Optica 5, 257–262 (2018) https://doi.org/10.1364/optica.5.000257

Large-scale free-electron laser and synchrotron facilities are excellent sources of high-energy X-rays. Meanwhile, smaller options, such as plasma-based X-ray lasers, enable smaller laboratory-scale facilities to conduct some experiments requiring relatively intense X-ray pulses. Now, Alex Rockwood and a team from Colorado State University and Oak Ridge National Laboratory have extended the wavelength range of compact gain-saturated X-ray lasers from around 9 nm, down to below 7 nm, by using an excited Ni-like Gd ion plasma formed by a combination of an optimized pre-pulse and a subpicosecond pump pulse. The pulses, from an 800-nm Ti:sapphire laser, irradiated 1–2 mm thick targets. First, the normal incidence ‘pre-pulse’ ionizes plasma to a Ni-like ionization regime, and the following subpicosecond pulse, at grazing incidence, heats the electrons and results in population inversion. In principle, different propagation velocities of the pump pulse and the amplified pulse limits amplification of the X-ray laser pulse, but the team was able to overcome this by using a reflection echelon with adjustable mirror segments, enabling adjustable excitation travelling wave velocity. The 6.85-nm Gd lasing was achieved with 2.5 Hz repetition. Even shorter lasing wavelengths were achieved, down to 5.85 nm, by using different Ni-like lanthanide ions and increasing pump pulse grazing incidence angle to yield higher plasma densities.