X-ray Holography: from nano-spectroscopy to nonlinear coherent diffraction

Andreas Scherz

European XFEL GmbH, Albert-Einstein-Ring 19, 22761 Hamburg, Germany

Holography is one of the key X-ray microscopy techniques for high-resolution imaging that is compatible with full-field single-shot imaging in intense coherent beams. Using X-ray free-electron lasers scientists are pushing towards the recording of real space snapshots and movies of transient phenomena in materials on nanometer length scales and femtosecond time scales such as laser-driven ultrafast phase transitions, melting and nucleation dynamics in complex systems.

The soft X-ray energy range covers absorption resonances that are extremely useful for material sciences. Those core level resonances not only strongly enhance the X-ray diffraction yield but also add element-specific contrast as well as chemical and spin sensitivity to the holographic methodology. Since every photon counts in X-ray holography, the use of intense X-ray laser pulses also enhances the signal-to-noise and spatial resolution.

The feasibility of X-ray spectro-holography with X-ray laser pulses could however be called into question by the high intensity. These techniques are inherently connected to the X-ray absorption and therefore, single-shot imaging becomes prone to X-ray field-induced electronic modifications. Nonlinear phenomena at X-ray wavelengths ranging from multi-ionization, self-induced X-ray transparency, self-amplified spontaneous X-ray emission, and stimulated X-ray Raman scattering have all been reported. The nonlinear electronic response of matter to highly intense X-ray pulses is therefore the electronic analogue to the ‘’diffraction before destruction’’ challenge, faced by crystallographers in the structural determination of biomolecules and proteins.

In the first part I will introduce the spectroscopy and phase recording aspects of X-ray holography and then cover advanced holographic techniques such as extended references for high-resolution imaging. In the second part I will give an introduction to nonlinear X-ray optics diffraction imaging.