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 x 1.20 mu m(2). The near 100% efficiency of this arrangement allows an enormous 40,000-fold increase in the fluence to a power density of 6 x 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 x 10(22) W cm(-2), which will play a crucial role in the advance of microscopic research towards ultimate angstrom resolution, as well as in the development of nonlinear optical sciences under extreme conditions.