Photoluminescence of Nitrogen-Vacancy Centers by Ultraviolet One- and Two-Photon Excitation of Fluorescent Nanodiamonds.

Fluorescent nanodiamonds contain nitrogen-vacancy (NV) centers as quantum defects. When exposed to a continuous-wave 325 nm laser or a femtosecond 344 nm laser, the particles emit red fluorescence from NV 0 centers at ∼620 nm. Power dependence measurements of the emission strength revealed a predominantly linear behavior at the laser peak intensity lower than 1 GW·cm -2 , contributed mainly by photoexcitation of electrons from the valence band of diamond to the NV 0 centers, followed by relaxation via electron-hole recombination. In the higher power regions, however, nonresonant two-photon interband excitation of the diamond matrix dominates the photoluminescence processes. Best fits of the experimental data to semiempirical models revealed an ionization coefficient of ∼1 cm -1 for the one-photon valence-to-defect excitation and a saturation intensity of 180 ± 60 GW·cm -2 for the two-photon interband excitation. The study provides new insight into the photoionization of NV 0 centers and the interband excitation properties of diamond in the UV region.