  Stimulated emission is the process fundamental to laser operation, thereby
producing coherent photon output. Despite negatively-charged nitrogen-vacancy
(NV$^-$) centres being discussed as a potential laser medium since the 1980's,
there have been no definitive observations of stimulated emission from
ensembles of NV$^-$ to date. Reasons for this lack of demonstration include the
short excited state lifetime and the occurrence of photo-ionisation to the
neutral charge state by light around the zero-phonon line. Here we show both
theoretical and experimental evidence for stimulated emission from NV$^-$
states using light in the phonon-sidebands. Our system uses a continuous wave
pump laser at 532 nm and a pulsed stimulating laser that is swept across the
phononic sidebands of the NV$^-$. Optimal stimulated emission is demonstrated
in the vicinity of the three-phonon line at 700 nm. Furthermore, we show the
transition from stimulated emission to photoionisation as the stimulating laser
wavelength is reduced from 700nm to 620 nm. While lasing at the zero-phonon
line is suppressed by ionisation, our results open the possibility of diamond
lasers based on NV centres, tuneable over the phonon-sideband. This broadens
the applications of NV magnetometers from single centre nanoscale sensors to a
new generation of ultra-precise ensemble laser sensors, which exploit the
contrast and signal amplification of a lasing system.
