It is generally accepted that nitrogen-vacancy (NV) defects in bulk diamond are bright sources of luminescence. However, the exact value of their internal quantum efficiency (IQE) has not been measured so far. Here we use an implementation of Drexhage's scheme to quantify the IQE of shallow-implanted NV defects in a single-crystal bulk diamond. Using a spherical metallic mirror with a large radius of curvature compared to the optical spot size, we perform calibrated modifications of the local density of states around NV defects and observe the change of their total decay rate, which is further used for IQE quantification. We also show that at the excitation wavelength of 532 nm, photo-induced relaxation cannot be neglected even at moderate excitation powers well below the saturation level. For NV defects shallow implanted 4.5 ± 1 and 8 ± 2 nm below the diamond surface, we determine the quantum efficiency to be 0.70 ± 0.07 and 0.82 ± 0.08, respectively.
It is generally accepted that nitrogen-vacancy (NV) defects in bulk diamond are bright sources of luminescence. However, the exact value of their internal quantum efficiency (IQE) has not been measured so far. Here we use an implementation of Drexhage's scheme to quantify the IQE of shallow-implanted NV defects in a single-crystal bulk diamond. Using a spherical metallic mirror with a large radius of curvature compared to the optical spot size, we perform calibrated modifications of the local density of states around NV defects and observe the change of their total decay rate, which is further used for IQE quantification. We also show that at the excitation wavelength of 532 nm, photo-induced relaxation cannot be neglected even at moderate excitation powers well below the saturation level. For NV defects shallow implanted 4.5 ± 1 and 8 ± 2 nm below the diamond surface, we determine the quantum efficiency to be 0.70 ± 0.07 and 0.82 ± 0.08, respectively.
Nitrogen–vacancy (NV) centers in diamond are generally recognized as highly promising as indefinitely stable highly efficient single-photon sources. We report an experimental quantification of the brightness, radiative decay rate, nonradiative decay rate and quantum efficiency of single NV centers in diamond nanocrystals. Our experiments show that the commonly observed large spread in fluorescence decay rates of NV centers in nanodiamond is inconsistent with the common explanation of large nanophotonic mode-density variations in the ultra-small high-index crystals at near-unity quantum efficiency. We report that NV centers in 25 nm nanocrystals are essentially insensitive to local density of optical states (LDOS) variations that we induce at a dielectric interface by using liquids to vary the refractive index, and propose that quantum efficiencies in such nanocrystals are widely distributed between 0 and 20%. For single NV centers in larger 100 nm nanocrystals, we show that decay rate changes can be reversibly induced by nanomechanically approaching a mirror to change the LDOS. Using this scanning mirror method, for the first time we report calibrated quantum efficiencies of NV centers, and show that different but nominally identical nanocrystals have widely distributed quantum efficiencies between 10 and 90%. Our measurements imply that nanocrystals that are to be assembled into hybrid photonic structures for cavity QED should first be individually screened to assess fluorescence properties in detail.
The nitrogen–vacancy centre (NV) has drawn much attention for over a decade, yet detailed knowledge of the photophysics needs to be established. Under typical conditions, the NV can have two stable charge states, negative (NV − ) or neutral (NV 0 ), with photo-induced interconversion of these two states. Here, we present detailed studies of the ionization dynamics of single NV centres in bulk diamond at room temperature during illumination and its dependence on the excitation wavelength and power. We apply a recent method which allows us to directly measure the charge state of a single NV centre, and observe its temporal evolution. We find that the steady-state NV − population is always ⩽75% for 450–610 nm excitation wavelength. In combination with saturation measurements, we show that the optimal excitation wavelength is around 510–540 nm. Furthermore, the relative absorption cross-section of NV − is determined for 540–610 nm, revealing a double-peak structure. Finally, the energy of the NV − ground state of 2.6 eV below the conduction band is measured. These results reveal new insights into the charge state dynamics of the NV centre.
This thesis decay rate enhancement and photophysics of nitrogen vacancy (NV) centers in diamond and quantum emitters in hexagonal boron nitride (hBN). The NV center in diamond is a unique physical system that finds use in many quantum technologies such as quantum sensing, quantum information processing and single photon generation. Consisting of a substitutional nitrogen atom next to a vacant lattice site, the NV center introduces discrete electronic energy levels within the wide bandgap of diamond. The NV center can be optically excited and will emit single photons upon relaxation that hold information of the spin state of the NV center. In this thesis, we use metallic structures to modify and enhance the decay rate of NV centers in bulk and nano diamonds in three different experiments. Such an enhancement is a way to improve the usability of NV as a single photon source. Using TiN films with metamaterial properties we observe a 2-fold decrease of the total excited state lifetime of NV centers in nano diamonds, when comparing to similar NV centers not influenced by the TiN films. For shallow implanted NV centers in bulk diamond we experimentally investigate how a decay rate enhancement influences the signal-to-noise ratio (SNR) in spin readout measurements. In this experiment we perform spin state readout before and after introducing a rate enhancement and find a 30 % drop in spin contrast. We model the spin readout measurements using a five-level model and use that to predict the SNR under higher rate enhancement. In the last NV related experiment we use a curved metallic mirror on top of shallow implanted NV centers, to measure the continuous lifetime modification as a function of distances to the mirror. This allows us to determine the internal quantum efficiency (IQE) by fitting the experimental data with a model that includes calculating the expected Purcell factor. We find IQE values of 0.70±0.07 and 0.82±0.08 for implantation depths of 4.5±1 and 8±2 nm, respectively. Last but not least, newly discovered bright single photon emitters in two dimensional hBN were investigated. Similar to the NV center they are believed to be crystal defects, but the defect configuration is not fully known. In this thesis, the photophysics of quantum emitters in hBN nano flakes are investigated. Intensity autocorrelation measurements indicate the presence of at least two metastable states, seen by distinct photon bunching levels on long time scales in the g(2)(τ)-functions. We also observe blinking behaviour, which we show can be explained by changes in the IQE. From rate equation fitting we find that transition rates scale linearly with excitation power. Based on the transition rates we estimate the IQE to be close to unity for excitation powers below saturation.
Efficient creation of shallow NV− ensembles by high-angle ion implantation
Negatively charged nitrogen-vacancy (NV−) centers located a few nanometers below the diamond surface are key quantum defects for nanoscale sensing of external spins. However, the creation of shallow NV− centers with high yield remains a materials challenge. Here, we demonstrate that high-angle ion implantation enhances the creation efficiency of shallow NV− centers. By implanting 15N ions at angles exceeding 60∘, we achieve high NV− yields approaching 10% with effective NV− depths below 10 nm. These yields are significantly higher than those typically reported for shallow NV− creation. The enhanced NV− yield is consistent with an increased vacancy-to-nitrogen ratio in the near-surface region, which is expected to promote NV formation during annealing. The created NV− ensembles show coherence properties comparable to those of single shallow NV− centers at similar depths, and allow detection of nuclear spins in van der Waals materials attached to the diamond surface. Our results establish geometric control of ion implantation as a simple and broadly applicable approach to engineer shallow vacancy-related quantum defects.
Although the spin properties of superficial shallow nitrogen-vacancy (NV) centers have been the subject of extensive scrutiny, considerably less attention has been devoted to studying the dynamics of NV charge conversion near the diamond surface. Using multicolor confocal microscopy, here we show that near-surface point defects arising from high-density ion implantation dramatically increase the ionization and recombination rates of shallow NVs compared to those in bulk diamond. Further, we find that these rates grow linearly, not quadratically, with laser intensity, indicative of single-photon processes enabled by NV state mixing with other defect states. Accompanying these findings, we observe NV ionization and recombination in the dark, likely the result of charge transfer to neighboring traps. Despite the altered charge dynamics, we show that one can imprint rewritable, long-lasting patterns of charged-initialized, near-surface NVs over large areas, an ability that could be exploited for electrochemical biosensing or to optically store digital data sets with subdiffraction resolution.
The nitrogen-vacancy (NV) centre in diamond is a unique optical defect that is used in many applications today and methods to enhance its fluorescence brightness are highly sought after. We observed experimentally an enhancement of the NV quantum yield by up to 7% in bulk diamond caused by an external magnetic field relative to the field-free case. This observation is rationalised phenomenologically in terms of a magnetic field dependence of the NV excited state triplet-to-singlet transition rate. The theoretical model is in good qualitative agreement with the experimental results at low excitation intensities. Our results significantly contribute to our fundamental understanding of the photophysical properties of the NV defect in diamond.