All Fiber Vector Magnetometer Based on Nitrogen-Vacancy Center

At a Glance

Metadata	Details
Publication Date	2023-03-06
Journal	Nanomaterials
Authors	Man Zhao, Qijing Lin, Qingzhi Meng, Wenjun Shan, Liangquan Zhu
Institutions	Yantai University, Xi’an Jiaotong University
Citations	14
Analysis	Full AI Review Included

Executive Summary

The research presents a robust and compact all-fiber vector magnetometer based on Nitrogen-Vacancy (NV) centers in micro-diamond, designed for practical engineering applications outside traditional laboratory settings.

Core Innovation: Replaced all conventional spatial optical elements (objective lenses, dichroic mirrors) with multi-mode fiber components, primarily utilizing a Wideband Multi-mode Circulator (WMC) for efficient laser excitation and fluorescence collection.
Performance Achievement: Demonstrated an experimental sensitivity of 0.73 nT/Hz^1/2, comparable to conventional confocal systems while offering superior portability and robustness.
Optical Efficiency: The all-fiber system achieved 1.87 times higher fluorescence intensity compared to a traditional confocal setup, resulting in a theoretical 27% improvement in sensitivity performance.
Vector Sensing: Enabled µm-scale vector magnetic field detection at the fiber tip using a simplified, rapid analysis method involving a single one-axis Helmholtz coil for calibration and distinguishing the four NV axes.
Diamond Material: Utilized Type-Ib HPHT micro-diamond (550 µm diameter) with a high NV center density (1.08 x 10¹⁸ cm^-3) created via 10 MeV electron irradiation and subsequent annealing.
Application Readiness: The compact and anti-interference nature of the all-fiber design makes it a strong candidate for magnetic endoscopy and remote sensing in complex physical environments.

Technical Specifications

Parameter	Value	Unit	Context
Fiber Magnetometer Sensitivity	0.73	nT/Hz^1/2	Photon shot-noise limited
Confocal Magnetometer Sensitivity	0.59	nT/Hz^1/2	Photon shot-noise limited
Fluorescence Intensity (Fiber/Confocal)	1.87	N/A	Ratio at 100 mW laser power
Laser Excitation Wavelength	532	nm	Green laser source
Laser Power (Source)	100	mW	Input power
Laser Power (at Diamond)	64.2	mW	Transmitted power after AOM/WMC
Fiber Core Diameter (Port 2)	200	µm	Multi-mode fiber
Fiber Numerical Aperture (NA₁)	0.39	N/A	Port 2
Fiber Fluorescence Collection Efficiency	0.607	%	Estimated geometric optics
Diamond Type/Size	Type-Ib HPHT	550 µm	Cubo-octahedron micro-diamond
Electron Irradiation Dose	1 x 10¹⁸	cm^-2	10 MeV energy
NV Center Density	1.08 x 10¹⁸	cm^-3	Achieved density
Annealing Temperature (Vacuum)	850	°C	First stage annealing
Annealing Temperature (Air)	500	°C	Second stage annealing
Zero-Field Splitting (D)	2871.5	MHz	Measured NV property
Lattice Stress Parameter (E)	3.5	MHz	Measured NV property

Key Methodologies

The methodology focuses on the fabrication of the fiber probe and the simplified vector magnetic field measurement technique (Pulsed ODMR).

1. NV Center Creation and Diamond Preparation

Material Selection: Used a high-pressure, high-temperature (HPHT) Type-Ib micro-diamond with a cubo-octahedron morphology (550 µm diameter).
Irradiation: The diamond was irradiated with a 10 MeV electron beam at a dose of 1 x 10¹⁸ cm^-2 to create vacancies.
Annealing: Vacancies were mobilized by annealing at 850 °C in vacuum, followed by 500 °C in air, resulting in an NV center density of approximately 1.08 x 10¹⁸ cm^-3.

2. All-Fiber Probe Assembly

Diamond Mounting: The micro-diamond was glued onto the tip of Port 2 of the Wideband Multi-mode Circulator (WMC).
Axial Alignment: A hexagon surface (corresponding to the <111> crystal plane) was aligned to coincide with the axial direction of the multi-mode optical fiber (200 µm core).
MW Antenna: An eight-turn copper coil antenna was wound around a ceramic ferrule near the diamond to deliver the microwave (MW) signal for spin manipulation.
Optical Path Integration: The WMC replaced the traditional dichroic mirror, separating the 532 nm excitation laser (Port 1 to Port 2) from the NV fluorescence (Port 2 to Port 3) with high transmission efficiency.

3. Vector Magnetic Field Measurement

Pulsed ODMR: The electron spin state was manipulated using pulsed Optically Detected Magnetic Resonance (ODMR) to avoid optical and MW power broadening, enhancing sensitivity.
Calibration Field (B_a): A one-axis Helmholtz coil was used to generate an auxiliary calibration magnetic field (B_a) at a known angle (45°) relative to the fiber axis.
NV Axis Determination: A single ODMR scan with B_a was used to determine the relative orientation (θ) of the four NV center axes (e.g., θ = 78.69°) and establish the transformation matrix (K) between the lab-based Cartesian system and the NV axis coordinate system.
Vector Extraction: ODMR scans were performed on the magnetic field to be measured (B_m) and the superimposed field (B_a + B_m). The vector components of B_m were calculated by analyzing the differential resonance peak shifts (dΩ_i) using the established transformation matrix K.

Commercial Applications

This robust, compact, and high-sensitivity all-fiber NV center magnetometer is highly valuable for applications requiring remote or localized magnetic field sensing in challenging environments.

Magnetic Endoscopy: Enables high-resolution, µm-scale magnetic field detection in inaccessible or complex physical spaces, such as inside machinery or biological systems.
Remote Sensing and Nondestructive Testing (NDT): Suitable for remote measurement of magnetic fields in industrial settings, including chip inspection, material characterization, and defect detection in advanced materials (e.g., steel damage imaging).
Biomedical Imaging: Potential use in bio-imaging and measurement of neural activity or magnetic nanoparticles within living cells, benefiting from the compact, non-invasive fiber probe design.
Condensed Matter Physics: Provides a portable tool for high-resolution magnetic field imaging of advanced materials at room temperature, complementing bulk laboratory setups.
Complex Environment Monitoring: Operates effectively in environments where traditional space light optics are unstable or susceptible to interference, such as high-temperature or high-pressure systems.

View Original Abstract

Magnetometers based on nitrogen-vacancy (NV) centers in diamonds have promising applications in fields of living systems biology, condensed matter physics, and industry. This paper proposes a portable and flexible all-fiber NV center vector magnetometer by using fibers to substitute all conventional spatial optical elements, realizing laser excitation and fluorescence collection of micro-diamond with multi-mode fibers simultaneously and efficiently. An optical model is established to investigate multi-mode fiber interrogation of micro-diamond to estimate the optical performance of NV center system. A new analysis method is proposed to extract the magnitude and direction of the magnetic field, combining the morphology of the micro-diamond, thus realizing μm-scale vector magnetic field detection at the tip of the fiber probe. Experimental testing shows our fabricated magnetometer has a sensitivity of 0.73 nT/Hz1/2, demonstrating its feasibility and performance in comparison with conventional confocal NV center magnetometers. This research presents a robust and compact magnetic endoscopy and remote-magnetic measurement approach, which will substantially promote the practical application of magnetometers based on NV centers.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano13050949

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