Amplifying copper Kα X-ray ultra short pulses

Reaching the milestone of X-ray lasers is something that X-ray physicists have dreamed of for many years. Recently developed X-ray free-electron lasers (XFEL) based on self-amplified spontaneous emission (SASE) constitute a very promising tool for future X-ray laser technologies. A team led by Professor H. Yoneda (University of Electro-Communications, Tokyo, Japan) has recently carried out very impressive experiments at Japanese XFEL facility, SACLA, on the same campus as SPring-8. Similar to an X-ray tube, the researchers employed a solid copper target to generate X-rays. However, their experiment is very unique in that two colors were employed by tuning undulator gaps; one energy is around 9 keV, i.e., above the Cu-K absorption edge, and the other energy is almost the same energy as Cu Kα₁ or Kα₂. While 9 keV photons ionize copper atoms and generate Cu K X-ray fluorescence spectra, the lower energy photons can amplify the X-ray fluorescence because of their temporal coherence. Their Cu Kα spectra are impressive, because Kα₁/Kα₂ can be controlled by tuning the second energy of XFEL pulses. The reason for the limited amplification is probably due to the energy band width of incoming temporary coherent X-ray photons. The researchers did not use any monochromators, but controlled only the undulator gaps. They have two different X-ray energies, but unfortunately monochromaticy has some limits and the band width is still quite wide. The present work could be a very important step toward achieving X-ray lasers by using atomic energy levels. For more information, see the paper, "Atomic inner-shell laser at 1.5-angstrom wavelength pumped by an X-ray free-electron laser", H. Yoneda et al., Nature 524, 446 (2015).