Effects of magnetic field on the low temperature emission of nitrogen vacancy centres in diamond

Under optical illumination in the blue or green, negatively charged nitrogen vacancy centres in diamond emit in the red. The intensity of this emission varies slightly depending on spin state occupation. Optical transitions occur predominantly without change of spin projection. However, excited m _s = ±1 states can decay non-radiatively to the ground m _s = 0 state via intermediary singlet states.With continuous excitation, this effect transfers most of the population to the m_s = 0 state resulting in decay becoming almost entirely radiative so that optical emission is stronger than when all spin states are occupied equally. However, under optical illumination the m_s = 0 polarization is reduced when an applied magnetic field induces avoided crossings between energy levels. The spin states are mixed and some population is diverted into the m_s = ±1 states with consequent reduction in optical emission. The change of emission can be calculated from a rate equation model involving the spin states of the ground and excited levels plus one singlet level [5]. The spin states of the excited levels are also affected by strain and in this work we calculate the variation in optical emission with changing magnetic field for various fixed values of strain.