Wide-field Fourier magnetic imaging with electron spins in diamond

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

Wide-field Fourier Magnetic Imaging (WFMI) in Diamond: Technical Analysis and 6CCVD Solutions

This document analyzes the research paper detailing the development of Wide-field Fourier Magnetic Imaging (WFMI) using Nitrogen-Vacancy (NV) centers in diamond, focusing on the technical specifications and providing material solutions available through 6CCVD.

Executive Summary

Super-Resolution Achievement: The WFMI technique successfully breaks the optical diffraction limit (>200 nm) in wide-field NV-diamond magnetic imaging.
Resolution Breakthrough: Spatial resolution was improved by a factor of 20, achieving a minimum resolution of 34.3 nm for single NV centers and 179 nm for ensemble NV layers.
Methodology: The technique integrates quantum sensing with Fourier encoding, utilizing wide-field pulsed magnetic field gradient encoding, quadrature phase detection, and parallel imaging via a camera detector.
Parallel Detection Advantage: The use of parallel imaging eliminates artifacts caused by reduced Field of View (FOV) encoding, allowing for efficient, large-scale imaging while maintaining high spatial resolution.
Material Requirements: The experiment relied on ultrapure CVD and HPHT diamond substrates optimized for long coherence times (T₂ up to 11.6 µs) and precise, shallow ion implantation.
Applications: This method is critical for efficient magnetic imaging of large-scale fine structures at the nanoscale, including nanoscale Magnetic Resonance Imaging (MRI) and characterization of 2D magnetic materials.

Technical Specifications

Parameter	Value	Unit	Context
Best Spatial Resolution	34.3	nm	Single NV center (y direction)
Ensemble Spatial Resolution	179	nm	WFMI of AC gradient field
Resolution Improvement	20	Factor	Compared to optical diffraction limit
Optical Diffraction Limit	>200	nm	Setup limitation
Ensemble NV T₂ Coherence Time	11.6	µs	¹⁴N⁺ implanted HPHT diamond
Single NV T₂ Coherence Time	9.7	µs	¹⁵N⁺ implanted CVD diamond
Ensemble Implantation Dose	1 x 10¹²	cm^-2	¹⁴N⁺, 40 keV
Single NV Implantation Dose	1 x 10¹⁰	cm^-2	¹⁵N⁺, 5 keV
CVD Diamond Thickness	0.1	mm	Substrate for single NV centers
HPHT Diamond Thickness	0.5	mm	Substrate for ensemble NV centers
Maximum Gradient Field (G)	1.5	G·µm^-1	Generated by microcoils at 4 A current
Axial Magnetic Field (B₀)	~340	G	Applied by permanent magnet
Microcoil Thermal Conductivity	1800	W·m^-1·k^-1	Polycrystalline diamond substrate
Microcoil Metal Stack Thickness	3420	nm	Ti/Au/Cu/Au (20/200/3000/200 nm)

Key Methodologies

The WFMI technique relies on integrating high-quality diamond substrates, precise microcoil fabrication, and advanced quantum control sequences:

Diamond Substrate Preparation:
- Ultrapure HPHT (3 x 3 x 0.5 mm) and CVD (2 x 2 x 0.1 mm) diamond were used to ensure low background noise and long T₂ times.
- NV centers were generated via ion implantation (¹⁴N⁺ or ¹⁵N⁺) at low energies (5 keV or 40 keV) to create shallow sensing layers, followed by 1000 °C ultrahigh vacuum annealing.
Gradient Microcoil Fabrication:
- Microcoils were fabricated on a high thermal conductivity polycrystalline diamond substrate using magnetron sputtering (Ti/Au adhesion layers) and electroplating (Cu/Au).
- The final metal stack was Ti/Au/Cu/Au (20/200/3000/200 nm) with a width of 10 µm, designed to generate a uniform gradient magnetic field (G) up to 1.5 G·µm^-1.
Quantum Control and Encoding (WFMI Pulse Sequence):
- NV centers are initialized using a 532 nm laser pulse.
- The WFMI sequence uses two independent Hahn echo sequences (0° and 90° microwave phases) for quadrature phase detection.
- A pulsed gradient magnetic field (G) is applied during the free precession time (τ) to encode the spatial position (r_i) into k-space phase (φ = 2πk·r_i).
Parallel Imaging and Readout:
- A camera acts as a parallel optical detector, simultaneously reading out the fluorescence from a dense layer of NV centers.
- Parallel imaging is used to eliminate artifacts resulting from the necessary reduced encoding FOV, which significantly shortens the acquisition time compared to conventional Fourier magnetic imaging.

6CCVD Solutions & Capabilities

This research highlights the critical need for high-purity, low-strain diamond materials and precise fabrication capabilities to push the boundaries of quantum sensing. 6CCVD is an expert supplier of MPCVD diamond engineered specifically for these advanced applications.

Applicable Materials for WFMI Replication

To replicate or extend this super-resolution magnetic imaging research, 6CCVD recommends materials optimized for maximum NV center coherence and minimal background noise:

Optical Grade Single Crystal Diamond (SCD):
- Requirement Match: Essential for achieving the long T₂ coherence times (9.7 µs to 11.6 µs) demonstrated in the paper. Our SCD features ultra-low nitrogen content (N < 1 ppb) and minimal lattice strain, providing the ideal quantum sensor platform.
- Thickness Control: We supply SCD plates in the required range, from ultra-thin 0.1 µm sensing layers up to 500 µm substrates, allowing precise control over NV depth and thermal management.
High Thermal Conductivity Polycrystalline Diamond (PCD):
- Requirement Match: The microcoils require a substrate with high thermal conductivity (1800 W·m^-1·k^-1) to handle the high currents (up to 4 A) needed for the gradient field. Our PCD substrates meet these demanding thermal specifications.

Customization Potential

The WFMI setup requires highly customized components, particularly regarding substrate dimensions and microcoil integration. 6CCVD offers comprehensive in-house engineering services to meet these needs:

Custom Requirement	Research Specification	6CCVD Capability
Custom Dimensions	2 x 2 mm and 3 x 3 mm plates	Custom plates and wafers up to 125 mm (PCD). We offer precision laser cutting for custom geometries of both SCD and PCD.
Metalization Services	Ti/Au/Cu/Au multi-layer stack	Internal capability for depositing custom metal stacks, including Au, Pt, Pd, Ti, W, and Cu. We can replicate the required microcoil stack thickness (e.g., 3420 nm total) with high precision.
Surface Finish	Low-strain surface for shallow NV centers	SCD polishing achieving Ra < 1 nm (atomic flatness), crucial for minimizing decoherence for near-surface NV centers generated by low-energy implantation (5 keV).
Substrate Optimization	Optimized for ion implantation	We provide SCD substrates specifically prepared for subsequent low-energy ion implantation, ensuring high NV yield and optimal depth control for nanoscale sensing.

Engineering Support

6CCVD’s in-house PhD team can assist researchers in material selection and optimization for similar Nanoscale Magnetic Resonance Imaging (MRI) and Quantum Diamond Microscopy projects. We ensure that the diamond material properties—including purity, T₂ time, and surface preparation—are perfectly matched to the specific demands of super-resolution quantum sensing protocols.

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

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.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41534-024-00818-9