Two-photon Excited Fluorescence of Nitrogen-Vacancy Centers in Proton-Irradiated Type Ib Diamond

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Abstract
Two-photon fluorescence spectroscopy of negatively charged nitrogen-vacancy [(N-V)-] centers in type Ib diamond single crystals have been studied with a picosecond (7.5 ps) mode-locked Nd:YVO4 laser operating at 1064 nm. The (N-V)- centers were produced by radiation damage of diamond using a 3 MeV proton beam, followed by thermal annealing at 800 °C. Prior to the irradiation treatment, infrared spectroscopy of the C−N vibrational modes at 1344 cm-1 suggested a nitrogen content of 109 ± 10 ppm. Irradiation and annealing of the specimen led to the emergence of a new absorption band peaking at ∼560 nm. From a measurement of the integrated absorption intensity of the sharp zero-phonon line (637 nm) at liquid nitrogen temperature, we determined a (N-V)- density of (4.5 ± 1.1) × 1018 centers/cm3 (or 25 ± 6 ppm) for the substrate irradiated at a dose of 1 × 1016 H+/cm2. Such a high defect density allowed us to observe two-photon excited fluorescence and measure the corresponding fluorescence decay time. No significant difference in the spectral feature and fluorescence lifetime was observed between one-photon and two-photon excitations. Assuming that the fluorescence quantum yields are the same for both processes, a two-photon absorption cross section of σTPA = (0.45 ± 0.23) × 10-50 cm4·s/photon at 1064 nm was determined for the (N-V)- center based on its one-photon absorption cross section of σOPA = (3.1 ± 0.8) × 10-17 cm2 at 532 nm. The material is highly photostable and shows no sign of photobleaching even under continuous two-photon excitation at a peak power density of 3 GW/cm2 for 5 min.