X-ray back diffraction from monolithic two silicon crystal plates of $25–150\mu \mathrm{m}$ thickness and a $40–150\mu \mathrm{m}$ gap using synchrotron radiation of energy resolution $\Delta E=0.36\mathrm{meV}$ at 14.4388 keV clearly show resonance fringes inside the energy gap and the total-reflection range for the (12 4 0) reflection. This cavity resonance results from the coherent interaction between the x-ray wave fields generated by the two plates with a gap smaller than the x-ray coherence length. This finding opens up new opportunities for high-resolution and phase-contrast x-ray studies, and may lead to new developments in x-ray optics.

We report the details of the recent x-ray back diffraction experiments, in which interference fringes due to x-ray cavity resonance are unambiguously observed. The Fabry-Pérot type cavities, the tested crystal devices of reflectivity $R\simeq 0.5$ and finesse $F\simeq 2.3$, consist of monolithic two-plate and eight-plate silicon crystals. They were prepared by using x-ray lithographic techniques. The thicknesses of the crystal plates and the gaps between the two adjacent plates are a few tens to hundreds $\mu \mathrm{m}$. The (12 4 0) back reflection and synchrotron x-radiation of energy resolution $\Delta E=0.36\mathrm{meV}$ at $14.4388\mathrm{keV}$ are employed. Interference fringes in angle- and photon-energy scans for two-plate and eight-plate cavities are shown. Considerations on the temporal and spatial coherence for observable resonance interference fringes using synchrotron x-rays are presented. The details about the accompanied simultaneous 24-beam diffraction in relation to x-ray photon energy are also described.