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

X-ray free-electron lasers 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. 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 μm2. The near 100% efficiency of this arrangement allows an enormous 40,000-fold increase in the fluence to a power density of 6 × 1017 W cm-2. This achievement is directly applicable to the generation of a nanometre-size beam with an extreme power density of >1× 1022 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. © 2013 Macmillan Publishers Limited.