The coherent X-rays can be produced the process of by high harmonic generation, where a laser beam is focused into a gas target. When the laser intensity is sufficiently high, part of the electron wavepacket, of the target gas atoms, can be driven away from the atom core and when the electric field switches polarity the electron wave has a certain probability to recombine with the atom core. This gives rise to an immensely energetic nonlinear polarization wave that produces the coherent X-ray light. If the laser intensity is high enough electrons from several ionization levels can contribute. However this can be efficient only when multi-cycle driving UV lasers are used. They takes advantage of the ultra-high X-ray emission per atom and ion species, low group and phase velocity mismatch. In addition, in this spectral region, the macroscopic coherent build-up of the X-rays is also favourable even at extremely high intensity.
The coherent X-rays emerge naturally as narrow line-width harmonics with short near transform-limited pulses. These X-rays are ideally suited for dynamic X-ray imaging techniques, for imaging and visualization of nano-scale and smaller objects and features through techniques as coherent diffraction imaging (CDI) of extended nano-, magnetic and bio-samples with nanometer spatial and femto-to-attosecond resolution. In addition, techniques like X-ray tomography can provide 3D structure.
Science 2015 Dec 3;350(6265):1225-31. Epub 2015 Dec 3.
JILA, University of Colorado, Boulder, CO 80309, USA.
High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching--the constructive addition of x-ray waves from a large number of atoms--favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth-limited pulse trains of ~100 attoseconds.
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