Little bits of diamond - Optically detected magnetic resonance of nitrogen-vacancy centers

At a Glance

Metadata	Details
Publication Date	2018-02-20
Journal	American Journal of Physics
Authors	Haimei Zhang, Carina Belvin, Wanyi Li, Jennifer Wang, Julia Wainwright
Institutions	Wellesley College
Citations	26
Analysis	Full AI Review Included

Technical Analysis: MPCVD Diamond for High-Sensitivity NV-Center ODMR

6CCVD provides high-purity Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) materials essential for advanced quantum applications like the Optically Detected Magnetic Resonance (ODMR) study of Nitrogen-Vacancy (NV) centers described in the attached research.

Executive Summary

The analyzed research details the construction and operation of a high-sensitivity ODMR apparatus leveraging high concentrations of NV centers in synthetic diamond, suitable for advanced magnetic sensing and quantum information science.

Core Achievement: Demonstration of optically detected magnetic resonance (ODMR) in ensemble NV centers, achieving magnetic field sensitivity capable of detecting Zeeman shifts caused by fields as low as a few tenths of a millitesla (mT).
Material Focus: Utilizes high-concentration NV- centers in large SCD samples (~1-5 mm), microcrystals (10-20 µm), and nanocrystals (100 nm).
Processing Requirement: High NV density was achieved via a two-step process: (1) High Pressure High Temperature (HPHT) growth to introduce nitrogen impurities (~100 ppm), followed by (2) 4.5 MeV electron irradiation and high-temperature (800 °C) vacuum annealing to create vacancies that migrate and form NV defects.
ODMR Spectroscopy: Measurements rely on manipulating the electron spin triplet ground state (D = 2.87 GHz splitting) using resonant microwave radiation (2.7-3.1 GHz) and monitoring changes in red fluorescence intensity (600-750 nm) upon 532 nm green laser excitation.
Critical Performance Metric: Single crystals exhibited high ODMR contrast (~8%) and narrow linewidths (Γ = 6.3 MHz), ideal for precision sensing applications.
Scale and Utility: The simple, low-cost apparatus allows undergraduate students to perform cutting-edge quantum measurements, powerfully connecting fundamental physics to next-generation electronic device concepts.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Band Gap	5.5	eV	Required for visible light transparency.
NV Ground State Splitting (D)	2.870	GHz	Zero magnetic field splitting (M_s = 0 to M_s = ±1).
NV Excited State Energy	1.945	eV	Corresponding to Zero Phonon Line (ZPL).
Excitation Wavelength (Green Laser)	532	nm	Photon energy ~2.4 eV.
Fluorescence Detection Range	600-750	nm	Phonon-assisted transitions (Red light).
Microwave Frequency Range (ODMR)	2.7-3.1	GHz	Tunable band around resonance.
Applied Static Magnetic Field (B₀)	0 to 1.2	mT	Used for Zeeman splitting experiments.
SCD ODMR Contrast (C)	0.08 (8)	% / Ratio	Fractional drop in fluorescence intensity at resonance.
Nanocrystal ODMR Contrast (C)	0.04 (4)	% / Ratio	Lower contrast due to inhomogeneous broadening.
SCD Linewidth (Γ)	0.0063 (6.3)	GHz (MHz)	Full width at half maximum of Lorentzian fit.
Nanocrystal Linewidth (Γ)	0.012 (12)	GHz (MHz)	Broader linewidth due to random orientation.
Electron Irradiation Dose	2 x 10¹⁸	e/cm²	Used to create vacancies for NV formation.
Annealing Temperature	800	°C	Vacuum condition (10^-4 Pa) for vacancy mobility.

Key Methodologies

The experiment successfully combined specialized diamond material preparation with a custom epifluorescence microscope setup and radio frequency (RF)/microwave control systems.

Material Synthesis & Post-Processing (NV Creation):
- Starting Material: High-purity single crystal diamond synthesized via HPHT with high concentrations of isolated nitrogen (~100 ppm) impurities.
- Vacancy Introduction: Samples were irradiated with 4.5 MeV electrons (dose 2 x 10¹⁸ e/cm²) over 2 hours to create lattice vacancies.
- NV Defect Formation: Samples were annealed in vacuum (approx. 10^-4 Pa) at 800 °C for several hours, allowing mobile vacancies to combine with substitutional nitrogen atoms, forming the NV- centers.
Optical Excitation and Detection:
- Microscope Configuration: Custom epifluorescence microscope setup utilizing a 10x objective for both 532 nm excitation focusing (~10 µm spot diameter) and fluorescence collection.
- Filtering: A dichroic mirror (550 nm cutoff) reflected the green excitation light while transmitting the red fluorescence. Additional edge filters (600 nm long pass, 750 nm short pass) limited detection to the NV fluorescence band.
- Stability: High laser power stability (< 1% fluctuation) was maintained using active temperature stabilization (Thorlabs TED200C) and constant current source (Thorlabs LDC210C) to ensure accurate ODMR signal detection (small 5-10% changes in intensity).
Microwave Spin Manipulation:
- Source: Voltage Controlled Oscillator (Mini-Circuits ZX95-3150+) driven by a ramp generator to sweep the frequency around 2.87 GHz (tuning range 2.7 GHz to 3.1 GHz).
- Antenna: A thin 40 µm diameter copper wire (5 mm long) was placed close to the sample to broadcast the microwave field ($B_{rf}$) necessary for driving spin transitions.
Magnetic Field Application:
- Static Field: Static magnetic fields (B₀) were generated either using small 20-turn coils wrapped around the microscope objective (0-1.2 mT) or by positioning a strong permanent magnet near the sample stage.

6CCVD Solutions & Capabilities

This research validates the critical role of highly engineered diamond material for advancing quantum sensing and computing. 6CCVD is uniquely positioned to supply and engineer the necessary material requirements, supporting researchers seeking to replicate or scale these high-performance ODMR systems.

Applicable Materials

To replicate the high-contrast ODMR achieved in the single crystal samples, 6CCVD recommends:

6CCVD Material Grade	Description and Application Relevance
High-Purity SCD (Single Crystal Diamond)	Essential starting material for achieving the highest coherence times (long T₂) and maximum contrast required for sensitive quantum applications. We supply plates/wafers with low inherent strain and optimal nitrogen incorporation for subsequent processing.
Nitrogen-Treated SCD Substrates	We offer SCD substrates pre-treated or grown specifically for high nitrogen incorporation, simplifying the researcher’s pathway to post-processing via electron irradiation and annealing. This material directly replaces the specialized HPHT diamond used in the study.
Optical Grade SCD Wafers	Recommended for the [111] orientation used in the study, featuring the ultra-low surface roughness (Ra < 1 nm) necessary for efficient epifluorescence collection via high-NA objectives.
PCD Substrates (for large area arrays)	For scaling up ensemble ODMR sensors beyond the 5 mm limit reported, our PCD wafers (up to 125 mm) provide cost-effective, large-area platforms, compatible with micro-scale antenna arrays (requires custom BDD layer for active circuitry).

Customization Potential

The success of the ODMR experiment hinges on precise spatial control of both the laser excitation and the microwave antenna. 6CCVD offers extensive customization services critical for advancing this platform into integrated sensor chips:

Service	Relevance to Research Paper	6CCVD Capability
Precision Dimensions & Cutting	The study used small crystals (~1-5 mm). Future research requires complex geometries and integrated components.	Custom Shapes/Plates: We provide plates/wafers up to 125 mm (PCD) and offer precise laser micro-machining and dicing to create custom chip sizes and integrated trenches for fiber optics or MW routing.
On-Chip Microwave Antenna Integration	The research used an external 40 µm copper wire antenna.	Custom Metalization: We offer internal capability for deposition and patterning of standard conductive tracks (Au, Cu, Ti/Pt/Au stacks) directly onto the diamond surface, creating integrated coplanar waveguides (CPWs) for superior microwave field confinement and efficiency.
Thickness Control	The experiment utilized bulk diamond.	Thickness Control: SCD/PCD layers available from 0.1 µm to 500 µm thickness, and substrates up to 10 mm, allowing optimization for specific ODMR depth profiles or integrated device packaging.
Surface Finish	Critical for minimizing scattering losses in epifluorescence setups.	Superior Polishing: Standard polishing achieves Ra < 1 nm (SCD) and Ra < 5 nm (Inch-size PCD), ensuring maximum optical throughput for the 532 nm excitation and 600-750 nm fluorescence collection.

Engineering Support

6CCVD’s in-house team of PhD material scientists and technical engineers possess deep expertise in CVD diamond growth, doping, and defect engineering. We can assist researchers targeting similar NV-center based quantum sensing and magnetometry projects by providing consultation on:

Nitrogen Doping Optimization: Tailoring the growth recipe to control isolated nitrogen content, which is the precursor for NV centers, ensuring optimal starting material for subsequent irradiation.
Post-Growth Treatment Guidance: Consulting on the appropriate electron beam irradiation dosage and annealing temperature/pressure protocols to maximize the conversion efficiency of nitrogen to the desired NV- charge state.
Strain Mitigation: Selecting low-strain SCD grades crucial for minimizing the zero-field splitting (E parameter) broadening observed in the microcrystal data, thereby enhancing spectral resolution.

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

View Original Abstract

We give instructions for the construction and operation of a simple apparatus for performing optically detected magnetic resonance measurements on diamond samples containing high concentrations of nitrogen-vacancy (NV) centers. Each NV center has a spin degree of freedom that can be manipulated and monitored by a combination of visible and microwave radiation. We observe Zeeman shifts in the presence of small external magnetic fields and describe a simple method to optically measure magnetic field strengths with a spatial resolution of several microns. The activities described are suitable for use in an advanced undergraduate lab course, powerfully connecting core quantum concepts to cutting edge applications. An even simpler setup, appropriate for use in more introductory settings, is also presented.

Tech Support

Original Source

DOI: https://doi.org/10.1119/1.5023389

References

1963 - Some experiments on nuclear magnetic resonance [Crossref]
1997 - Scanning confocal optical microscopy and magnetic resonance on single defect centers [Crossref]
2007 - Spins in few-electron quantum dots [Crossref]
2014 - Atom-like crystal defects: From quantum computers to biological sensors [Crossref]
2010 - Quantum computers [Crossref]
2014 - Deterministic electrical charge-state initialization of single nitrogen-vacancy center in diamond [Crossref]
2008 - Nanoscale magnetic sensing with an individual electronic spin in diamond [Crossref]