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arXiv:1107.4968 · Physical Review B

Anomalous saturation effects due to optical spin depolarization in nitrogen-vacancy centers in diamond nanocrystals

Robert Chapman, Taras Plakhotnik

Published Jul 10, 2012• 21 citations
arXivHTMLPDF

Abstract

The photoluminescence of the negatively charged nitrogen-vacancy (${\mathrm{NV}}^{\ensuremath{-}}$) center displays anomalous saturation behavior at high excitation pulse energies. Where the luminescence is expected to approach a maximum value asymptotically, we have found that it instead drops by as much as a factor of two. In this report, we present evidence that these effects are caused by optical spin depolarization of the ${\mathrm{NV}}^{\ensuremath{-}}$ center. We show that the presence of an external magnetic field results in a marked decrease in the anomalous character of the ${\mathrm{NV}}^{\ensuremath{-}}$ centers' luminescence and demonstrate that low-energy pulses applied after a strong depolarizing pulse can repolarize the spin of the center. We also offer a model and derive important parameters describing dynamics in the NV center and its interaction with light, which quantitatively explain all the observations.

DOI
10.1103/physrevb.86.045204

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The photoluminescence of the negatively charged nitrogen-vacancy (NV\ensuremath−{\mathrm{NV}}^{\ensuremath{-}}NV\ensuremath−) center displays anomalous saturation behavior at high excitation pulse energies. Where the luminescence is expected to approach a maximum value asymptotically, we have found that it instead drops by as much as a factor of two. In this report, we present evidence that these effects are caused by optical spin depolarization of the NV\ensuremath−{\mathrm{NV}}^{\ensuremath{-}}NV\ensuremath− center. We show that the presence of an external magnetic field results in a marked decrease in the anomalous character of the NV\ensuremath−{\mathrm{NV}}^{\ensuremath{-}}NV\ensuremath− centers' luminescence and demonstrate that low-energy pulses applied after a strong depolarizing pulse can repolarize the spin of the center. We also offer a model and derive important parameters describing dynamics in the NV center and its interaction with light, which quantitatively explain all the observations.
The electron spin of the negatively charged the nitrogen vacancy center $({\mathrm{NV}}^{\ensuremath{-}}$) in diamond can be optically polarized through intersystem crossing, which enables the defect to be used for quantum computation and metrology. In this work, we studied the electron spin depolarization effect of the NV center induced by charge state conversion, which was proven to be a spin-independent process. The spin-state initialization fidelity was largely affected by the charge state conversion process. As a result, the optical polarization of the electron spin decreased about 1414%(31%14) with a high-power continuous-wave (pulsed) green laser. Moreover, the undefined fluorescence anomalous saturation effect of the NV center was analyzed and explained in detail based on the spin depolarization. The results demonstrated that a weak laser should be used for initialization of the NV center. In addition, the power and polarization of a laser for NV spin detection should be carefully adjusted to obtain the highest fluorescence signal. Our work also provided information that can increase the understanding of the charge state conversion and spin polarization processes of the NV center for quantum information and sensing.
We investigate the magnetic field dependent photophysics of individual nitrogen-vacancy (NV) color centers in diamond under cryogenic conditions. At distinct magnetic fields, we observe significant reductions in the NV photoluminescence rate, which indicate a marked decrease in the optical readout efficiency of the NV's ground state spin. We assign these dips to excited state level anticrossings, which occur at magnetic fields that strongly depend on the effective, local strain environment of the NV center. Our results offer new insights into the structure of the NVs' excited states and a new tool for their effective characterization. Using this tool, we observe strong indications for strain-dependent variations of the NV's orbital g factor, obtain new insights into NV charge state dynamics, and draw important conclusions regarding the applicability of NV centers for low-temperature quantum sensing.
The nitrogen vacancy (NV) center in diamond is the most widely studied color center in diamond for its wide range of applications in both physics and biology. The negatively charged state ${\mathrm{NV}}^{\ensuremath{-}}$ is often favored for its better spin-optical properties, whereas the presence of the neutral charge state NV0{\mathrm{NV}}^{0}NV0 is often minimized or neglected by using a single optimized excitation wavelength. In many cases, the use of additional laser wavelengths has an important impact on the NV optical properties and can lead to a dramatically quenched fluorescence. Through measurements of the photophysics of the NV center we show in this work that this quenching mechanism is mediated by charge state interconversion providing additional channels for nonradiative decay. Our results show that this mechanism is also sensitive to the spin state of the ${\mathrm{NV}}^{\ensuremath{-}}$. A better understanding of this physical mechanism will help mitigate the spin and charge state depolarization effects that are detrimental for quantum technology applications.
We report a study of the magnetic field dependence of the photo-luminescence of NV−^-− centers (negatively charged nitrogen-vacancy centers) in diamond single crystals. In such a magnetic field dependence characteristic lines are observed, which are coming from Level Anti-Crossings (LACs) in the coupled electron-nuclear spin system. For enhancing the sensitivity, we used lock-in detection to measure the photo-luminescence intensity and observed a remarkably strong dependence of the LAC-derived lines on the modulation frequency. Upon decreasing of the modulation frequency from 12 kHz to 17 Hz the amplitude of the lines increases by approximately two orders of magnitude. To take a quantitative account for such effects, we developed a theoretical model, which describes the spin dynamics in the coupled electron-nuclear spin system under the action of an oscillating external magnetic field. Good agreement between experiments and theory allows us to conclude that the observed effects are originating from coherent spin polarization exchange in the NV−^-− center. Our results are of great practical importance allowing one to optimize the experimental conditions for probing LAC-derived lines in diamond defect centers.
The photophysics and charge state dynamics of the nitrogen vacancy (NV) center in diamond has been extensively investigated, but is still not fully understood. In contrast to previous work, we find that NV0{\mathrm{NV}}^{0}NV0 converts to ${\mathrm{NV}}^{\ensuremath{-}}$ under excitation with low power near-infrared (1064-nm) light, resulting in increased photoluminescence from the ${\mathrm{NV}}^{\ensuremath{-}}$ state. We used a combination of spectral and time-resolved photoluminescence experiments and rate-equation modeling to conclude that NV0{\mathrm{NV}}^{0}NV0 converts to ${\mathrm{NV}}^{\ensuremath{-}}$ via absorption of 1064-nm photons from the valence band of diamond. We report fast quenching and recovery of the photoluminescence from both charge states of the NV center under low power 1064-nm laser excitation, which has not been previously observed. We also find, using optically detected magnetic resonance experiments, that the charge transfer process mediated by the 1064-nm laser is spin dependent.
Nitrogen vacancy (NV) centers in diamond have been identified over the past few years as promising systems for a variety of applications, ranging from quantum information science to magnetic sensing. This relies on the unique optical and spin properties of the negatively charged NV. Many of these applications require shallow NV centers, i.e., NVs that are close (a few nm) to the diamond surface. In recent years there has been increasing interest in understanding the spin and charge dynamics of NV centers under various illumination conditions, specifically under infrared (IR) excitation, which has been demonstrated to have significant impact on the NV centers' emission and charge state. Nevertheless, a full understanding of all experimental data is still lacking, with further complications arising from potential differences between the photodynamics of bulk and shallow NVs. Here we suggest a generalized quantitative model for NV center spin- and charge-state dynamics under both green and IR excitation. We experimentally extract the relevant transition rates, providing a comprehensive model which reconciles all existing experimental results in the literature, except for highly nonlinear regimes. Moreover, we identify key differences between the photodynamics of bulk and shallow NVs, and use them to significantly enhance the initialization fidelity of shallow NVs to the useful negatively charged state.