Table-top soft X-ray laser

High-harmonic generation (HHG) is a universal response of atoms and molecules in strong femtosecond laser fields, and can be used to generate coherent photons in the soft X-ray region. Simply speaking, HHG is the coherent version of an X-ray tube; instead of accelerating thermal electrons emitted from the filament and generating incoherent X-rays by hitting a metallic target, HHG begins with tunnel ionization of an atom in a strong laser field. The portion of the electron wave function that escapes the atom is accelerated by the laser electric field and, when driven back to its parent ion by the laser, can coherently convert its kinetic energy into a high-harmonic photon. So far, for many cases, around 100 near-infrared laser photons have been combined to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms is added constructively. Recently, a team led by Professor H. C. Kapteyn and Professor M. M. Murnane (University of Colorado at Boulder, USA) have employed a mid-infrared femtosecond laser in a high-pressure gas, and succeeded in getting ultrahigh harmonics up to orders greater than 5000, resulting in a bright continuum spectra ranging from 0.2 to around 1.6 keV. The energy has still not yet reached the hard X-ray regime, but this would be a very attractive coherent ultra short pulse source for soft X-rays. For more information, see the paper, "Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers", T. Popmintchev et al., Science, 336, 1287 (2012).