Nonlinear reversal of photoexcitation on the attosecond time scale improves ultrafast X-ray diffraction images
- Anatoli Ulmer
- Phay J. Ho
- Bruno Langbehn
- Stephan Kuschel
- Linos Hecht
- Razib Obaid
- Simon Dold
- Taran Driver
- Joseph Duris
- Ming-Fu Lin
- David Cesar
- Paris Franz
- Zhaoheng Guo
- Philip A. Hart
- Andrei Kamalov
- Kirk A. Larsen
- Xiang Li
- Michael Meyer
- Kazutaka Nakahara
- Robert G. Radloff
- River Robles
- Lara Rönnebeck
- Nick Sudar
- Adam M. Summers
- Linda Young
- Peter Walter
- James P. Cryan
- Christoph Bostedt
- Daniela Rupp
- Agostino Marinelli
- Tais Gorkhover
2026-07-28
The complex refractive index of a material governs its light-matter interactions, with intense light fields enabling tailored nonlinear optical responses. In the X-ray regime, rapid photoionization limits the potential of nonlinear techniques by inducing irreversible electronic damage. Here we demonstrate that intense, sub-femtosecond X-ray pulses, shorter than typical Auger decay times, can partially reverse photoexcitation via stimulated emission near atomic resonances. By analyzing thousands of coherent diffraction patterns and ion spectra from neon nanoparticles exposed to sub-fs and 15-fs pulses, we observe enhanced X-ray diffraction alongside reduced energy absorption for sub-fs pulses. Theoretical modeling attributes this to dynamics akin to Rabi flopping that prolong the lifetime of resonant states and suppress electronic bleaching. These findings suggest that ultrashort, intense X-ray pulses enable active control of X-ray refractive index and damage pathways, opening avenues for improved high-resolution imaging and nonlinear spectroscopy in complex nanoscale systems.