Zhao Chen, Daniel Higley, Markus Hantschmann, Martin Beye, Tianmin Liu, Virat Mehta, Kristof Lodewijks, William Schlotter, Joachim Stohr

Stanford University

We present direct evidence of resonant stimulated X-Ray scattering in magnetically patterned Co/Pd multilayers, which were measured simultaneously in the X-Ray transmission and X-Ray scattering geometries at LCLS. We transmit ultrafast (~2fs) SASE X-Ray pulses at the Co L3 edge for varying fluences of up to 2 J/cm^2/fs through our Co/Pd samples. We record with higher incoming fluences both a nonlinear decrease in Bragg scattering intensity together with a nonlinear increase in transmitted forward intensity in a spectrometer set behind the sample. At high enough fluences (>1 J/cm^2/fs), sample absorption and scattering are both attenuated by orders of magnitude, leaving the sample effectively transparent. Together, these two effects are characteristic of elastic stimulated scattering, which boosts transmitted intensity in the forward direction at the cost of intensity scattered in all other directions. We show that our data is quantitatively very well described by the optical Bloch equations, and from theoretical simulations also extract new insight into the importance of temporal coherence in stimulated processes. Our dual measurement provides an unprecedentedly complete, direct look at stimulated processes at XFELs, and paves the way for experiments on realizing powerful X-Ray techniques such as stimulated RIXS.