  Single-crystal diamond, with its unique optical, mechanical and thermal
properties, has emerged as a promising material with applications in classical
and quantum optics. However, the lack of heteroepitaxial growth and scalable
fabrication techniques remain major limiting factors preventing more
wide-spread development and application of diamond photonics. In this work, we
overcome this difficulty by adapting angled-etching techniques, previously
developed for realization of diamond nanomechanical resonators, to fabricate
racetrack resonators and photonic crystal cavities in bulk single-crystal
diamond. Our devices feature large optical quality factors, in excess of 10^5,
and operate over a wide wavelength range, spanning visible and telecom. These
newly developed high-Q diamond optical nanocavities open the door for a wealth
of applications, ranging from nonlinear optics and chemical sensing, to quantum
information processing and cavity optomechanics.
