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

The free-electron laser, first proposed by Madey(1) in 1971, has significantly reduced laser wavelengths to the vacuum ultraviolet(2,3) and soft X-ray regions(4). Recently, an X-ray free-electron laser (XFEL) was operated at 1.2 angstrom at the Linac Coherent Light Source (LCLS)(5). Here, we report the successful generation of sub-angstrom laser light using a compact XFEL source, combining a short-period undulator with an 8 GeVelectron beam. The shortest wavelength attained-0.634 angstrom (63.4 pm)-is four orders of magnitude smaller than the 694 nm generated by Maiman's first laser(6). 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.