Focusing of X-ray free-electron laser pulses using multilayer Laue lenses

Achieving the highest possible intensities of pulses at X-ray free-electron laser(XFEL)facilitiesentailsfocusingtheX-raybeamtothesmallestpossibledimensions. MultilayerLauelensesarevolumediffractiveopticsthatholdpromisetoachievehighintensitiesduetotheirhighnumericalapertureathardX-raywavelengths. Theselensesaremadebylayerdepositionandhence have a small aperture that is usually less than 100µm. Consequently, they must withstandhigh intensities and heat loads if used to focus pulses of high energies. Here, we demonstrate thefocusingofXFELpulsesusinglensesmadefromlayersofMo2CandSiC,whichwerechosenfortheirhighefficiencyandlowbeamheating. WeshowthatlensalignmentandcharacterisationcanbecarriedoutusingattenuatedXFELpulsesusingthemethodofptychographicX-rayspeckletracking, which also provides an approach for high-resolution projection imaging with XFELpulses. The approach also gives quantitative information about the distribution of intensityin the focal plane, which is analysed here using plots of the encircled energy describing theproportionofthebeamenergyconcentratedintoacertaindiameter,showingthepossibilitytoreach 1.8×1020 Wcm−2 at a photon energy of 16.9keV. No measurable change in the wavefrontaberrationsofthelensescouldbedetectedaftertheiruseinanexperimentformanydayswithunattenuated XFEL pulses, showing that the materials and mounting scheme presented heremake theselenses suitablefor sustained use at XFEL facilities.