Atomic inner-shell laser at 1.5-angstrom wavelength pumped by an X-ray free-electron laser

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(1,2); 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 emission(3,4) have made it possible to generate a hard-X-ray laser (that is, the photon energy is of the order of ten kiloelectronvolts) in anngstrom-wavelength regime(5,6), 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 angstroms. X-ray free-electron laser pulses with an intensity of about 10(19) watts per square centimetre(7,8) tuned to the copper K-absorption edge produced sufficient population inversion to generate strong amplified spontaneous emission on the copper Ka lines. Furthermore, we operated the X-ray free-electron laser source in a two-colour mode(9), with one colour tuned for pumping and the other for the seed (starting) light for the laser.