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Chinese Journal of Chemical Physics

Enhancing fluorescence of shallow nitrogen-vacancy centers in diamond by surface coating with titanium oxide layers

Wenlong Zhang, Liren Lou, Wei Zhu, Guanzhong Wang

Published Jul 29, 2019• 4 citations
Publisher

Abstract

We present an enhancement of the fluorescence of shallow (<10 nm) nitrogen-vacancy (NV−) centers by using atomic layer deposition to deposit titanium oxide layers on the diamond surface. In this way, the shallow NV− center charge states were stabilized, leading to the increasing fluorescence intensity of about 2 times. This surface coating technique could produce a protective layer of controllable thickness without any damages to the solid-state quantum system surface, which might be an approach to the further passivation or packaging techniques for the solid-state quantum devices.

DOI
10.1063/1674-0068/cjcp1904069

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We present an enhancement of the fluorescence of shallow (<10 nm) nitrogen-vacancy (NV−) centers by using atomic layer deposition to deposit titanium oxide layers on the diamond surface. In this way, the shallow NV− center charge states were stabilized, leading to the increasing fluorescence intensity of about 2 times. This surface coating technique could produce a protective layer of controllable thickness without any damages to the solid-state quantum system surface, which might be an approach to the further passivation or packaging techniques for the solid-state quantum devices.
We present an enhancement of spin properties of the shallow (<5nm) NV centers by using ALD to deposit titanium oxide layer on the diamond surface. With the oxide layer of an appropriate thickness, increases about 2 up to 3.5 times of both relaxation time and evolution time were achieved and the shallow NV center charge states stabilized as well. Moreover, the coherence time kept almost unchanged. This surface coating technique could produce a protective coating layer of controllable thickness without any damages to the solid quantum system surface, making it possible to prolong T1 time and T2* time, which would be a possible approach to the further packaging technique for the applicating solid quantum devices.
We realize anti-reflection (AR) coatings for optical excitation and fluorescence measurements of nitrogen-vacancy (NV) color centers in bulk diamond by depositing quarter-wavelength thick silica layers on the diamond surface. These AR coatings improve NV-diamond optical measurements by reducing optical reflection at the diamond-air interface from ≈17% to ≈2%, which allows more effective NV optical excitation and more efficient detection of NV fluorescence. We also show that diamond AR coatings eliminate standing-wave interference patterns of excitation laser intensity within bulk diamond, and thereby greatly reduce spatial variations in NV fluorescence, which can degrade spatially resolved magnetic field sensing using NV centers.
The negatively charged nitrogen-vacancy (NV−) center in diamond is a promising platform for quantum sensing. However, fluorescence from the NV− centers suffers large energy loss at the diamond–air interface. Here, we propose a broadband antireflection coating to enhance the fluorescence intensity by simultaneously reducing the energy loss of the excitation laser and the fluorescence. The reflectance for normal-incidence light decreases from nearly 17% for bared diamond to below 0.33% for coated diamond in the wavelength range 500 nm–800 nm. The reflectance averaged over the fluorescence bandwidth is below 3% for angles of incidence less than 20°. The measured emitted fluorescence for the coated diamond is 1.44 times that of uncoated diamond, corresponding to nearly 20% improvement in the measurement sensitivity. The proposed method is significant for enhancing the signal-to-noise ratio of NV−-based sensors.
High Resolution Image Download MS PowerPoint Slide We study the photophysical stability of ensemble near-surface nitrogen vacancy (NV) centers in diamond under vacuum and air. The optically detected magnetic resonance contrast of the NV centers was measured following exposure to laser illumination, showing opposing trends in air compared to vacuum (increasing by up to 9% and dropping by up to 25%, respectively). Characterization using X-ray photoelectron spectroscopy (XPS) suggests a surface reconstruction: In air, atmospheric oxygen adsorption on a surface leads to an increase in NV – fraction, whereas in vacuum, net oxygen desorption increases the NV 0 fraction. NV charge state switching is confirmed by photoluminescence spectroscopy. Deposition of ∼2 nm alumina (Al 2 O 3 ) over the diamond surface was shown to stabilize the NV charge state under illumination in either environment, attributed to a more stable surface electronegativity. The use of an alumina coating on diamond is therefore a promising approach to improve the resilience of NV sensors.
We demonstrate nanometer-precision depth control of nitrogen-vacancy (NV) center creation near the surface of synthetic diamond using an in situ nitrogen delta-doping technique during plasma-enhanced chemical vapor deposition. Despite their proximity to the surface, doped NV centers with depths (d) ranging from 5 to 100 nm display long spin coherence times, T2 > 100 μs at d = 5 nm and T2 > 600 μs at d ≥ 50 nm. The consistently long spin coherence observed in such shallow NV centers enables applications such as atomic-scale external spin sensing and hybrid quantum architectures.
Abstract We investigated the charge state stability and coherence properties of near-surface single nitrogen vacancy (NV) centers in 12 C-enriched diamond for potential use in nanoscale magnetic field sensing applications. The stability of charge states in negatively charged NV centers (NV − ) was evaluated using one of the pulsed optically detected magnetic resonance measurements, Rabi oscillation measurements. During the accumulation of Rabi oscillations, an unstable shallow NV − was converted to a neutral state. As a result, the contrast of Rabi oscillations degraded, depending on charge state stability. We stabilized the NV − state of very shallow NV centers (∼2.6 ± 1.1 nm from the surface) created by 1.2 keV nitrogen ion implantation by diamond surface modification, UV/ozone exposure, and oxygen annealing. This improvement indicates that we can suppress the upward surface band bending and surface potential fluctuations through Fermi level pinning originating from oxygen-terminated diamond surfaces.