Nature 481, 488–491 (2012)

X-ray free-electron laser (FEL) facilities provide pulses of soft and hard X-rays that are bright and short but suffer from unstable emission spectra. Nina Rohringer and colleagues from the USA and Germany have now found a way to improve the fluctuating spectrum, noise and limited coherence of an FEL. The team used the FEL at the Linac Coherent Light Source in the USA to pump a hard-X-ray atom laser and obtain narrowband and coherent output. The team used 40–80 fs pulses with photon energies of around 960 eV (wavelength of 1.28 nm) to illuminate a neon gas with a 1–2 μm spot size and thus produce a plasma column of core-excited ions. Amplified stimulated emission occurred at the front of the plasma column when photons emitted from previously excited atoms encountered atoms prepared in an excited state due to the FEL pulse. The researchers detected atomic radiation at 849 eV (wavelength of 1.45 nm) using a grating spectrometer and a CCD. To confirm exponential amplification of the X-rays along the gain medium, the researchers doubled the FEL energy from 0.12 mJ to 0.24 mJ, which increased the output by four orders of magnitude. Single-shot experiments provided a peak output of around 1.1 μJ and an effective gain coefficient of approximately 70 cm−1.