Direct writing of high-density nitrogen-vacancy centers inside diamond by femtosecond laser irradiation

High-density nitrogen-vacancy (NV) centers exceeding 1016/cm3 inside a diamond can be achieved by femtosecond laser irradiation. The number of pulses and pulse energy were adjusted considering the increasing trend of the NV concentration and damage generation. The NV concentration first grew as the number of laser pulses was increased, and then the concentration was temporarily saturated or decreased. By increasing the number of laser pulses more than 2.5–5 × 107, the concentration increased again. The Hahn-echo measurement revealed that electron spins of the substitutional nitrogen center (P1) were dominant source for decoherence and the number of paramagnetic defects induced by the laser irradiation was not significant for degradation of spin coherence of the laser-induced NV centers even without a post-annealing process. Such high-dense ensemble NV centers are crucial for a high-sensitive quantum sensor.