  Optical resonators are increasingly important tools in science and
technology. Their applications range from laser physics, atomic clocks,
molecular spectroscopy, and single-photon generation to the detection, trapping
and cooling of atoms or nano-scale objects. Many of these applications benefit
from strong mode confinement and high optical quality factors, making small
mirrors of high surface-quality desirable. Building such devices in silicon
yields ultra-low absorption at telecom wavelengths and enables integration of
micro-structures with mechanical, electrical and other functionalities. Here,
we push optical resonator technology to new limits by fabricating
lithographically aligned silicon mirrors with ultra-smooth surfaces, small and
wellcontrolled radii of curvature, ultra-low loss and high reflectivity. We
build large arrays of microcavities with finesse greater than F = 500,000 and a
mode volume of 330 femtoliters at wavelengths near 1550 nm. Such high-quality
micro-mirrors open up a new regime of optics and enable unprecedented
explorations of strong coupling between light and matter.
