  Pulsed laser excitation causes the luminescence of Nitrogen Vacancy centers
in diamond to unexpectedly decrease with increasing pulse energy. This decrease
is observed in both the negatively charged and neutral centers and is caused by
shortening of the luminescence lifetimes of the centers of both types. In
darkness, the luminescence does not show any recovery on a time scale of 10
microseconds but as little as three low-intensity pulses can return the
luminescence to its previous, brighter state. An external magnetic field
reduces the magnitude of the effect. A possible mechanism for these phenomena
based on optical polarisation and depolarisation of an electronic spin is
proposed.
