We report for the first time saturable absorption in the soft x-ray regime: by photoionizing L-shell core electrons we observed on a 15fs timescale a multifold increase of transmission through an aluminium foil. While saturable absorption is a phenomenon readily seen in the optical and infrared wavelengths, it has never been observed in a core electron transition due to the short lifetimes of the created excited states and the high intensities of the soft x-rays that are needed.
The experiments were performed at the XUV Free Electron Laser FLASH and used record high intensities. After the FEL pulse has passed, the aluminum sample is in an exotic state where all the aluminum atoms have a L-shell hole, and the conduction band has a 9eV temperature, while the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to Warm Dense Matter condition, at 20eV temperatures.
The saturable absorption allows for a very homogeneous and efficient heating. Therefore the method is an ideal candidate to study homogeneous Warm Dense Matter, highly relevant to planetary science, astrophysics and inertial confinement fusion.