Direct Observation of X-ray Resonance Effects in Crystal Cavity Using Sub-meV Resolution Synchrotron Radiation*

X-ray resonator, or cavity, has long been proposed and considered as a first step in realizing an X-ray laser for more than three decades. Attempt to realize the resonance in a crystal cavity has been pursued from time to time but with limited success. The difficulty arises mainly from lack of a sufficient energy resolution of X-rays and of a small-gap crystal cavity. That is, the required temporal coherence is not retained. With a high energy resolution of $\Delta $E=0.36 meV at 14.438 keV of X-ray synchrotron radiation and crystal plates with 100$\sim $150 $\mu $m gaps prepared by the microelectronic lithography technique, here we report the realization of a Fabry-Perot resonator for hard X-rays. Interference fringes inside the total reflection range in angle scans and inside the energy gap in energy scans near and at the (12 4 0) reflection position for two- and eight-plate silicon crystal cavities are clearly observed. This finding suggests many fundamental investigations and applications in high-resolution X-ray optics.