Röntgen materials for x-ray lasers-on-a-chip

Compact X-Ray laser is a hot topic in the field of laser research, enabling 24/7 advanced spectroscopy and overcoming the beamline bottleneck. The investigated systems are either scaled-down replicas of accelerators, or tabletop architectures based on high-harmonic generation, plasmas, or wakefield acceleration. Ideally, one would enable a large range of applications if the X-Ray source would be portable. For that, some groups are working on accelerators-on-a-chip. A new class of active materials exploiting distributed feedback was proposed 50 years ago, as a candidate for an X-Ray laser gain medium. A Fabry-Perot analysis of a selection of "röntgen materials", based on their refractive index, Bragg's coupling coefficient, and threshold gain, is presented. The alkaline earth metal oxide showed the highest gain value of all the materials considered in this work. A relationship between the refractive index of the material and the threshold gain value is given. In addition, details on the geometry of the gain medium are discussed. Theoretical analysis revealed that alkaline earth metal oxides are a promising material with a higher gain coefficient of about 77.4 nm-1 for a 0.001 μm3 crystal and the highest of all the materials investigated in this work. Except for alkaline earth metal oxide, all other oxide materials, such as transition and lanthanide metal oxide, have the lowest gain value. While nitrides, carbides, and compound semiconductors outperform oxide materials in terms of gain, they have still one order of magnitude less gain than alkaline earth metal oxide. The details of röntgen material calculations and design parameters are covered in depth.