We are inviting bachelor and master students within the scope of the DESY summer student program.

The ultrashort high-intensity pulses from x-ray free-electron lasers can drive the matter in novel states. In our group, we investigate the x-ray self-amplifying state that transiently appears after massive inner-shell photoionization. In these conditions, spontaneously emitted x-ray fluorescence develops into collective emission resulting in short and intense x-ray bursts – that can be used as a spectroscopic tool or as an x-ray source with unique properties. Theoretical description of this process is challenging – it requires quantum-mechanical treating of the initial stage, dealing with the macroscopic amount of emitters, and accounting for field propagation effects. Within the summer student programme, we propose to start with theoretical modeling of collective emission from few (two-level) atoms – a solvable case where one can obtain insights into the collective emission process. As a next step, ways to generalize to the macroscopic amount of atoms can be considered and crossover to continuous description can be studied. The results of the project may help in understanding the role of a local arrangement of atoms on collective x-ray emission properties, and could be investigated further jointly. We expect from the candidates the knowledge of quantum mechanics (quantum optics) and basic skills in numerical modeling (e.g., in Python). A detailed escription of the project can be found here.

For further information, please contact Dr. Andrei Benediktovitch.

We have PhD positions open within Max Planck School of Photonics!

  • X-ray diffraction from population-inverted atoms: opportunities for single-particle imaging

X-rays provide a unique opportunity to obtain the structure of matter at atomic resolution. In crystals (periodic arrangement of atomic or molecular constituents) x-ray diffraction is successfully used over more than 100 years to unravel the atomic and electronic structure with applications ranging from simple materials to large biological complexes. Despite the advent of novel, ultrabright x-ray sources -- x-ray free-electron lasers (XFELs) -- the study of single particles of biological interest remains challenging. The challenge manifests itself in the inherently small elastic x-ray scattering strength (giving rise to diffraction) combined with strong competing processes such as ionization and/or Compton scattering. In this project, we will develop a novel imaging technique, relying on two-color pulses of XFELs: The first x-ray pulse will prepare atoms of the sample in core-excited states by promoting an electron of the inner-most electronic shell into a valence shell. The second x-ray pulse, tuned to an inner-shell transition (for example K-alpha transition), will elastically scatter on a set of atoms in states of population inversion. Two effects will enhance the scattering signal: On resonance, anomalous x-ray scattering gives an enhancement of the scattering strength. Moreover, scattering on core-inverted atoms can result in stimulated emission, that eventually gives rise to an exponentially enhanced signal amplification. The signal from population-inverted atoms can be analyzed together with non-resonant scattering from other atoms of the object, thus enhancing the contrast. The successful candidate will develop the concept and theory of the novel approach and in the later stage of the project, will participate in proof-of-concept experiments at XFEL sources.

  • Source of entangled photon pairs and triplets via parametric down conversion in photonic crystals: opportunities from many-beam diffraction

Generation of entangled-photon pairs via parametric down-conversion (PDC) is an essential ingredient of numerous quantum-optics and quantum-information setups. To achieve the PDC generation, nonlinear response of the crystal in combination with phase-matching conditions is employed. The generation of entangled photon-triplets (for example, Greenberger–Horne–Zeilinger states) would provide further opportunities, however is even more challenging due to higher nonlinearities needed and further restrictions on phase-matching. In the current project, we propose to use photonic crystals (structures with artificially created spatial periodicity) to enhance the generation of PDC photons. Namely, we aim at a systematic exploration of the additional degrees of freedom emerging thanks to artificial periodicity (orientation of reciprocal lattice vectors) that can be used to steer the phase-matching conditions. The shaping of the electromagnetic field in the periodic structures (many-beam diffraction) was well-studied for x-rays and natural crystals. In this project, we aim at transferring the concepts from x-ray crystallography to photonic-crystals optics in order to find opportunities for enhanced production of entangled photons.

If you are interested in these topics – or in other research projects within the scope of activity of our group – you are welcome to contact us!