Generation of transient nuclear inversion using XFEL

We collaborate with Adriana Pálffy-Buß from university Würzburg and show that transient nuclear population inversion can be coherently generated by a focused x-ray free electron laser pulse in a tapered waveguide where a nuclear sample is placed at the focal point. We theoretically show that elliptical waveguides using a cladding material with a high atomic number, such as platinum, can maintain an x-ray intensity of up to three orders of magnitude larger than in free space. This feature can be used to place a nuclear sample in the waveguide focal area and drive nuclear Mössbauer transitions up to transient nuclear population inversion. The latter is a long-standing goal related to gamma-ray lasers or nuclear state population control for energy storage. We show that inverted nuclei numbers of up to approximately 2×10^5 are achievable in the realistic region of longitudinal x-ray-free-electron-laser coherence time ≤ 10fs. Our results anticipate the important role of tapered x-ray waveguides and strategically embedded samples in the field of x-ray quantum optics. This work is published in Phys. Rev. Research 4, L032007 (2022).