm and 4μm at photon energies of 0.3 keV and 1 keV, respectively. The beam seems to be diffraction-limited to within a factor of five. The spectrum of the generated radiation was observed by an energy-dispersive X-ray spectrometer with some filters. The results are really exciting, because they could detect the copper L-edges (~950 eV)! One would notice that the energy of photons produced by laser technologies has been increasing every year - the main idea behind this progress is the creation of time-gradient in the driving pulse, which allows some 25% of the helium atoms to be ionized within half a cycle before the pulse peak. The electrons detached within this time are pushed in the most intense half-cycle back to the atomic core. For more information, see the paper, "Source of coherent kiloelectronvolt X-rays", J. Seres et al., Nature, 433, 596 (2005). C. Streli and P. Wobrauschek (Atominstitut der Osterreichischen Universtitaten, Technische Universitat Wien) were the co-authors of this paper.