Development of portable nanofocusing optics for X-ray free-electron laser pulses

We present the development of a portable and compact nanofocusing system utilizing Kirkpatrick-Baez optics for X-ray free-electron lasers (XFELs). The system has a total length of merely 340 mm from the initial elliptical mirror to the focal point. With this setup, we achieve focusing capabilities reaching dimensions as small as 150 nm horizontally and 220 nm vertically, resulting in an intense beam with an intensity of 2.5 × 10¹⁹ W cm^-2 for a 10 keV XFEL with a pulse energy of 110 µJ. As a demonstration of its applicability in X-ray nonlinear optics, we successfully observed an 8.64 keV Zn Kα laser by irradiating a Zn foil with the focused XFEL beam. The attained photon energy is currently the highest photon energy for amplified spontaneous emission via bound-bound transitions of electrons in the world. We anticipate that this portable and compact nanofocusing system will pave the way for new frontiers in X-ray nonlinear optics and high-energy density science, owing to its adaptability to various experimental conditions.