Optically Induced Polarization and Depolarization of an Electron Spin in   a Nitrogen-Vacancy Center of Diamond Nano-crystals
  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. 
Robert Chapman and Taras Plakhotnik
cond-mat.mes-hall quant-ph