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Probing electron and hole colocalization by resonant four-wave mixing spectroscopy in the extreme ultraviolet

Rottke, Horst; Engel, Robin Y.; Schick, Daniel; Schunck, Jan O.; Miedema, Piter S.; Borchert, Martin C.; Kuhlmann, Marion; Ekanayake, Nagitha; Dziarzhytski, Siarhei; Brenner, Günter; Eichmann, Ulrich; Korff Schmising, Clemens von; Beye, Martin; Eisebitt, Stefan

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.
Published in: Science Advances, 10.1126/sciadv.abn5127, American Association for the Advancement of Science