Near-field radio-frequency imaging by spin-locking with a nitrogen-vacancy spin sensor

At a Glance

Metadata	Details
Publication Date	2021-07-09
Journal	Journal of Applied Physics
Authors	Shintaro Nomura, Koki Kaida, Hideyuki Watanabe, Satoshi Kashiwaya
Institutions	Nagoya University, National Institute of Advanced Industrial Science and Technology
Citations	11
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a novel near-field radio-frequency (RF) imaging technique utilizing Nitrogen-Vacancy (NV) centers in diamond, achieving micrometer-scale resolution and high sensitivity.

Frequency Range Extension: The method successfully extends the lower bound of near-field electromagnetic imaging down to the MHz range (demonstrated at 15 MHz) by adopting the spin-locking quantum sensing technique, overcoming the typical gigahertz (GHz) limitation of previous NV methods.
High Spatial Resolution: Spatial resolution is determined by the optical microscope resolution, which is significantly higher than existing RF imaging methods relying on wired sensor arrays. The ultimate limit is the NV layer depth (~10 nm).
Enhanced Fidelity via Composite Pulses: The SCROFULOUS composite pulse sequence is implemented for spin manipulation, reducing the sensitivity to systematic microwave pulse amplitude errors (g) across the field of view.
Error Reduction Performance: The SCROFULOUS sequence reduced the signal intensity drop due to a 25% amplitude error (g = -0.25) from 19% (simple π/2 pulses) to only 2.5%.
RF Field Mapping: The technique successfully mapped the RF magnetic field emitted from a 10 µm wide Au/Ti wire, showing field intensity maxima near the wire edges, consistent with FDTD simulation results.
Broad Applicability: The method is suitable for material characterization (polar molecules, polymers), RF device testing (substrates, shields), and medical fields (magnetocardiography).

Technical Specifications

Parameter	Value	Unit	Context
Imaging Resolution	Micrometer (µm)	Length	Spatial resolution set by optical microscope.
Ultimate Resolution Limit	~10	nm	Distance of implanted NV layer from diamond surface.
Detected RF Frequency	15	MHz	Target frequency used for imaging the Au/Ti wire.
RF Power Applied	-30	dBm	Power fed to the Au/Ti wire structure.
Diamond Type	(100) CVD Type IIa	N/A	Ultra-pure diamond chip used as sensor substrate.
Diamond Size	2.0 x 2.0 x 0.5	mm³	Physical dimensions of the diamond chip.
NV Implantation Depth	~10	nm	Depth of ¹⁵N₂⁺ ion implantation.
Annealing Temperature	800	°C	Post-implantation thermal treatment.
Inhomogeneous Dephasing Time (T₂*)	0.8	µs	Measured spin coherence time.
Hahn-Echo Dephasing Time (T₂)	4	µs	Measured spin coherence time.
Rotating-Frame Relaxation Time (T_1ρ)	640	µs	Long component, intrinsic to the measurement.
Spin-Lattice Relaxation Time (T₁)	0.86	ms	Measured using phase cycling.
Objective Lens Numerical Aperture (NA)	0.73	N/A	Used for photoluminescence (PL) collection.
Objective Lens Working Distance (WD)	4.7	mm	Used for PL collection at room temperature.
Microwave Field Variation (Image Area)	< 14	%	Maximum variation in the driving microwave field intensity.
Detected RF Field Underestimation (SCROFULOUS)	1.2	%	Error due to pulse amplitude variation (compared to 6.6% for simple π/2).
Relative Permittivity (Si)	11.20	N/A	Value used in Finite-Difference Time-Domain (FDTD) simulation.
Relative Permittivity (Diamond)	5.68	N/A	Value used in FDTD simulation.

Key Methodologies

The near-field RF imaging relies on optically detected magnetic resonance (ODMR) using NV ensembles, enhanced by spin-locking and composite pulse sequences.

NV Center Preparation:
- A (100) CVD Type IIa ultra-pure diamond chip (2.0 x 2.0 x 0.5 mm³) was used.
- NV centers were created via ¹⁵N₂⁺ ion implantation, targeting a depth of approximately 10 nm below the surface.
- The chip was subsequently annealed at 800°C and subjected to acid treatment.
Experimental Setup (Wide-Field Microscopy):
- A home-made wide-field microscope was used for photoluminescence (PL) imaging.
- A temperature-stabilized pulsed laser diode (520 nm) initialized the NV spins to the |0> state.
- PL was collected using a 100x objective (NA 0.73, WD 4.7 mm) and filtered (cut-off wavelength 650 nm) before detection by a scientific CMOS camera.
- A static magnetic field was applied along the [111] direction using Nd₂Fe₁₄B permanent magnets.
Microwave and RF Field Generation:
- Microwave pulses for spin manipulation were generated by up-converting baseband I and Q pulses from an arbitrary wave generator using a double-balanced mixer and a local oscillator.
- A spatially homogeneous microwave field was applied via a planar ring antenna placed above the diamond chip.
- The target RF field (15 MHz) was generated by feeding a continuous RF signal to a photolithographically defined 10 µm wide Au/Ti wire placed below the diamond chip.
Spin-Locking Measurement Sequence:
- The standard spin-locking sequence involves: Laser initialization -> π/2 pulse (rotation about x-axis) -> y-driving microwave field (duration τ) -> second π/2 pulse -> Laser readout.
- A phase cycling scheme (Pattern A: +π/2, Pattern B: -π/2) was used to eliminate common-mode noise and enhance the signal.
SCROFULOUS Composite Pulse Implementation:
- To mitigate errors from inhomogeneous microwave fields, the SCROFULOUS (SCROtated FULl OUScillation) composite pulse sequence was adopted for the initial and final π/2 rotations.
- The sequence is composed of three pulses (θ₁φ₁ - θ₂φ₂ - θ₃φ₃).
- Nominal π/2 Rotation Parameters: θ₁ = 115.2°, φ₁ = 62.0°, θ₂ = 180°, φ₂ = 280.6°, and θ₃ = 115.2°, φ₃ = 62.0°.
- Shaped pulses were used to ensure accurate control of pulse widths and reduce baseband bandwidths.
RF Image Acquisition:
- The amplitude of the driving microwave field (Rabi frequency Ω_d) was scanned (10 to 21 MHz).
- The spin-locking signal (I_SL) was calculated at each pixel (I_SL = I_PL(C) - [I_PL(A) + I_PL(B)]/2).
- The maximum I_SL was searched across the frequency scan for each pixel to obtain the RF field image, which is proportional to the square of the RF magnetic field magnitude (|B^RF|²).

Commercial Applications

The demonstrated near-field RF imaging technology, leveraging the high sensitivity and spatial resolution of NV centers in diamond, is highly relevant to several advanced engineering and scientific fields:

RF Device Characterization and Testing:
- Substrate and Shield Analysis: Non-destructive characterization of RF components, including mapping magnetic field leakage or distribution around microstrip lines, substrates, and RF shields.
- Quality Control: High-resolution testing of photolithographically defined metal structures and integrated circuits for defects or performance anomalies.
Material Science and Characterization:
- Molecular Analysis: Characterization of materials such as polar molecules, polymers, and proteins by detecting their associated magnetic noise spectra in the MHz range.
- Defect Analysis: Imaging localized magnetic fields generated by current flows or magnetic impurities within materials.
Medical and Biological Fields:
- Biomagnetic Measurement: Applications requiring high sensitivity magnetic field detection, such as early diagnosis of disease and magnetocardiography (MCG).
- Miniaturized Sensing: The optical readout eliminates the need for complex electric wiring arrays, enabling highly dense, non-invasive sensor platforms.
Quantum Sensing Technology:
- Sensor Development: Provides a robust platform for developing and testing advanced quantum sensing protocols (like spin-locking and composite pulses) suitable for noisy, room-temperature environments.

View Original Abstract

We present results of near-field radio-frequency (RF) imaging at micrometer resolution using an ensemble of nitrogen-vacancy (NV) centers in diamond. The spatial resolution of RF imaging is set by the resolution of an optical microscope, which is markedly higher than the existing RF imaging methods. High sensitivity RF field detection is demonstrated through spin locking. SCROFULOUS composite pulse sequence is used for manipulation of the spins in the NV centers for reduced sensitivity to possible microwave pulse amplitude error in the field of view. We present procedures for acquiring an RF field image under spatially inhomogeneous microwave field distribution and demonstrate a near-field RF imaging of an RF field emitted from a photolithographically defined metal wire. The obtained RF field image indicates that the RF field intensity has maxima in the vicinity of the edges of the wire, in accord with a calculated result by a finite-difference time-domain method. Our method is expected to be applied in a broad variety of application areas, such as material characterizations, characterization of RF devices, and medical fields.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0052161

References

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