title: Atomic inner-shell laser at 1.5-ångström wavelength pumped by an X-ray free-electron laser.
abstract: Since the invention of the first lasers in the visible-light region, research has aimed to produce short-wavelength lasers that generate coherent X-rays; 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 emission 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 regime, 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 nanometres. 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 10(19) watts per square centimetre 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 mode, with one colour tuned for pumping and the other for the seed (starting) light for the laser.
authors: Institute for Laser Science, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan.|RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan.
authors: Hitoki
authors: H
authors: Yoneda
authors: RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan.|Japan Synchrotron Radiation Research Institute (JASRI), Sayo, Hyogo 679-5198, Japan.
authors: Yuichi
authors: Y
authors: Inubushi
authors: Institute for Laser Science, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan.
authors: Kazunori
authors: K
authors: Nagamine
authors: Institute for Laser Science, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan.
authors: Yurina
authors: Y
authors: Michine
authors: RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan.|Japan Synchrotron Radiation Research Institute (JASRI), Sayo, Hyogo 679-5198, Japan.
authors: Haruhiko
authors: H
authors: Ohashi
authors: Japan Synchrotron Radiation Research Institute (JASRI), Sayo, Hyogo 679-5198, Japan.
authors: Hirokatsu
authors: H
authors: Yumoto
authors: RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan.|Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871 Japan.
authors: Kazuto
authors: K
authors: Yamauchi
authors: RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan.|Department of Precision Engineering, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan.
authors: Hidekazu
authors: H
authors: Mimura
authors: Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors: Hikaru
authors: H
authors: Kitamura
authors: Japan Synchrotron Radiation Research Institute (JASRI), Sayo, Hyogo 679-5198, Japan.
authors: Tetsuo
authors: T
authors: Katayama
authors: RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan.
authors: Tetsuya
authors: T
authors: Ishikawa
authors: RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan.
authors: Makina
authors: M
authors: Yabashi