  Photoluminescence (PL) spectra of single silicon vacancy (SiV) centers
frequently feature very narrow room temperature PL lines in the near-infrared
(NIR) spectral region, mostly between 820 nm and 840 nm, in addition to the
well known zero-phonon-line (ZPL) at approx. 738 nm [E. Neu et al., Phys. Rev.
B 84, 205211 (2011)]. We here exemplarily prove for a single SiV center that
this NIR PL is due to an additional purely electronic transition (ZPL). For the
NIR line at 822.7 nm, we find a room temperature linewidth of 1.4 nm (2.6 meV).
The line saturates at similar excitation power as the ZPL. ZPL and NIR line
exhibit identical polarization properties. Cross-correlation measurements
between the ZPL and the NIR line reveal anti-correlated emission and prove that
the lines originate from a single SiV center, furthermore indicating a fast
switching between the transitions (0.7 ns). g(2) auto-correlation measurements
exclude that the NIR line is a vibronic sideband or that it arises due to a
transition from/to a meta-stable (shelving) state.
