Probing electron and hole colocalization by resonant four-wave mixing spectroscopy in the extreme ultraviolet
H. Rottke, Robin Y. Engel, Daniel Schick, Jan O. Schunck, Piter S. Miedema, Martin Borchert, Marion Kuhlmann, Nagitha Ekanayake, Siarhei Dziarzhytski, Günter Brenner, U. Eichmann, Clemens von Korff Schmising, Martin Beye, Stefan Eisebitt
Abstract
Extending nonlinear spectroscopic techniques into the x-ray domain promises unique insight into photoexcited charge dynamics, which are of fundamental and applied interest. We report on the observation of a third-order nonlinear process in lithium fluoride (LiF) at a free-electron laser. Exploring the yield of four-wave mixing (FWM) in resonance with transitions to strongly localized core exciton states versus delocalized Bloch states, we find resonant FWM to be a sensitive probe for the degree of charge localization: Substantial sum- and difference-frequency generation is observed exclusively when in a one- or three-photon resonance with a LiF core exciton, with a dipole forbidden transition affecting details of the nonlinear response. Our reflective geometry-based approach to detect FWM signals enables the study of a wide variety of condensed matter sample systems, provides atomic selectivity via resonant transitions, and can be easily scaled to shorter wavelengths at free-electron x-ray lasers.