Nuclear excitation and collective effects with coherent X-ray light

With present and upcoming X-ray light sources such as the X-ray Free Electron Laser (XFEL), the direct interaction between nuclei and super-intense laser fields has become feasible, opening the new field of nuclear quantum optics. One of the main difficulties with driving nuclear transitions arises from the weak coupling between nuclei and the electromagnetic field leading to very narrow bandwidths. In solid state targets that allow recoilless nuclear excitation and decay, the excitation caused by a single photon may be shared by a large number of nuclei, forming a collective excited state with a sometimes significantly larger decay width than that of a single nucleus. Following this line, in this work we investigate theoretically the direct laser-nucleus interaction taking into account for the first time the line broadening due to collective effects. Furthermore, we update former estimates considering the experimental advances of the XFEL regarding photon frequency, focus and intensity. Our results show an enhancement by several orders of magnitude of the excited state population compared to previously reported values.