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Fluorescence imaging of nonlinear x-ray propagation and lasing

Svyatoslav Blinov1,2, Pavel Krasnov1,2, Faris Gelmukhanov1,2,3, Jan-Erik Rubensson4, Sergey Polyutov1,2, and Victor Kimberg3,*

  • 1International Research Center of Spectroscopy and Quantum Chemistry—IRC SQC, Siberian Federal University, 660041 Krasnoyarsk, Russia
  • 2Federal Siberian Research Clinical Centre under the Federal Medical Biological Agency, 660037 Krasnoyarsk, Russia
  • 3Theoretical Chemistry and Biology, KTH Royal Institute of Technology, 10691 Stockholm, Sweden
  • 4Department of Physics and Astronomy, Uppsala University, Box 516, S-751 20 Uppsala, Sweden

  • *Contact author: kimberg@kth.se
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Phys. Rev. Research 8, 013110 – Published 30 January, 2026

DOI: https://doi.org/10.1103/5b65-yjmp

Abstract

We introduce a general approach for probing nonlinear x-ray propagation by imaging secondary fluorescence emitted transverse to the driving field. When a short, intense x-ray pulse excites a deep 1s core orbital, subsequent Kα emission from spin-orbit-split 2p states can undergo stimulated amplification. This nonlinear process reshapes the relative populations of the 2p1/2 and 2p3/2 levels along the propagation path, leaving distinct signatures in the delayed L-edge fluorescence. By solving the coupled density-matrix and Maxwell equations, we show that these fluorescence signals provide a direct and experimentally accessible probe of x-ray amplification dynamics. We demonstrate the concept for argon atoms and extend it to molecular systems containing third-row elements, where competing effects of lifetimes, transition intensities, and nonresonant absorption determine the efficiency of stimulated emission. Our results establish L-edge fluorescence as a broadly applicable diagnostic of nonlinear x-ray phenomena, opening opportunities for studying light-matter interactions in regimes where direct detection of amplified x-ray signals is technically challenging.

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Phys. Rev. Research 8, 013110– Published 30 January, 2026
  • Received 29 August 2025
  • Accepted 31 December 2025
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