Single-mode selection for hard x-ray cavity resonance.

Single-mode selection is realized for hard x-ray cavity resonance using a three-mirror crystal device. The developed device consists of two coupled Si Fabry-Perot resonators (FPRs) and uses (12 4 0) backward diffraction to reflect back and forth the incident 14.4388 keV x-ray beam. The coupling between the two cavities gives an effective single-mode spectrum with a bandwidth of 0.81 meV. This method can be used to enhance the longitudinal coherent length without affecting transverse coherence, and is potentially useful in generating nearly total coherent beams in synchrotron or free-electron laser facilities.