Realization of a Monolithic High-Reflectivity Cavity Mirror from a Single Silicon Crystal

We report on the first experimental realization of a high-reflectivity cavity mirror that solely consists of a single silicon crystal. Since no material was added to the crystal, the urgent problem of `` coating thermal noise'' that currently limits classical as well as quantum measurements is avoided. Our mirror is based on a surface nanostructure that creates a resonant surface waveguide. In full agreement with a rigorous model we realized a reflectivity of (99.79 +/- 0.01)% at a wavelength of 1.55 mu m, and achieved a cavity finesse of 2784. We anticipate that our achievement will open the avenue to next generation high-precision experiments targeting fundamental questions of physics.