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Description
The XFELs with an anomalously high peak brilliance are opening the way to a number of novel X-ray photon research paths. At SPring-8 Angstrom Compact Free-Electron Laser (SACLA) [1], the XFEL pulses with high stability and short pulse duration (6-7 fs) have been regularly provided thanks to the unique electron gun, accelerator, and undulator systems [2]. By focusing these XFELs to 1um-100nm, the peak intensity has been dramatically increased and new phenomena in hard X-ray nonlinear optics have been explored, such as observation of saturable absorption [3], two-photon absorption [4], and the atomic inner-shell laser emission [5]. To further promote the study in the ultra-intense X-ray laser field, we have developed a focusing system that achieves sub-10nm spot size and 1022 W/cm2 intensity.
For the sub-10 nm focusing optics, an advanced Kirkpatrick-Baez (AKB) mirror system based on Wolter-type III geometry [6] has been adopted. The AKB consisting of one-dimensional Wolter mirrors can satisfy Abbe’s sine condition, which leads to a reduced coma aberration and a high tolerance to the incident angle error. We have designed and developed the AKB mirror system for SACLA BL3-EH4c at a photon energy of 9.1 keV. One of the remarkable challenges for the development was the fabrication of the mirrors with 1-nm accuracy. We applied an X-ray wavefront correction scheme [7] for the precise fabrication, and achieved wavefront accuracy of λ/15 rms which satisfies Maréchal’s criterion. Ptychographic probe measurements revealed the focusing spot size of 6.6 nm (horizontal) × 7.1 nm (vertical), indicating eventually attained focused intensity of 1.21 × 1022 W/cm2.
References:
[1] T. Ishikawa et al., Nat. Photon. 6 (2012).
[2] For example, I. Inoue et al., Phys. Rev. Lett. 127 (2021). & T. Osaka et al., Phys. Rev. Research 4 (2022).
[3] H. Yoneda et al., Nat. Commun. 5 (2014).
[4] K. Tamasaku et al. Nat. Photon. 8 (2014). & K. Tamasaku et al., Phys. Rev. Lett 121 (2018).
[5] H. Yoneda et al., Nature 524 (2015).
[6] J. Yamada et al., Opt. Express 3 (2019).
[7] S. Matsuyama et al., Sci. Rep. 8 (2018).
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