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	University of Tsukuba, Nagoya University
Citations	11
Analysis	Full AI Review Included

Technical Documentation & Analysis: Near-Field RF Imaging via NV Spin-Locking

This document analyzes the research demonstrating high-resolution near-field radio-frequency (RF) imaging using Nitrogen-Vacancy (NV) centers in diamond via the spin-locking technique. The findings validate the need for ultra-pure, high-quality Single Crystal Diamond (SCD) substrates, a core specialization of 6CCVD.

Executive Summary

Quantum Sensing Achievement: Demonstrated near-field RF imaging at micrometer resolution using an ensemble of NV centers in CVD diamond, leveraging the spin-locking technique for enhanced sensitivity.
Resolution & Frequency: Achieved spatial resolution set by the optical microscope, significantly higher than existing RF imaging methods. Successfully detected RF fields at 15 MHz.
Error Mitigation: Employed the SCROFULOUS composite pulse sequence to reduce sensitivity to microwave pulse amplitude errors across the field of view, improving fidelity.
Performance Metrics: The intrinsic rotating-frame relaxation time (T_1ρ) was measured at 640 µs, significantly longer than T₂* (0.8 µs) and T₂ (4 µs), confirming the high sensitivity enabled by the spin-locking method.
Material Requirement: The experiment relied on a high-purity, (100) oriented CVD Type IIa diamond chip, highlighting the critical role of ultra-low nitrogen concentration substrates for long coherence times.
Applications: The method is highly applicable to material characterization (polar molecules, polymers), characterization of RF devices (substrates, shields), and advanced medical fields (magnetocardiography).

Technical Specifications

The following hard data points were extracted from the experimental setup and results:

Parameter	Value	Unit	Context
Diamond Material	CVD Type IIa Ultra-Pure SCD	N/A	(100) orientation required for optimal NV alignment
Diamond Chip Size	2.0 x 2.0 x 0.5	mm³	Substrate dimensions used in the setup
NV Center Depth	~10	nm	Created via ¹⁵N⁺ ion-implantation
Post-Implantation Annealing	800	°C	Required for NV center activation
Inhomogeneous Dephasing Time (T₂*)	0.8	µs	Baseline coherence time
Hahn-Echo Dephasing Time (T₂)	4	µs	Improved coherence time
Spin-Locking Relaxation Time (T_1ρ)	640	µs	Long component, crucial for high sensitivity
Target RF Field Frequency	15	MHz	Detected frequency from the metal wire
Microwave Rabi Frequency (Ω_d)	26.8	MHz	Fixed frequency for y-driving field
Metal Wire Structure	Ti/Au (10 µm width)	N/A	Photolithographically defined RF source
Pulse Sequence Fidelity Improvement	19% reduced to 2.5%	%	Signal drop reduction at g = -0.25 using SCROFULOUS

Key Methodologies

The experiment successfully combined advanced MPCVD diamond material science with robust quantum control techniques:

Substrate Selection: Utilized a high-quality, ultra-pure (100) CVD diamond chip to minimize background noise and maximize intrinsic T_1ρ.
Shallow NV Creation: NV centers were created via ¹⁵N⁺ ion-implantation, targeting a shallow layer (~10 nm) below the surface to maximize near-field coupling, followed by 800°C annealing and acid treatment.
Spin Initialization and Readout: NV spins were initialized and read out using a 520 nm pulsed laser diode, collecting photoluminescence (PL) via wide-field microscopy.
Spin-Locking Implementation: The spin-locking technique was adopted to extend the measurable frequency range down to the MHz regime, significantly enhancing sensitivity compared to Rabi oscillation methods.
Composite Pulse Control: The SCROFULOUS composite pulse sequence (composed of three shaped pulses: θ₁Φ₁-θ₂Φ₂-θ₃Φ₃) was implemented to perform desired spin rotation while maintaining high fidelity despite spatial inhomogeneities in the microwave field amplitude.
RF Field Generation: A continuous RF field (15 MHz) was applied to a photolithographically defined 10 µm wide Au/Ti wire placed beneath the diamond chip.
Imaging Acquisition: PL intensity images (I_A and I_B) were acquired using a scientific CMOS camera under a phase cycling scheme, and the normalized difference (I_A - I_B)/I_B was calculated to map the RF field intensity.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the foundational materials and custom engineering required to replicate, scale, and advance this high-sensitivity NV sensing research.

Applicable Materials

To achieve the long T_1ρ (640 µs) and high sensitivity demonstrated, the research requires the highest quality diamond available.

Material Recommendation: Optical Grade Single Crystal Diamond (SCD)
- Purity: Ultra-low nitrogen concentration ([N] < 1 ppb) is essential to minimize spin decoherence and maximize T_1ρ.
- Orientation: We provide standard (100) oriented SCD plates, matching the substrate used for optimal NV alignment.
- Surface Quality: Our SCD is polished to an atomic-scale finish (Ra < 1 nm), crucial for minimizing surface noise and enabling precise shallow NV implantation.

Customization Potential

6CCVD’s in-house manufacturing capabilities directly address the specific physical and structural requirements of advanced quantum sensing devices.

Research Requirement	6CCVD Capability	Technical Advantage
Custom Dimensions	Plates/wafers up to 125 mm (PCD) and custom SCD sizes.	We can supply the exact 2.0 x 2.0 x 0.5 mm³ chips used, or scale up to larger SCD wafers for array development.
Thickness Control	SCD thickness control from 0.1 µm up to 500 µm.	Allows researchers to precisely control the bulk material volume and optimize heat dissipation or optical path length.
Integrated Metalization	Internal capability for depositing Au, Pt, Pd, Ti, W, Cu.	We can deposit the required Ti/Au microstrip structures (or alternative stacks) directly onto the diamond surface via photolithography, eliminating the need for external Si chips and improving alignment precision.
Surface Preparation	Precision polishing (Ra < 1 nm for SCD).	Ensures an ideal surface for subsequent ion-implantation processes (e.g., ¹⁵N⁺) and minimizes surface defects that contribute to noise.

Engineering Support

Replicating this research requires precise control over both the material properties and the post-growth processing.

NV Center Optimization: 6CCVD’s in-house PhD team provides consultation on material selection tailored for specific NV creation methods (e.g., ion-implantation dose, energy, and subsequent annealing protocols up to 1500 °C) to achieve optimal shallow NV layers (~10 nm depth) for near-field RF sensing projects.
RF Device Characterization: We offer expert guidance on selecting the appropriate diamond grade (SCD vs. PCD) and metalization stack for similar RF device characterization and material characterization applications.
Global Logistics: Global shipping is available (DDU default, DDP available) to ensure rapid delivery of high-value, custom diamond substrates worldwide.

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

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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