Dynamic control of X-ray core-exciton resonances by Coulomb screening in photoexcited semiconductors

Excitonics is an emerging field focused on exploiting and manipulating excitons generated through light-matter interactions. Advancing the field into X-ray excitonics requires precise energy and time control over core-exciton resonances. Such control enables the use of core excitons to enhance non-linear optical effects, including X-ray second harmonic generation, and has broad applications in material characterization and quantum information processing. To achieve these objectives, it is essential to comprehend the role of many-body effects governing core-exciton dynamics. In this work, we address this challenge by combining experiments with a novel \emph{ab initio} approach specifically developed to interpret pump-probe excitations. Applied to the prototypical wide-bandgap semiconductor ZnO, first-principles calculations reproduce experimental results and unveil how the density and distribution of photoexcited carriers can tune the dynamical Coulomb screening, thereby controlling core-exciton binding energies, while Pauli blocking remains negligible. These insights lead us to propose a method for dynamically controlling core-exciton resonances at absorption edges, achieving either a uniform spectral blue shift, driven by thermalized carrier distributions on picosecond timescales, or individual spectral blue shifts, driven by time-dependent carrier distributions on femtosecond timescales.