  Synthetic single-crystal diamond has recently emerged as a promising platform
for Raman lasers at exotic wavelengths due to its giant Raman shift, large
transparency window and excellent thermal properties yielding a greatly
enhanced figure-of-merit compared to conventional materials. To date, diamond
Raman lasers have been realized using bulk plates placed inside macroscopic
cavities, requiring careful alignment and resulting in high threshold powers
(~W-kW). Here we demonstrate an on-chip Raman laser based on fully-integrated,
high quality-factor, diamond racetrack micro-resonators embedded in silica.
Pumping at telecom wavelengths, we show Stokes output discretely tunable over a
~100nm bandwidth around 2-{\mu}m with output powers >250 {\mu}W, extending the
functionality of diamond Raman lasers to an interesting wavelength range at the
edge of the mid-infrared spectrum. Continuous-wave operation with only ~85 mW
pump threshold power in the feeding waveguide is demonstrated along with
continuous, mode-hop-free tuning over ~7.5 GHz in a compact, integrated-optics
platform.
