Temporal Shaping of X-ray Pulses via Saturable Absorption

To shorten the x-ray pulse duration, we present a nonlinear optical technique using core-hole atoms generated by x-ray irradiation on matter. The weak Coulomb screening in the core-hole atoms results in deterioration of absorption properties for the photon energy immediately above the absorption edge. By employing this phenomenon, referred to as saturable absorption, we successfully reduce the duration of x-ray free-electron laser pulses (photon energy: 9.000 keV, duration: 6-7 fs, fluence: 2.0-3.5\times10^5 J/cm^2) by \~35%. This finding that core-hole atoms are applicable to nonlinear x-ray optics is an essential stepping stone for extending nonlinear technologies commonplace at optical wavelengths to the x-ray region.