Wide-Field Dynamic Magnetic Microscopy Using Double-Double Quantum Driving of a Diamond Defect Ensemble
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-05-14 |
| Journal | Physical Review Applied |
| Authors | Zeeshawn Kazi, Isaac M. Shelby, Hideyuki Watanabe, Kohei M. Itoh, V. Shutthanandan |
| Institutions | National Institute of Advanced Industrial Science and Technology, Keio University |
| Citations | 25 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThe research introduces the Double-Double Quantum (DDQ) driving technique to significantly enhance the performance of wide-field Nitrogen Vacancy (NV) ensemble magnetic microscopy for dynamic applications.
- Core Innovation: DDQ driving utilizes a four-tone Radio Frequency (RF) pulse sequence to suppress spurious contrast generated by pixel-to-pixel variations in the NV resonance curve shape (specifically, optical contrast $C$ and linewidth $\delta\nu$).
- Performance Enhancement: DDQ successfully mitigates non-magnetic gradients (strain, temperature, electric field) and eliminates the dominant source of imaging noise caused by inhomogeneous broadening in state-of-the-art NV surfaces.
- Dynamic Capability: The technique enables high-frame-rate imaging (demonstrated at 15.6 Hz) of time-dependent magnetic fields, a capability previously limited by the slow acquisition required for full resonance curve fitting.
- Sensitivity: The sensor platform achieved an average volume-normalized DC magnetic sensitivity of 31 nT Hz-1/2 ”m3/2.
- Application Proof: DDQ was used to image the dynamic reorientation of a single ferromagnetic nanoparticle tethered by a DNA molecule, demonstrating its efficacy for micron-scale dynamic magnetometry in single-molecule biophysics.
- Operational Simplicity: The DDQ method requires only a two-image sequence and no phase control of the RF excitation, simplifying implementation compared to other quantum control schemes.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Layer Thickness | 150 | nm | 15N doped layer grown by CVD |
| Substrate Purity | 99.999 | % | 12C electronic-grade diamond substrate |
| Implantation Species/Energy | He+ / 25 | keV | Used for vacancy formation |
| Implantation Dose | 5 x 1011 | ions/cm2 | Vacancy creation density |
| NV Ensemble Density | 1.7 x 1016 | cm-3 | Resulting density in the active layer |
| Spin Coherence Time (T2) | 2.5 | ”s | Ensemble spin coherence time |
| Volume-Normalized Sensitivity (ηNV) | 31 | nT Hz-1/2 ”m3/2 | Average DC magnetic sensitivity |
| Optical Excitation Wavelength | 532 | nm | Laser used for pumping and readout |
| Optical Pulse Duration | 500 | ns | Pulsed excitation scheme |
| RF Ï-Pulse Duration | 3500 | ns | Pulsed RF control |
| External Static Magnetic Field (Bext) | 1 | mT | Applied along the (111) NV orientation |
| 15N Hyperfine Splitting | 3.05 | MHz | Separation between RF tones |
| Dynamic Imaging Frame Rate | 15.6 | Hz | Achieved rate (64 ms exposure per frame) |
| Nanoparticle-Sensor Distance | ~400 | nm | Approximate distance in the Tethered-Particle-Motion (TPM) assay |
Key Methodologies
Section titled âKey MethodologiesâThe wide-field magnetic particle imaging (magPI) platform relies on precise sensor fabrication and a novel quantum control sequence:
- Diamond Sensor Fabrication: A 150 nm thick layer of 15N doped, isotope-purified (99.999% 12C) diamond was grown via Chemical Vapor Deposition (CVD) on an electronic-grade substrate.
- Vacancy Creation and Annealing: Vacancies were created using 25 keV He+ implantation (5 x 1011 ions/cm2). The sample underwent a two-step anneal: 900 °C in vacuum (2 hours) for NV formation, followed by 425 °C in O2 (2 hours) for charge state stabilization.
- Pulsed ODMR Readout: Optical and RF fields are applied separately in a pulsed sequence (”s-scale) to eliminate optical power broadening. A 532 nm laser pumps the NV ensemble, and emitted photoluminescence (PL) is captured by a sCMOS camera.
- RF Excitation Scheme: RF excitation is delivered via a broadband microwave antenna. Each required RF frequency is mixed to create two equal tones separated by 3.05 MHz, simultaneously driving the two 15N-NV hyperfine transitions.
- Double-Double Quantum (DDQ) Driving: The DDQ technique employs four distinct RF tones (f1, f2, f3, f4) applied in a two-image sequence.
- The first image, Ion(f1, f4), uses RF applied at the outer inflection points of the two resonance curves.
- The second image, Ion(f2, f3), uses RF applied at the inner inflection points.
- DDQ Signal Calculation: The DDQ difference image (DI) is calculated as a normalized ratio: DDQ = 2 * [Ion(f1, f4) - Ion(f2, f3)] / [Ion(f1, f4) + Ion(f2, f3)]. This normalization cancels out common-mode shifts and minimizes the dependence on local contrast (C) and linewidth ($\delta\nu$) variations.
- Dynamic Imaging Demonstration: The DDQ method was applied to a Tethered-Particle-Motion (TPM) assay, imaging the magnetic field produced by a 500 nm ferromagnetic nanoparticle tethered by a single DNA molecule as it reorients under fluid flow.
Commercial Applications
Section titled âCommercial ApplicationsâThe DDQ technique significantly improves the reliability and speed of NV ensemble magnetometry, opening doors for high-performance commercial applications:
- Biomagnetic Sensing and Diagnostics: High-frame-rate tracking of magnetic labels (nanoparticles) in biological systems, enabling dynamic studies of molecular motors, fluid mechanics, and single-molecule biophysics without complex per-pixel calibration.
- Spintronics and Materials Science: Wide-field mapping of local magnetic fields and strain in novel magnetic materials, antiferromagnets, and spintronic devices, crucial for quality control and fundamental research.
- Integrated Circuit (IC) Analysis: Non-invasive, room-temperature magnetic imaging of current flow and magnetic leakage fields in microelectronics, facilitating failure analysis and device characterization at high speeds.
- Quantum Sensor Development: DDQ provides a robust quantum control method for mitigating fabrication inhomogeneities, leading to more uniform and reliable NV ensemble sensors suitable for large-area commercial deployment.
- Geomagnetism and Paleomagnetism: High-resolution mapping of magnetic fields in geological samples, where rapid acquisition is necessary to characterize large areas or time-sensitive processes.
View Original Abstract
Wide-field magnetometry can be realized by imaging the optically-detected magnetic resonance of diamond nitrogen vacancy (NV) center ensembles. However, NV ensemble inhomogeneities significantly limit the magnetic-field sensitivity of these measurements. We demonstrate a double-double quantum (DDQ) driving technique to facilitate wide-field magnetic imaging of dynamic magnetic fields at a micron scale. DDQ imaging employs four-tone radio frequency pulses to suppress inhomogeneity-induced variations of the NV resonant response. As a proof-of-principle, we use the DDQ technique to image the dc magnetic field produced by individual magnetic-nanoparticles tethered by single DNA molecules to a diamond sensor surface. This demonstrates the efficacy of the diamond NV ensemble system in high-frame-rate magnetic microscopy, as well as single-molecule biophysics applications.