  A study of the photophysical properties of nitrogen-vacancy (NV) color
centers in diamond nanocrystals of size of 50~nm or below is carried out by
means of second-order time-intensity photon correlation and cross-correlation
measurements as a function of the excitation power for both pure charge states,
neutral and negatively charged, as well as for the photochromic state, where
the center switches between both states at any power. A dedicated three-level
model implying a shelving level is developed to extract the relevant
photophysical parameters coupling all three levels. Our analysis confirms the
very existence of the shelving level for the neutral NV center. It is found
that it plays a negligible role on the photophysics of this center, whereas it
is responsible for an increasing photon bunching behavior of the negative NV
center with increasing power. From the photophysical parameters, we infer a
quantum efficiency for both centers, showing that it remains close to unity for
the neutral center over the entire power range, whereas it drops with
increasing power from near unity to approximately 0.5 for the negative center.
The photophysics of the photochromic center reveals a rich phenomenology that
is to a large extent dominated by that of the negative state, in agreement with
the excess charge release of the negative center being much slower than the
photon emission process.
