Probing electron and hole co-localization by resonant four-wave mixing in the extreme-ultraviolet

TL;DR

This paper demonstrates that resonant four-wave mixing in the extreme-ultraviolet can probe charge localization in materials, revealing differences between core exciton and delocalized states with potential applications in condensed matter studies.

Contribution

It introduces a novel reflection-geometry method for resonant FWM in the x-ray domain, enabling sensitive detection of charge localization in condensed matter systems.

Findings

Resonant FWM is sensitive to charge localization in LiF.

Significant FWM signals occur only at resonances with core exciton states.

The method is scalable to shorter wavelengths at free-electron x-ray lasers.

Abstract

The extension of nonlinear spectroscopic techniques into the x-ray domain is in its infancy but holds the promise to provide unique insight into the dynamics of charges in photoexcited processes, which are of fundamental as well as applied interest. We report on the observation of a third order nonlinear process in lithium fluoride at a free-electron laser. Exploring the yield of four wave mixing (FWM) in resonance with transitions to strongly localized core exciton states vs. 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 reflection-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.