Wide-field Fourier magnetic imaging with electron spins in diamond
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2024-02-21 |
| Journal | npj Quantum Information |
| Authors | Zhongzhi Guo, You Huang, Mingcheng Cai, Chunxing Li, M. Shen |
| Institutions | University of Science and Technology of China |
| Citations | 5 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research introduces Wide-field Fourier Magnetic Imaging (WFMI), a novel technique leveraging Nitrogen-Vacancy (NV) centers in diamond to achieve super-resolution magnetic mapping over a large field of view (FOV).
- Resolution Breakthrough: WFMI overcomes the optical diffraction limit (typically >200 nm), achieving a demonstrated spatial resolution down to 34.3 nm.
- Performance Gain: This represents a resolution improvement factor of approximately 20 times compared to the optical resolution limit (estimated at 700 nm in the setup).
- Core Methodology: The technique integrates wide-field pulsed magnetic field gradient encoding (k-space sampling) with parallel imaging, utilizing the camera pixels as effective spatial filters to eliminate Fourier artifacts caused by reduced FOV encoding.
- Wide-Field Capability: Unlike scanning super-resolution methods (e.g., STED), WFMI maintains the large FOV necessary for assessing the overall performance of large-scale materials and devices.
- Demonstration: Successfully demonstrated wide-field super-resolution magnetic imaging of a one-dimensional AC gradient magnetic field using a dense NV ensemble layer.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Highest Spatial Resolution | 34.3 | nm | Achieved in y-direction for single NV center (NV2) |
| Resolution Improvement Factor | ~20 | Factor | Relative to optical resolution limit (~700 nm) |
| Optical Diffraction Limit (Setup) | 700 | nm | Estimated sensitive area diameter of a single camera pixel |
| NV Ensemble T2 Coherence Time | 11.6 | µs | 14N+ implanted HPHT diamond |
| Single NV T2 Coherence Time | 9.7 | µs | 15N+ implanted CVD diamond |
| NV Ensemble Implantation Dose | 1 x 1012 | cm-2 | 14N+, 40 keV, (100) surface |
| Single NV Implantation Dose | 1 x 1010 | cm-2 | 15N+, 5 keV, (100) surface |
| NV Layer Density (Ensemble) | ~1 x 1010 | cm-2 | Estimated density for 2D thin layer experiment |
| Static Magnetic Field (B0) | ~340 | G | Applied by permanent magnet |
| Maximum Gradient Strength | 1.5 | G·µm-1 | At 4 A current in microcoils |
| Phase Encoding Time (τ) | 10.73 | µs | Used for single NV center demonstration |
| Microcoil Thermal Conductivity | 1800 | W·m-1·k-1 | Polycrystal diamond substrate |
| Microcoil Metal Layer Thickness | 3000 | nm | Cu layer thickness (part of Ti/Au/Cu/Au stack) |
| Microcoil Width | 10 | µm | Final electrode width |
| Objective Lens | x60 | N/A | Numerical Aperture (NA) = 0.7 |
Key Methodologies
Section titled “Key Methodologies”The WFMI technique relies on precise diamond sample preparation, microcoil fabrication, and a specialized quantum pulse sequence combining sensing and k-space encoding.
1. Diamond Sample Preparation (NV Center Generation)
Section titled “1. Diamond Sample Preparation (NV Center Generation)”- Ensemble NV Centers:
- Substrate: 3 x 3 x 0.5 mm HPHT ultrapure diamond, (100) surface.
- Implantation: 14N+ ions.
- Parameters: Dose: 1 x 1012 cm-2; Energy: 40 keV.
- Annealing: 1000 °C under ultrahigh vacuum (UHV).
- Single NV Centers:
- Substrate: 2 x 2 x 0.1 mm ultrapure CVD diamond, (100) surface, arranged in a nanopillar array (2 µm spacing).
- Implantation: 15N+ ions.
- Parameters: Dose: 1 x 1010 cm-2; Energy: 5 keV.
2. Gradient Microcoils Fabrication
Section titled “2. Gradient Microcoils Fabrication”- Substrate: Polycrystal diamond (1800 W·m-1·k-1 thermal conductivity).
- Adhesion/Seed Layers: 20 nm Ti and 200 nm Au deposited via magnetron sputtering.
- Lithography: Photolithography using MA6 (AZ4620).
- Electroplating: Deposition of 3 µm Cu and 200 nm Au electrodes.
- Final Stack: Ti/Au/Cu/Au (20/200/3000/200 nm thickness) with a 10 µm width.
- Shielding: A silicon oxide-titanium-silicon oxide film (400/200/100 nm thickness) was applied to isolate scattered light and impurities.
3. Wide-field Fourier Magnetic Imaging (WFMI) Sequence
Section titled “3. Wide-field Fourier Magnetic Imaging (WFMI) Sequence”The sequence is a modified Hahn echo (spin echo) sequence incorporating pulsed gradients and quadrature phase detection.
- Initialization: 532 nm laser pulse initializes NV centers to the |0> state.
- Sensing/Encoding:
- A spin echo sequence is used: (π/2)0 - τ/2 - π0 - τ/2 - (π/2)0/90.
- A pulsed gradient magnetic field (G) is applied during the free precession periods (τ/2) to encode spatial information into k-space phase (φ = 2πk·ri).
- Quadrature phase detection (using 0° and 90° final π/2 pulses) generates a complex k-space signal Si(k).
- Parallel Imaging & Artifact Elimination:
- A reduced FOV is used during k-space sampling to minimize acquisition time.
- The camera acts as a parallel detector, where each pixel’s sensitive area (700 nm diameter) functions as a spatial filter.
- Fourier transformation is performed, and the spatial filter is applied to the extended full FOV data to eliminate artifacts (aliasing) caused by the reduced FOV encoding.
- Readout: Final laser pulse reads out the population on the |0> state via fluorescence detection using a camera.
Commercial Applications
Section titled “Commercial Applications”The ability of WFMI to provide high-resolution magnetic maps over large areas makes it highly valuable for advanced characterization in several high-tech sectors.
| Industry/Sector | Application Area | Technical Advantage |
|---|---|---|
| Micro/Nanoelectronics | Current Path Imaging & Electrical Characterization | Mapping current flow in 2D materials (e.g., graphene) and microelectronic circuits; characterizing nanoscale conductive networks. |
| Advanced Materials Science | Magnetic Domain and Stress Mapping | Imaging domain reversal in ultrathin Van der Waals ferromagnets; mapping stress and magnetism at high pressures. |
| Nanoscale MRI & Biomagnetism | High-Resolution Sensing | Potential for nanoscale Magnetic Resonance Imaging (MRI) of nuclear and electron spins; imaging magnetic fields generated by biological cells (e.g., neural networks). |
| Quantum Computing/Sensing | Spin Qubit Characterization | Highly efficient detection and spatial mapping of electronic and nuclear spin qubits inside diamond. |
| Solid-State Devices | Performance Assessment | Assessing the overall performance and structural integrity of devices by probing multiple structural units over a wide area. |
View Original Abstract
Abstract Wide-field magnetic imaging based on nitrogen-vacancy (NV) centers in diamond has been shown the applicability in material and biological science. However, the spatial resolution is limited by the optical diffraction limit (>200 nm) due to the optical real-space localization and readout of NV centers. Here, we report the wide-field Fourier magnetic imaging technique to improve spatial resolution beyond the optical diffraction limit while maintaining the large field of view. Our method relies on wide-field pulsed magnetic field gradient encoding of NV spins and Fourier transform under pixel-dependent spatial filters. We have improved spatial resolution by a factor of 20 compared to the optical resolution and demonstrated the wide-field super-resolution magnetic imaging of a gradient magnetic field. This technique paves a way for efficient magnetic imaging of large-scale fine structures at the nanoscale.