Since the invention of the first lasers in the visible-light region, research has aimed to produce short-wavelength lasers that generate coherent X-rays1,2; the shorter the wavelength, the better the imaging resolution of the laser and the shorter the pulse duration, leading to better temporal resolution in probe measurements. Recently, free-electron lasers based on self-amplified spontaneous emission3,4 have made it possible to generate a hard-X-ray laser (that is, the photon energy is of the order of ten kiloelectronvolts) in an ångström-wavelength regime5,6, enabling advances in fields from ultrafast X-ray spectrosopy to X-ray quantum optics. An atomic laser based on neon atoms and pumped by a soft-X-ray (that is, a photon energy of less than one kiloelectronvolt) free-electron laser has been achieved at a wavelength of 14 nanometres7. Here, we use a copper target and report a hard-X-ray inner-shell atomic laser operating at a wavelength of 1.5 ångströms. X-ray free-electron laser pulses with an intensity of about 1019 watts per square centimetre7,8 tuned to the copper K-absorption edge produced sufficient population inversion to generate strong amplified spontaneous emission on the copper Kα lines. Furthermore, we operated the X-ray free-electron laser source in a two-colour mode9, with one colour tuned for pumping and the other for the seed (starting) light for the laser.
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The experiment was performed at SACLA with the approval of JASRI and the program review committee (grant numbers 2012B8014, 2013A8013, 2013B8020, 2014A8008). We acknowledge the supporting members of the SACLA facility. This research was partially supported by Grant-in-Aids for Scientific Research (A) (25247093) and (S) (23226004), by the Photon Frontier Network Program and by the Global COE Program ‘Center of Excellence for Atomically Controlled Fabrication Technology’ from the Ministry of Education, Sports, Culture, Science and Technology, Japan (MEXT).
The authors declare no competing financial interests.
Extended data figures and tables
a, The waveforms of the seeded output pulses at different ratios of the peak intensities between the output and input pulses. The waveform of the pump pulse is shown as a reference. b, A measured spectrum at Ip = 4.3 × 1019 W cm-2 and its decomposition into several Lorentzian functions. The lines labelled K1 and K2 are the unshifted components of Kα1 and Kα2, respectively. The lines S1 to S4 that have the same widths as K1 are the shifted components with a constant interval 3.2 eV in the energy shift. The total ratio of the shifted components is estimated to be about 37%.
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Yoneda, H., Inubushi, Y., Nagamine, K. et al. Atomic inner-shell laser at 1.5-ångström wavelength pumped by an X-ray free-electron laser. Nature 524, 446–449 (2015). https://doi.org/10.1038/nature14894
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