Sub-second temporal magnetic field microscopy using quantum defects in diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-05-24 |
| Journal | Scientific Reports |
| Authors | Madhur Parashar, Anuj Bathla, Dasika Shishir, Alok Gokhale, Sharba Bandyopadhyay |
| Institutions | Indian Institute of Technology Kharagpur, Indian Institute of Technology Bombay |
| Citations | 29 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research demonstrates a breakthrough in diamond Nitrogen Vacancy (NV) center magnetometry, achieving dynamic, widefield magnetic field imaging at sub-second timescales.
- Core Achievement: Successfully transitioned widefield NV magnetometry from a temporally static technique (minutes per image) to a dynamic one, achieving imaging frame rates between 50 and 200 frames per second (fps).
- Methodology: Implemented a novel per-pixel lock-in detection protocol using a high-frame-rate lock-in camera synchronized with frequency-modulated NV Photo-Luminescence (fm-ODMR).
- Performance: Achieved a median per-pixel magnetic field sensitivity of 731 nT/sqrt(Hz).
- Temporal Resolution: Demonstrated millisecond-scale magnetic field snapshots (e.g., 4.8 ms acquisition time per frame) capable of tracking periodic (1.26 Hz to 41.52 Hz) and arbitrary current waveforms.
- Spatial Resolution: Maintained microscale spatial resolution (1.33 ”m/pixel to 1.7 ”m/pixel) while imaging intricate current flow paths in microcoils and microwires.
- Significance: This technique marks a significant improvement over conventional ODMR imaging, enabling the investigation of dynamically varying microscale magnetic field processes.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Imaging Frame Rate | 50 - 200 | fps | Dynamic magnetic field imaging |
| Single Frame Acquisition Time | 4.8 | ms | At 208 fps rate (n=15 iterations) |
| Median Per-Pixel Sensitivity | 731 | nT/sqrt(Hz) | At 6.25 kHz modulation frequency |
| Spatial Resolution (Microwire) | 1.33 | ”m/pixel | Effective magnification 30X |
| Spatial Resolution (Microcoil) | 1.7 | ”m/pixel | Effective magnification 23.5X |
| NV Layer Thickness | 1 | ”m | Implanted layer |
| NV Concentration | 1 - 2 | ppm | NV- concentration |
| Diamond Thickness | 500 | ”m | Isotopically pure crystal |
| Excitation Wavelength | 532 | nm | Green laser (continuous wave) |
| Excitation Power | 1.5 | W | Entering objective back aperture |
| Microwave Frequency Range | 2.5 - 3.2 | GHz | Applied for ODMR |
| Lock-in Modulation Frequency (w_mod) | 6.25 - 8.33 | kHz | Camera demodulation rate |
| Camera External Trigger Frequency | 2 * w_mod | kHz | Synchronization signal |
| Sample Stand-off Distance | ~13 to ~14 | ”m | Limited by glue layer layer |
| Peak Current (Dynamic Tests) | 500 | ”A | Produces ~5 to 6 ”T field |
| Microcoil Dimensions | 100 ”m x 125 ”m | ”m | 10 ”m track width |
Key Methodologies
Section titled âKey MethodologiesâThe experimental protocol relies on synchronizing frequency-modulated NV PL with the fast demodulation cycles of a commercial lock-in camera.
- Diamond and Sample Integration: An isotopically pure diamond crystal with a 1 ”m NV- implanted layer was glued onto a custom microwave loop PCB. Microscale conductive structures (Au microwire and spiral microcoil) were fabricated on silicon and mounted adjacent to the diamond surface, separated by a ~13-14 ”m standoff layer (glue).
- Optical Excitation: Continuous 532 nm green laser light (1.5 W) was focused onto the NV layer via a 100X objective to continuously excite the NV centers.
- Frequency Modulated ODMR (fm-ODMR): Microwave (MW) resonant frequencies (2.5 to 3.2 GHz) were applied using a frequency shift keying waveform (square wave envelope) at a modulation frequency (w_mod). This modulates the NV red PL intensity at w_mod.
- PL Collection and Filtering: The emitted red PL was collected through the objective and filtered (above 567 nm) to reject the green excitation light.
- Lock-in Camera Synchronization: A high-speed TTL pulse generator (PulseBlaster ESR-PRO) generated the MW modulation waveform and a synchronized external reference signal (at 2 * w_mod). This reference signal was fed to the lock-in camera (Heliotis Helicam C3) trigger input.
- Per-Pixel Demodulation: The external trigger defined four quarter periods for light integration (S1, S2, S3, S4) within the camera. The camera internally calculated the In-phase (I = S1 - S3) and Quadrature (Q = S2 - S4) signals for every pixel simultaneously.
- Dynamic Imaging: The I and Q signals were averaged over N cycles to form a single IQ frame pair. By setting the modulation frequency (w_mod) and the number of averaging cycles (n_cyc), frame rates up to 200 fps were achieved.
- Magnetic Field Tracking: The applied MW frequency was fixed at the zero-crossing point of the ODMR curve. Time-dependent magnetic field B(t) was estimated by scaling the measured lock-in pixel intensity v(t) by the independently determined zero-crossing slope (dV_lock/df) and the gyromagnetic ratio (gamma).
Commercial Applications
Section titled âCommercial ApplicationsâThe demonstrated dynamic widefield NV magnetometry technique is critical for applications requiring high spatial resolution combined with millisecond-scale temporal tracking.
- Neuroscience and Bio-Sensing:
- Real-time tracking of magnetic nano-particle motion (Magnetic Nanotweezers, MNTs) in cellular environments.
- Spatiotemporal mapping of action potential associated magnetic fields in cultured neuronal networks.
- High-speed in-vivo nanoscale thermometry using fluorescent nanodiamonds (FNDs) to monitor cellular metabolic activities.
- Micro-Electronics and Device Testing:
- Dynamic magnetic field fingerprinting and fault analysis of integrated circuits (ICs) and semiconductor devices.
- Real-time imaging of current flow and vortex dynamics in microscale devices and quantum materials (e.g., superconductors, graphene).
- Quantum Technology Development:
- Advancing the temporal resolution and Signal-to-Noise Ratio (SNR) of ensemble NV quantum sensors.
- Enabling dynamic T1 relaxometry protocols for free radical measurements in biological systems.
View Original Abstract
Abstract Wide field-of-view magnetic field microscopy has been realised by probing shifts in optically detected magnetic resonance (ODMR) spectrum of Nitrogen Vacancy (NV) defect centers in diamond. However, these widefield diamond NV magnetometers require few to several minutes of acquisition to get a single magnetic field image, rendering the technique temporally static in itâs current form. This limitation prevents application of diamond NV magnetometers to novel imaging of dynamically varying microscale magnetic field processes. Here, we show that the magnetic field imaging frame rate can be significantly enhanced by performing lock-in detection of NV photo-luminescence (PL), simultaneously over multiple pixels of a lock-in camera. A detailed protocol for synchronization of frequency modulated PL of NV centers with fast camera frame demodulation, at few kilohertz frequencies, has been experimentally demonstrated. This experimental technique allows magnetic field imaging of sub-second varying microscale currents in planar microcoils with imaging frame rates in the range of 50-200 frames per s (fps). Our work demonstrates that widefield per-pixel lock-in detection of frequency modulated NV ODMR enables dynamic magnetic field microscopy.