Microwave-Assisted Spectroscopy Technique for Studying Charge State in Nitrogen-Vacancy Ensembles in Diamond

At a Glance

Metadata	Details
Publication Date	2020-07-06
Journal	Physical Review Applied
Authors	D. P. L. Aude Craik, P. Kehayias, A. S. Greenspon, X. Zhang, M J Turner
Institutions	Harvard University, Massachusetts Institute of Technology
Citations	23
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Contrast NV Magnetometry via Microwave-Assisted Spectroscopy

Executive Summary

This research introduces a powerful microwave-assisted spectroscopy technique that fundamentally improves the performance of Nitrogen-Vacancy (NV) ensemble magnetometers by addressing the critical issue of charge-state instability and background noise in CVD diamond.

Core Innovation: A microwave-assisted spectroscopy method determines the relative concentrations of negatively-charged (NV⁻) and neutrally-charged (NV⁰) centers in situ, accounting for sample-specific material properties and environmental variations (e.g., local strain).
Performance Enhancement: The technique enables background-free Optically-Detected Magnetic Resonance (ODMR), yielding a measured 4.8-fold enhancement in ODMR contrast in high-NV-density diamond samples.
Material Requirement: The method relies on high-quality, CVD-grown Single Crystal Diamond (SCD) with precisely controlled nitrogen doping (10 ppm ¹⁴N) and post-processing (irradiation and annealing).
Scientific Discovery: The analysis revealed evidence for a previously unidentified spin-dependent ionization pathway from the NV⁻ singlet state, crucial for optimizing NV charge stability through diamond engineering.
Application Impact: The high-contrast ODMR methods presented here are essential for achieving significant sensitivity improvements in NV magnetometers, particularly for near-surface ensembles where NV⁰ populations are typically large.
Versatility: The methodology is demonstrated to be applicable to isolating spectral signatures of other fluorescent solid-state defects, such as V2-type silicon vacancies (SiV) in 4H SiC.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Material	SCD (CVD-grown)	N/A	Electronic-grade substrate with active layer.
NV Layer Thickness	10	µm	High-density ensemble layer.
Nitrogen Concentration	10	ppm ¹⁴N	Controlled doping for NV creation.
Carbon Purity	>99.95	% ¹²C	High isotopic purity.
Electron Irradiation Dosage	6 x 10¹⁸	electrons/cm²	Required for vacancy creation.
Annealing Temperatures	800 and 1000	°C	Two-stage annealing protocol (12 hours each).
Excitation Wavelength	532	nm	Continuous illumination for ODMR/PL.
Laser Power (ODMR Measurement)	7.3	mW	Power used for 4.8-fold contrast enhancement demonstration.
Microwave Drive Frequency	2.87	GHz	Resonant with NV spin transition (m_s=0 to m_s=±1).
ODMR Contrast Enhancement	Up to 4.8	fold	Achieved using the fitting method.
Target Sensitivity Range	~1	pT/√Hz	Current state-of-the-art sensitivity for ensemble magnetometers.

Key Methodologies

The experiment relies on precise material engineering and advanced optical/microwave control to isolate the spectral contributions of the two NV charge states.

Material Synthesis and Post-Processing:
- A 10 µm thick NV layer was grown on an electronic-grade SCD substrate using Chemical Vapor Deposition (CVD).
- The sample underwent high-dosage electron irradiation (6 x 10¹⁸ electrons/cm²) followed by two-stage high-temperature annealing (800 °C and 1000 °C) to maximize NV center formation.
Microwave Delivery System:
- A 2.87 GHz microwave drive was delivered to the NV ensemble via an omega-loop stripline fabricated by gold deposition on a silicon carbide carrier.
- A TTL-triggered microwave switch was used to rapidly alternate between microwaves ON ($S_{MWon}$) and OFF ($S_{MWoff}$) states.
Data Acquisition:
- Photoluminescence (PL) spectra were acquired under continuous 532 nm illumination using a confocal microscope and a grating spectrometer/CCD setup.
- Approximately 20,000 pairs of ON/OFF spectra were averaged to suppress shot-to-shot laser intensity drift and technical noise.
Charge-State Spectral Isolation:
- The difference spectrum ($S_{diff} = S_{MWoff} - S_{MWon}$) was calculated, which primarily isolates the spectral shape of the NV⁻ contribution (since NV⁰ fluorescence is spin-independent).
- An iterative scaling and fitting procedure was used to determine the correct scale factor ($k_0$) that minimizes the residual NV⁻ Zero Phonon Line (ZPL) signature in the extracted NV⁰ spectrum, thereby yielding the pure $S_{NV^{-}}$ and $S_{NV^{0}}$ basis spectra in situ.
High-Contrast ODMR:
- The extracted $S_{NV^{-}}$ and $S_{NV^{0}}$ spectral shapes were used to fit subsequent ODMR scans, allowing the spin-independent NV⁰ background fluorescence to be mathematically discarded, resulting in a significant increase in the signal-to-noise ratio (SNR) and ODMR contrast.

6CCVD Solutions & Capabilities

The research highlights the critical need for high-quality, custom-engineered CVD diamond to advance quantum sensing applications. 6CCVD is uniquely positioned to supply the materials and processing services required to replicate and extend this high-sensitivity magnetometry work.

Research Requirement	6CCVD Solution & Capability	Technical Advantage
High-Purity SCD Substrates	Optical Grade Single Crystal Diamond (SCD)	Provides the lowest defect density and highest crystalline quality necessary for long NV coherence times ($T_2$) and minimal local strain, which is crucial for stable charge states.
Custom NV Layer Engineering	Precision Nitrogen-Doped SCD	We offer precise control over ¹⁴N or ¹⁵N doping concentrations (ppm level) and layer thickness (SCD up to 500 µm) to optimize the NV ensemble density and depth for specific sensing modalities (e.g., bulk vs. near-surface).
Large-Area Scaling	PCD Wafers up to 125 mm Diameter	For scaling high-sensitivity ensemble magnetometers to commercial or industrial applications, 6CCVD provides large-area Polycrystalline Diamond (PCD) plates with superior polishing (Ra < 5 nm).
Microwave Circuit Integration	Custom Metalization Services (Au, Ti, Pt)	We offer in-house deposition of thin-film metals (e.g., Ti/Pt/Au stacks) directly onto the diamond surface, enabling the fabrication of high-frequency microwave striplines and omega-loops for optimal spin control.
Near-Surface Sensing Optimization	Ultra-Smooth Polishing (Ra < 1 nm SCD)	Achieving high sensitivity in near-surface NV ensembles requires minimizing surface defects. Our state-of-the-art polishing ensures roughness below 1 nm (SCD), reducing surface-induced NV⁰ formation and charge instability.
Post-Processing Expertise	Engineering Support for Irradiation & Annealing	6CCVD’s expert material scientists can consult on optimizing post-growth processing parameters (irradiation dosage and multi-stage annealing temperatures/durations) to maximize the yield of the desired NV⁻ charge state.

Engineering Support

6CCVD’s in-house PhD team specializes in the physics and engineering of MPCVD diamond defects. We can assist researchers and engineers in selecting the optimal material specifications (doping, thickness, purity) and processing protocols required for high-sensitivity NV Magnetometry and Spin-Dependent Ionization projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

We introduce a microwave-assisted spectroscopy technique to determine the\nrelative concentrations of nitrogen vacancy (NV) centers in diamond that are\nnegatively-charged (NV${}^-$) and neutrally-charged (NV${}^0$), and present its\napplication to studying spin-dependent ionization in NV ensembles and enhancing\nNV-magnetometer sensitivity. Our technique is based on selectively modulating\nthe NV${}^-$ fluorescence with a spin-state-resonant microwave drive to\nisolate, in-situ, the spectral shape of the NV${}^-$ and NV${}^0$ contributions\nto an NV-ensemble sample’s fluorescence. As well as serving as a reliable means\nto characterize charge state ratio, the method can be used as a tool to study\nspin-dependent ionization in NV ensembles. As an example, we applied the\nmicrowave technique to a high-NV-density diamond sample and found evidence for\na new spin-dependent ionization pathway, which we present here alongside a\nrate-equation model of the data. We further show that our method can be used to\nenhance the contrast of optically-detected magnetic resonance (ODMR) on NV\nensembles and may lead to significant sensitivity gains in NV magnetometers\ndominated by technical noise sources, especially where the NV${}^0$ population\nis large. With the high-NV-density diamond sample investigated here, we\ndemonstrate up to a 4.8-fold enhancement in ODMR contrast. The techniques\npresented here may also be applied to other solid-state defects whose\nfluorescence can be selectively modulated by means of a microwave drive. We\ndemonstrate this utility by applying our method to isolate room-temperature\nspectral signatures of the V2-type silicon vacancy from an ensemble of V1 and\nV2 silicon vacancies in 4H silicon carbide.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.14.014009

References

2017 - High Sensitivity Magnetometers, Smart Sensors, Measurement and Instrumentation