Focusing of X-ray free-electron laser pulses with reflective optics

X-ray free-electron lasers 1 , 2 produce intense femtosecond pulses that have applications in exploring new frontiers in science. The unique characteristics of X-ray free-electron laser radiation can be enhanced significantly using focusing optics 3 . However, with such an optical device, even a slight deviation from the ideal design can lead to considerable errors in the focusing properties. Here, we present reflective optics comprising elliptically figured mirrors with nanometre accuracy to preserve a coherent wavefront, successfully focusing a 10 keV X-ray free-electron laser to the small area of 0.95 × 1.20 µm 2 . The near 100% efficiency of this arrangement allows an enormous 40,000-fold increase in the fluence to a power density of 6 × 10 17 W cm −2 . This achievement is directly applicable to the generation of a nanometre-size beam with an extreme power density of >1 × 10 22 W cm −2 , which will play a crucial role in the advance of microscopic research towards ultimate ångstrom resolution, as well as in the development of nonlinear optical sciences under extreme conditions. Researchers focus 10 keV X-ray free-electron laser radiation to an area of 0.95 µm × 1.20 µm with near-100%-efficiency using reflective optics. This approach increases the fluence by a factor of 40,000 and provides a power density of 6 × 10 17 W cm −2 .