A compact X-ray free-electron laser emitting in the sub-?ngstrom region

The free-electron laser, first proposed by Madey in 1971, has significantly reduced laser wavelengths to the vacuum ultraviolet and soft X-ray regions. Recently, an X-ray free-electron laser (XFEL) was operated at 1.2 Å at the Linac Coherent Light Source (LCLS). Here, we report the successful generation of sub-ångström laser light using a compact XFEL source, combining a short-period undulator with an 8 GeV electron beam. The shortest wavelength attained--0.634 Å (63.4 pm)--is four orders of magnitude smaller than the 694 nm generated by Maiman's first laser. The maximum power exceeded 10 GW with a pulse duration of 10-14 s. This achievement will contribute to the widespread use of XFEL sources and provide broad opportunities for exploring new fields in science.