Benny Wu, SIMES Research
Coherent x-ray diffraction is a fundamental technique for the study of materials on the nanoscale. Key to this technique is the close relationship between the far field diffraction of a sample and its spatially dependent refractive index. The emergence of x-ray free electron laser (XFEL) facilities, such as the Linac Coherent Light Source (LCLS), with its femtosecond x-ray pulses orders of magnitude more intense than third generation synchrotrons, has enabled us to directly image ultrafast processes on nanometer length scales with a combination of coherent diffraction and pump-probe techniques.
In this talk, I will discuss coherent x-ray diffraction experiments on perpendicularly magnetized Co/Pd multilayers performed at the LCLS. Utilizing the incredible brightness of LCLS, we were able to perform single shot imaging of the nanoscale magnetic order with Fourier Transform Holography. An even more interesting observation is the surprising nonlinear dependence of the refractive index on the x-ray intensity, which resulted in dramatic changes to the charge and magnetic scattering at sufficiently high intensities. I will show that the observed changes can be well explained by a nonlinear contribution to the refractive index through the third order susceptibility. The onset of nonlinear effects sets the boundaries on the application of coherent diffraction for reliable structural determination. At the same, these effects may foster the development of nonlinear spectroscopic x-ray techniques at XFELs.
