Description
We introduce a general approach for probing nonlinear x-ray propagation by imaging secondary fluorescence emitted transverse to the driving field [1]. 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 $2p_{1/2}$ and $2p_{3/2}$ 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.
[1] Blinov, S., Krasnov, P., Gelmukhanov, F., Rubensson, J. E., Polyutov, S., & Kimberg, V. (2026). Fluorescence imaging of nonlinear x-ray propagation and lasing. Physical Review Research, 8(1), 013110.
