Abstract
The success1,2 of X-ray free-electron lasers (XFELs) has extended the frontier of nonlinear optics into the hard X-ray region. Recently, sum-frequency generation3 has been reported, as well as parametric downconversion4,5,6. These are of the lowest (second) order, and higher-order processes remain unexplored. Here, we report the first observation of a third-order process: two-photon absorption of a 5.6 keV XFEL beam by germanium. We find that two-photon absorption competes with single and sequential multiphoton processes7,8, but we successfully determine the intrinsic cross-section by analysing the pulse-energy dependence. We also discuss the two-photon absorption cross-section by comparing a new mechanism unique to X-rays with the conventional mechanism and show that the latter is consistent with the present result. The experimental determination and understanding of the cross-section would allow two-photon absorption spectroscopy. Our result indicates that X-ray analogues of other third-order nonlinear optical processes9, such as nonlinear Raman and optical Kerr effects, are available for XFEL applications in spectroscopy, imaging and beam control.
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Acknowledgements
The authors thank H. Yoneda, T. Hara, T. Tanaka and T. Hatsui for helpful discussions. This work was supported by a grant-in-aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan (23360038 and 23226004). The experiments were performed with the approval of JASRI (proposal no. 2012B8006).
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K.T., E.S. and Y.I. designed the experiment. H.Y., H.O., H.M. and K.Y. designed the focusing optics and the sample chamber. K.T., E.S., Y.I. and T.K. acquired the experimental data. K.T. and K.S. performed theoretical calculations. K.T. analysed data and wrote the manuscript. All authors discussed the results and commented on the manuscript.
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Tamasaku, K., Shigemasa, E., Inubushi, Y. et al. X-ray two-photon absorption competing against single and sequential multiphoton processes. Nature Photon 8, 313–316 (2014). https://doi.org/10.1038/nphoton.2014.10
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DOI: https://doi.org/10.1038/nphoton.2014.10
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