Journal
NATURE PHYSICS
Volume 11, Issue 11, Pages 964-970Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/NPHYS3452
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Funding
- US Department of Energy (DOE), Office of Basic Energy Sciences (BES)
- Volkswagen Foundation
- DOE Office of Science Graduate Fellowship Program
- Division of Materials Sciences and Engineering, BES, DOE [51 DE-AC02-76SF00515]
- AMOS program within the Chemical Sciences, Geosciences, and Biosciences Division, DOE, BES, DOE
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X-ray scattering is typically used as a weak linear atomic-scale probe of matter. At high intensities, such as produced at free-electron lasers, nonlinearities can become important, and the probe may no longer be considered weak. Here we report the observation of one of the most fundamental nonlinear X-ray-matter interactions: the concerted nonlinear Compton scattering of two identical hard X-ray photons producing a single higher-energy photon. The X-ray intensity reached 4 x 10(20) W cm(-2), corresponding to an electric field well above the atomic unit of strength and within almost four orders of magnitude of the quantum-electrodynamic critical field. We measure a signal from solid beryllium that scales quadratically in intensity, consistent with simultaneous non-resonant two-photon scattering from nearly-free electrons. The high-energy photons show an anomalously large redshift that is incompatible with a free-electron approximation for the ground-state electron distribution, suggesting an enhanced nonlinearity for scattering at large momentum transfer.
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