A matter of brightness: table-top X-ray generation inside waveguides and X-ray holography with single free-electron laser pulses

Abstract

X-ray microscopy delivers insights into the structure of optically opaque bulk specimens with high spatial resolution. The source brightness poses a limit on the achievable resolution, however. While table-top X-ray sources are readily available but provide only low brightness, large-facility sources, such as synchrotrons and X-ray free-electron lasers (XFEL), generate radiation with high brightness and high coherence, but are not easily accessible. In this work, we report on and experimentally demonstrate a novel table-top X-ray source concept to generate spatially coherent X-rays with high brightness, that are emitted directly into the modes of a waveguide. Our estimate of the achievable gain increase demonstrates a substantial brightness improvement with respect to other table-top X-ray sources. In another set of experiments, we make use of the high peak brilliance of an XFEL to observe transient states of water under extreme conditions. In a pump-probe scheme, an infrared laser pulse generates a plasma after optical breakdown to seed a cavitation bubble, which we image with a single XFEL pulse. To get access to the pressure distribution within the shockwave of the cavitation bubbles, we calculate the quantitative phase shift based on a tailored phase-retrieval approach. We further complement nanofocus X-ray holography with time-resolved X-ray diffraction to obtain information on the molecular structure of water after dielectric breakdown. This combined approach delivers quantitative information from microscopic to molecular length scales with high temporal resolution.