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Laser Physics

Homogeneity of the negatively charged assembly of nitrogen vacancy centres in diamonds using the Quasi-finite-element optical scanning position method

Doudou Zheng, Zongmin Ma, Yangang Zhang, Yueping Fu, Jiuyan Wei, Hua Yuan, Li Qin, Yunbo Shi, Jun Tang, Jun Liu, Yan Jun Li, Yasuhiro Sugawara

Published Feb 25, 2021• 1 citation
Publisher

Abstract

Abstract We developed a versatile Quasi-finite-element scanning method to analyse the concentration and homogeneity of the ensembles of nitrogen-vacancy (NV − ) centres in block diamonds. In this method, 13 μ m × 13 μ m area of the bulk diamond with high-density NV − centres was divided into small pieces at the nanoscale by using a focused light beam. The concentration of NV − centres was obtained and imaged through the fluorescence intensity of each pixel by employing a home-built spectrometer. In addition, global and local homogeneities were introduced to evaluate the NV − centre distribution. NV − centres in a specific pixel exhibited preferred direction growth peaks attributed to the diamond crystal orientation. Moreover, the uniformity of NV − centres was not proportional to the electron irradiation duration, but an optimal time was observed. This study can be beneficial for enhancing the sensitivities of bulk magnetometry based on confocal systems.

DOI
10.1088/1555-6611/abe46e

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Abstract We developed a versatile Quasi-finite-element scanning method to analyse the concentration and homogeneity of the ensembles of nitrogen-vacancy (NV − ) centres in block diamonds. In this method, 13 μ m × 13 μ m area of the bulk diamond with high-density NV − centres was divided into small pieces at the nanoscale by using a focused light beam. The concentration of NV − centres was obtained and imaged through the fluorescence intensity of each pixel by employing a home-built spectrometer. In addition, global and local homogeneities were introduced to evaluate the NV − centre distribution. NV − centres in a specific pixel exhibited preferred direction growth peaks attributed to the diamond crystal orientation. Moreover, the uniformity of NV − centres was not proportional to the electron irradiation duration, but an optimal time was observed. This study can be beneficial for enhancing the sensitivities of bulk magnetometry based on confocal systems.
Abstract In order to enhance the spin-dependent fluorescence signal coupled with outer fields, accurate positioning of the center of nitrogen-vacancy (NV) in bulk diamond is important in enhancing the sensitivity of the solid-state quantum magnetometry. In this paper, we have developed a new approach of three-dimensional optical scanning techniques for the imaging of variable-density of high concentration of NV centers in the interior of the diamond at the micrometer scale. The research defined the definite trajectories of electrons with different energies to generate NV – centers in diamond based on Monte-Carlo simulation results. Moreover, it also confirmed experimentally that the accurate location for the center of NV – in the diamond is at the depth of 100 μ m, using three-dimensional optical vertical scanning techniques. This research paves the way to produce various depth-confined NV centers, with high coherence, important in wide-field magnetometry applications.
Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.
Individual, scannable nitrogen vacancy (NV) centers in single crystal diamond nanostructures enable nanoscale, quantitative imaging of magnetic stray fields. Nevertheless, important parameters like distance between the NV center and the sample and the orientation of the NV high symmetry axis are often not known precisely and enter data evaluation as free fitting parameters. We here use scanning NV imaging on micro-patterned, perpendicularly magnetized stripes and discs. From these measurements, we directly infer NV - sample distance d_NV and the NV's azimuthal orientation without the need for an external vector magnet control. We determine d_NV = 31.5 nm, while we infer the azimuthal orientation with a precision of 3°. We additionally employ commercially available silicon needles to image the apex topography of our diamond nanostructures to detect surface contamination. Simultaneously, monitoring NV fluorescence as a function of the needle's position allows us to estimate the lateral placement of the NV inside the diamond nanostructure.
Nitrogen vacancy (NV) centres are a type of defect in diamond that exhibit a number of interesting properties. NV centres can be optically excited around 450-650 nm and emit fluorescence at around 550-800 nm [1]. The optical excitation also spin polarises the negatively charged NV centres, permitting optical detection of magnetic resonance [2]. This allows nanotesla scale sensitive magnetometry to be performed with nanometre scale spatial resolution [3].To date, a direct measurement of stimulated emission from ensembles of NV centres in diamond has yet to be demonstrated experimentally. Despite this, stimulated emission depletion microscopy has been utilised in superresolution imaging that relies on the stimulated emission of single NV centres to quench fluorescence [4]. Recently, there has also been similar indirect experimental evidence of stimulated emission in NV ensembles [5]. We thus aimed to show both optical gain of a probe beam in a pumped NV-rich diamond sample and a modulation of this gain via microwave excitation of its ground level spin transitions. The experimental set-up consisted of a pump (532 nm) and probe (723-950 nm) laser beam combined with a dichroic mirror and focused into a bulk diamond specimen to a spot-size of around 90 microns. Pump excitation powers of up to 3 W and probe powers up to 0.5 W were used. The beams were then collimated and separated with a prism and the probe power measured with a photodiode. The diamond specimen was placed on an antenna to allow simultaneous excitation with microwaves. A double modulation scheme was employed to measure either the change in probe power due to the pump laser or the microwave excitation during any one experiment.
We created dense ensembles of negatively charged nitrogen-vacancy (NV-) centers in diamond by neutron and electron irradiation for applications in hybrid quantum systems and magnetometry. We characterize fluorescence intensity, optical and coherence properties of the resulting defects by confocal microscopy, UV/Vis and FTIR spectroscopy, optically detected magnetic resonance and small angle X-ray scattering. We find the highest densities of NV- at neutron fluences on the order of 10^17 cm^-2 and electron doses of 10^18 cm^-2, with spin resonance linewidths of 6 MHz. Lower electron energies increase the ratio of centers in the desired negative charge state to those in the neutral one. Annealing at 900 °C during the irradiation reduces the spin resonance linewidth. Electron irradiation furthermore results in substantially higher optical transparency compared to neutron irradiation.
Ensembles of nitrogen-vacancy (NV) centers in diamond are versatile quantum sensors with broad applications in the physical and life sciences. The concentration of neutral substitutional nitrogen ( [ N s 0 ]) strongly influences NV electronic spin coherence times, sensitivity, and optimal sensing strategies. Diamonds with [ N s 0 ] ∼ 1–10 ppm are a focus of recent material engineering efforts, with higher concentrations being favorable for continuous-wave optically detected magnetic resonance (CW-ODMR) and lower concentrations expected to benefit pulsed magnetometry techniques through extended NV spin coherence times and improved sensing duty cycles. In this work, we synthesize and characterize low- [ N s 0 ] ( ∼ 0.8 ppm), NV-enriched diamond material, engineered through low-strain chemical vapor deposition (CVD) growth on high-quality substrates, 12 C isotopic purification, and controlled electron irradiation and annealing. Our results demonstrate good strain homogeneity in diamonds grown on CVD substrates and spin-bath-limited NV dephasing times. By measuring NV spin and charge properties across a wide range of optical NV excitation intensity, we provide direct comparisons of photon-shot-noise-limited magnetic field sensitivity between the current low- [ N s 0 ] and previously studied higher- [ N s 0 ] ( ∼ 14 ppm) NV-diamond sensors. We show that low- [ N s 0 ] diamond can outperform higher- [ N s 0 ] diamond at moderate and low optical NV excitation intensity. Our results provide practical benchmarks and guidance for selecting NV-diamond sensors tailored to specific experimental constraints and sensing requirements.