A fiber based diamond RF B-field sensor and characterization of a small helical antenna

At a Glance

Metadata	Details
Publication Date	2018-09-24
Journal	Applied Physics Letters
Authors	Dong Mm, Z. Z. Hu, Y. Liu, B. Yang, Y.J. Wang
Institutions	Nanjing University of Posts and Telecommunications
Citations	33
Analysis	Full AI Review Included

Technical Documentation & Analysis: Fiber-Based Diamond RF B-Field Sensor

Executive Summary

This research successfully demonstrates a highly sensitive, fiber-based microwave B-field scanning system utilizing Nitrogen Vacancy (NV) centers in diamond microcrystals. This technology is critical for non-invasive characterization of complex microwave structures.

High Spatial Resolution: Achieves micrometer-scale spatial resolution (sub-100 µm crystal size) essential for near-field mapping of Monolithic Microwave Integrated Circuits (MMICs) and small antennas.
Quantum Sensing Technique: Employs a novel pulsed lock-in Optically Detected Magnetic Resonance (ODMR) technique to achieve robust operation and high Signal-to-Noise Ratio (SNR).
High Sensitivity: The system demonstrates a magnetic field sensitivity of 5 nT/√Hz, enabling the detection of weak microwave fields.
Detection Limit: A minimum resolvable modulation depth of ~20 ppm is achieved with a 3 Hz Resolution Bandwidth (RBW).
Non-Invasive Probe: The diamond microcrystal is fixed to a tapered fiber tip, creating a small, robust probe that minimizes invasiveness when mapping fields inside structures like helical antennas.
Key Application: Successfully characterized the standing-wave B-field distribution inside and the near-field pattern outside a helical antenna operating at 2.857 GHz.

Technical Specifications

The following hard data points were extracted from the research detailing the sensor performance and experimental parameters.

Parameter	Value	Unit	Context
Sensor Material Type	HPHT Diamond Microcrystal	N/A	Ion implanted, containing NV centers
Crystal Size (Diameter)	Sub-100	µm	Milled to approximately 60 µm for high resolution
Spatial Resolution	Micrometer scale	N/A	Enabled by small crystal size
NV Zero-Field Splitting	2.87	GHz	Ground state magnetic sub-levels
Excitation Wavelength	532	nm	Used for NV center polarization and readout
Laser Pulse Duration (ON/OFF)	500 / 500	nanosecond	Optimized for efficient spin state manipulation
Microwave Pulse Length	50	nanosecond	Used in pulsed ODMR sequence
Modulation Frequency (f_m)	1	kHz	Used for amplitude modulation of MW B-field
Minimum Resolvable Modulation Depth	~20	ppm	Achieved with 3 Hz Resolution Bandwidth (RBW)
Magnetic Field Sensitivity	5	nT/√Hz	Calculated sensitivity at the weak field limit
Minimum Resolvable Field	0.89e-4	Gauss	Corresponds to 1% modulation depth (weak field limit)
Field Measurement Dynamic Range	54	dB	Dynamic range in microwave power

Key Methodologies

The experiment relied on precise material preparation and advanced quantum control techniques to achieve high-resolution field mapping.

Diamond Preparation: Commercially available High Pressure High Temperature (HPHT) diamond was ion implanted to create NV centers, then milled to microcrystal sizes (approx. 60 µm).
Fiber Probe Fabrication: A standard optical fiber was tapered by heating and stretching to form a desired geometry, resulting in a 70 µm end diameter.
Crystal Integration: A sub-100 µm diamond microcrystal was precisely glued onto the tapered fiber tip using UV curable adhesive under high magnification (< 500x).
Static Field Application: A permanent magnet was manually positioned to apply a static magnetic field (B₀), lifting the degeneracy of the m_s = ±1 states and resolving eight ODMR peaks.
Pulsed ODMR Sequence: The system utilized staggered laser (500/500 ns ON/OFF) and microwave (50 ns pulse length) pulses to polarize and read out the NV ground spin state.
Pulsed Lock-in Detection: The microwave B-field was amplitude modulated at a low frequency (f_m = 1 kHz). Demodulation at the sideband frequency provided a significantly improved Signal-to-Noise Ratio (SNR) against excitation laser intensity fluctuations.
Field Mapping: The Device Under Test (DUT, helical antenna) was scanned in a 2D plane using a motorized XY stage, while the fixed diamond probe measured the B-field strength, which was linearly proportional to the sideband signal amplitude in the short pulse (linear) regime.

6CCVD Solutions & Capabilities

6CCVD provides the high-quality, custom diamond materials and precision fabrication services necessary to replicate, optimize, and scale this advanced NV-based quantum sensing technology.

Applicable Materials

To maximize the performance of NV-based quantum sensors, high-purity, low-strain diamond is essential.

Single Crystal Diamond (SCD) - Quantum Grade:
- Advantage: While the paper used HPHT, 6CCVD’s MPCVD SCD offers superior purity and lower strain, which directly translates to longer spin coherence times (T₂) and significantly higher magnetic field sensitivity (nT/√Hz).
- Recommendation: Use high-purity SCD with controlled nitrogen doping (via gas phase or implantation) for optimal NV ensemble creation.
Polycrystalline Diamond (PCD) - Large Area Sensing:
- Advantage: For applications requiring large-area mapping arrays or cost-effective ensemble sensing, 6CCVD offers PCD wafers up to 125 mm in diameter, providing a scalable platform for high-throughput device characterization.

Customization Potential

The success of this fiber-based probe relies on precise micro-sized diamond components. 6CCVD specializes in meeting these stringent dimensional requirements.

Customization Service	Relevance to NV Sensor Research	6CCVD Capability
Custom Dimensions & Cutting	Replicating the sub-100 µm microcrystal size for fiber tip attachment.	Precision laser cutting and micro-milling services for SCD plates up to 500 µm thick, ensuring exact dimensions for high spatial resolution probes.
Thickness Control	Fabricating ultra-thin sensing layers for near-field applications.	SCD and PCD wafers available from 0.1 µm to 500 µm thickness, allowing optimization of NV layer depth relative to the device under test.
Surface Engineering	Minimizing optical losses during laser coupling and fluorescence collection.	Optical Grade Polishing: Achieves surface roughness Ra < 1 nm (SCD) and Ra < 5 nm (Inch-size PCD), critical for efficient light transmission through the tapered fiber interface.
Metalization for Integration	Creating microwave strip lines or alignment marks directly on the diamond surface.	Internal capability for custom metalization (Au, Pt, Pd, Ti, W, Cu) for integrating microwave delivery structures or bonding the diamond to advanced substrates.

Engineering Support

6CCVD’s in-house PhD team provides expert consultation to accelerate research and development in quantum sensing and microwave diagnostics.

Material Selection: Assistance in choosing the optimal diamond grade (SCD vs. PCD) and nitrogen concentration for specific NV creation methods (e.g., ion implantation vs. in-situ doping) to maximize T₂ and sensitivity.
Fabrication Optimization: Support for designing custom geometries and surface treatments necessary for integrating diamond sensors into complex systems like fiber probes or on-chip MMIC diagnostic tools.
Global Logistics: Reliable global shipping (DDU default, DDP available) ensures rapid delivery of custom diamond components worldwide.

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

View Original Abstract

We present a microwave B-field scanning imaging technique using a diamond micro-crystal containing a nitrogen vacancy center that is attached to a fiber tip. We propose a pulsed modulation technique, enabling the implementation of a variety of pulsed quantum algorithms for state manipulation and fast readout of the spin state. A detailed mapping of the magnetic B-field distribution of a helical antenna with sub-100 μm resolution is presented and compared with numerical simulations. This fiber based microwave B-field probe has the advantages of minimized invasiveness and small overall size and will boost broad interest in a variety of applications where near field distribution is essential to device characterization, to name a few, antenna radiation profiling, monolithic microwave integrated circuit failure diagnosis, electromagnetic compatibility test of microwave integrated circuits, and microwave cavity field mode mapping.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.5047495