Observation of a Topological Edge State in the X‐ray Band

The possibility of obtaining edge states of light that are robust against disorder by mimicking the topological properties of a solid-state system has brought a profound impact on optical sciences. In the short-wavelength region, X-ray science and technology is undergoing tremendous development. It requires high-precision optics with exquisite control of light properties and robustness against structural perturbations. Therefore, it is very attractive to extend the concept of optical topological manipulation to the X-ray regime. Herein, the topological edge state is theoretically proposed and experimentally demonstrated at the interface of two kinds of photonic crystals having different bandgap topological characteristics in the X-ray regime. Remarkably, this topologically protected edge state is immune to the thickness disorder as long as the zero-average-effective-mass condition is satisfied. This work extends the topological photonics to the X-ray regime and paves the way for the development of novel X-ray optics such as high-resolution X-ray filters/monochromators, or X-ray quantum optics, to realize the phenomena such as Rabi splitting that are robust against disorders.