Near-Field Microwave Imaging Method of Monopole Antennas Based on Nitrogen-Vacancy Centers in Diamond

At a Glance

Metadata	Details
Publication Date	2024-05-22
Journal	Micromachines
Authors	Xuguang Jia, Yue Qin, Zhengjie Luo, Shining Zhu, Xin Li
Institutions	North University of China
Citations	3
Analysis	Full AI Review Included

Executive Summary

This research introduces a highly efficient, non-contact method for near-field microwave imaging of monopole antennas, leveraging Nitrogen-Vacancy (NV) centers in diamond.

Non-Invasive Wide-Field Imaging: The system uses a whole diamond chip as a combined probe and camera, enabling wide-field imaging (5 x 5 mm) without the need for mechanical scanning, which significantly reduces measurement time and eliminates probe displacement errors.
High Resolution and Speed: Achieved a high spatial resolution of 3 µm, allowing for detailed visualization of micro-monopole antenna fields (100 µm diameter). Measurement time is fixed at 20 s regardless of resolution (up to 150 x 150), drastically outperforming traditional scanning methods (e.g., 112 s for 100 x 100 resolution).
Accurate Field Characterization: The diamond NV probe is non-metallic and non-interfering, providing accurate near-field distribution maps, including 3D field reconstruction via layered scanning.
Dynamic Phase Tracking: The method successfully tracks dynamic phase changes in the monopole antenna, achieving an optimal input microwave phase resolution of 0.52° at 0.8494 W.
Mechanism: Microwave intensity is derived from the contrast of the Optically Detected Magnetic Resonance (ODMR) spectrum, which is read out optically via red fluorescence captured by a CMOS camera.
Operating Range: The system demonstrated effective imaging within a bandwidth of 2.7 to 3.2 GHz.

Technical Specifications

Parameter	Value	Unit	Context
Sensing Element	Nitrogen-Vacancy (NV) Centers	N/A	Embedded in diamond chip
Diamond Chip Size	4 x 4	mm	Square chip used as probe
Monopole Antenna Size	100 µm diameter, 15 mm length	µm, mm	Antenna under test
Field of View (FOV)	5 x 5	mm	Imaging area
Spatial Resolution	3	µm	Achieved resolution limit
Imaging Bandwidth	2.7-3.2	GHz	Operating frequency range
Optimal Input Phase Resolution	0.52	°	Achieved at optimal power
Optimal Input Microwave Power	0.8494	W	Power level for best phase resolution
Excitation Wavelength	532	nm	Green laser source
Measurement Time (50x50 Res.)	20	s	Diamond method (vs 65 s traditional)
Measurement Time (100x100 Res.)	20	s	Diamond method (vs 112 s traditional)
Pixel Averaging (M value)	50	N/A	Number of pixels averaged per unit for ODMR calculation

Key Methodologies

The near-field microwave imaging system relies on three main components: an optical system, a microwave system, and a data acquisition system, following these key steps:

Experimental Setup: A square diamond chip containing NV centers is positioned directly above the monopole antenna under test. A copper wire (100 µm radius) is placed on the diamond surface and connected to the microwave source.
Uniform Illumination: A 532 nm green laser is expanded, collimated, and passed through a Fly’s Eye Homogenizer to ensure uniform Gaussian illumination across the 4 x 4 mm diamond surface.
Fluorescence Acquisition: The NV centers are excited by the laser and the microwave field (MW) from the antenna. The resulting red fluorescence is collected, filtered by a bandpass filter, and captured by a CMOS camera.
Data Synchronization: A synchronization pulse sequence coordinates the laser, MW source, and external camera trigger to ensure chronological saving of fluorescence signals.
Image Segmentation and Averaging: The captured fluorescence image is segmented into multiple spatial units. Data from M=50 pixels within each unit are averaged to improve the signal-to-noise ratio (SNR) and system stability.
ODMR Spectrum Generation: Continuous Wave Optically Detected Magnetic Resonance (CW-ODMR) experiments are performed. The fluorescence data collected across N frames for each spatial unit are used to compute the unit’s ODMR spectrum.
System Calibration: A microwave power characteristic curve is established by measuring the ODMR spectral contrast under varying, uniform microwave powers. This curve relates contrast directly to microwave field intensity.
Field Reconstruction: The ODMR contrast obtained from the antenna measurement is inversely solved using the calibration curve to determine the microwave field intensity at each spatial unit, thereby reconstructing the near-field image.
3D and Phase Imaging: Three-dimensional field maps are generated by performing layered scanning, precisely controlling the vertical distance (z-axis) between the diamond and the antenna. Phase tracking is achieved by varying the input microwave phase via a phase shifter and observing the resulting field distribution changes.

Commercial Applications

This diamond NV center-based imaging technology offers significant advantages in speed, resolution, and non-invasiveness for several high-tech sectors:

Quantum Sensing and Metrology: Core application for developing high-sensitivity, room-temperature quantum sensors for magnetic and electric field mapping.
RF and Microwave Component Testing: Essential for the design, manufacturing, and quality control of miniaturized antennas (like micro-monopoles), integrated circuits (ICs), and high-frequency components.
Non-Destructive Evaluation (NDE): Provides non-contact, non-invasive testing of microwave devices, crucial for components that cannot tolerate traditional metallic probes.
Semiconductor and Integrated Circuit (IC) Analysis: High-resolution imaging (3 µm) allows for the visualization of microwave fields generated by active circuits within integrated chips, aiding in failure analysis and performance optimization.
5G/6G Technology Development: Characterization of high-frequency, small-scale antennas and transmission lines required for next-generation wireless communication systems.

View Original Abstract

Visualizing the near-field distribution of microwave field in a monopole antenna is very important for antenna design and manufacture. However, the traditional method of measuring antenna microwave near field distribution by mechanical scanning has some problems, such as long measurement time, low measurement accuracy and large system volume, which seriously limits the measurement effect of antenna microwave near field distribution. In this paper, a method of microwave near-field imaging of a monopole antenna using a nitrogen-vacancy center diamond is presented. We use the whole diamond as a probe and camera to achieve wide-field microwave imaging. Because there is no displacement structure in the system, the method has high time efficiency and good stability. Compared with the traditional measurement methods, the diamond probe has almost no effect on the measured microwave field, which realizes the accurate near-field imaging of the microwave field of the monopole antenna. This method achieves microwave near-field imaging of a monopole antenna with a diameter of 100 µm and a length of 15 mm at a field of view of 5 × 5 mm, with a spatial resolution of 3 µm and an imaging bandwidth of 2.7~3.2 GHz, and an optimal input microwave phase resolution of 0.52° at a microwave power of 0.8494 W. The results provide a new method for microwave near-field imaging and measurement of monopole antennas.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi15060679

References

2005 - A small wideband microstrip-fed monopole antenna [Crossref]
2022 - A Bayesian Compressive Sensing Approach to Robust Near-Field Antenna Characterization [Crossref]
2022 - An Effective Antenna Pattern Reconstruction Method for Planar Near-Field Measurement System [Crossref]
2022 - An Adaptive Data Acquisition Technique to Enhance the Speed of Near-Field Antenna Measurement [Crossref]
2014 - Fast Antenna Testing with Reduced Near Field Sampling [Crossref]
2022 - Single-Cut Phaseless Near-Field Measurements for Fast Antenna Testing [Crossref]
2022 - Photonics-Based Near-Field Measurement and Far-Field Characterization for 300-GHz Band Antenna Testing [Crossref]