Research at the European XFEL on multiphoton collective lambshift in nuclear resonant scattering

Ralf Röhlsberger from Friedrich Schiller University Jena examines special light-matter interaction. The observation of the collective Lambshift (CLS) of Mössbauer nuclei has recently opened the field of experimental nuclear quantum optics. The origin of the CLS lies in the exchange of virtual photons within an ensemble of closely spaced identical atoms, e.g., in an x-ray waveguide, which leads to an effective dipole-dipole interaction of the atoms resulting in a shift of the collective resonance energy. At present-day synchrotron radiation sources only the single-photon collective Lambshift is experimentally accessible.
The situation has changed significantly with the availability of hard X-ray free-electron lasers, where single pulses can contain several hundred resonant photons able to excite ensembles of 57Fe nuclei. After successful detection of first nuclear resonant scattering at the European XFEL in the previous experiment, in this continuation experiment the CLS as function of photon number was determined at the Materials Imaging & Dynamics instrument (MID).