The invention of optical lasers led to a revolution in the field of optics and to the creation of such fields of research as quantum optics. The reason was their unique statistical and coherence properties. The emerging, short-wavelength free-electron lasers (FELs) are sources of very bright coherent extreme-ultraviolet and X-ray radiation with pulse durations on the order of femtoseconds, and are presently considered to be laser sources at these energies. FELs are highly spatially coherent to the first-order but in spite of their name, behave statistically as chaotic sources. Here, we demonstrate experimentally, by combining Hanbury Brown and Twiss interferometry with spectral measurements that the seeded XUV FERMI FEL-2 source does indeed behave statistically as a laser. The results may be useful for quantum optics experiments and for the design and operation of next generation FEL sources.

Using coherent X-ray diffraction imaging (CXDI) as an in situ tool, we determined the shape and strain state of a platinum nanoparticle with ≈160 nm diameter supported by a strontium titanate substrate. The experiment was performed at a temperature of 400 K under continuous gas flow conditions of pure Ar and Ar/CO mixtures. The nanoparticle was preselected by scanning electron microscopy (SEM) and postanalyzed by atomic force microscopy (AFM). We obtain a very good agreement between the overall nanoparticle size, shape, and defect structure as determined by CXDI and AFM. In addition, we compare the strain state in the nanoparticle near surface region and its bulk: For pure Ar flow, we find a slight compressive strain in the nanoparticle bulk compared to an expansion in the near surface region. We ascribe the latter to the presence of high index vicinal surfaces. Our analysis suggests that under mixed Ar/CO flow at 400 K reshaping of the nanoparticle occurred. New high index facets developed, leading to a stronger lattice expansion, also propagating into the nanoparticle bulk. Our high-resolution experiments pave the way for future CXDI experiments under operando catalytic reaction conditions.

In III-V nanowires the energetic barriers for nucleation in the zinc blende or wurtzite arrangement are typically of a similar order of magnitude. As a result, both arrangements can occur in a single wire. Here, we investigate the evolution of this polytypism in self-catalyzed GaAs nanowires on Si(111) grown by molecular beam epitaxy with time-resolved in situ x-ray diffraction. We interpret our data in the framework of a height dependent Markov model for the stacking in the nanowires. In this way, we extract the mean sizes of faultless wurtzite and zinc blende segments—a key parameter of polytypic nanowires—and their temporal evolution during growth. Thereby, we infer quantitative information on the differences of the nucleation barriers including their evolution without requiring a model of the nucleus.

Here we present the results of a classical ghost imaging experiment accomplished at an XUV free-electron laser (FEL). To perform such experiment at an FEL source each x-ray pulse was transmitted through a moving diffuser, which created a noncorrelated speckled beam. This beam was then split in two identical branches by introducing a beam splitter in the form of a transmission grating. In one of these branches the sample was positioned. We demonstrate the possibility of image formation, a double bar in our case, in the beam that has never interacted with the sample. With this experiment we extend the quantum optics methodology to the FEL community.