High-frequency resolution diamond nitrogen-vacancy center wide-spectrum imaging technology
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-01-01 |
| Journal | Acta Physica Sinica |
| Authors | Yuanyuan Shen, Bo Wang, Dongqian Ke, Doudou Zheng, Zhonghao Li |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details the development and validation of a quantum diamond nitrogen-vacancy (NV) center system capable of high-resolution, wide-spectrum microwave imaging.
- Core Achievement: Successfully achieved 1 Hz frequency resolution across a wide operational spectrum ranging from 900 MHz to 6.0 GHz.
- Methodology: Combines spatial encoding via a magnetic field gradient (Zeeman effect) for wideband coverage with a continuous wave-mixing (heterodyne) technique for ultra-high frequency resolution.
- Resolution Enhancement: The continuous wave-mixing method uses a slightly detuned auxiliary microwave signal to generate an AC fluorescence signal, significantly enhancing the NV magnetometerâs response to weak microwave fields.
- Spatial Encoding: A spherical magnet generates a gradient up to 10.5 T/m, spatially mapping the NV center resonance frequency across the diamond surface, which is captured via CMOS imaging.
- Material Basis: Utilizes a single-crystal CVD diamond sample (110 orientation) prepared through high-energy electron irradiation (10 MeV) and high-temperature vacuum annealing (up to 800 °C).
- System Limitation Overcome: The heterodyne approach mitigates the frequency resolution degradation caused by high microwave power and non-uniform broadening inherent to wideband measurements.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Wideband Spectrum Range | 900 MHz - 6.0 GHz | Frequency | Operational range achieved via spatial encoding. |
| Frequency Resolution (Achieved) | 1 Hz | Frequency | Achieved using continuous wave-mixing (heterodyne). |
| Frequency Step Size (Tested) | 1 MHz | Frequency | Step size for independent resolution measurement points. |
| NV Center Material | Single-crystal CVD Diamond | Material | (110) orientation. |
| Sample Dimensions | 3.5 x 3.5 x 0.2 | mm | Sample size. |
| Initial Nitrogen Concentration | < 1 x 10-4 | Concentration | Prior to NV creation. |
| Laser Wavelength | 532 nm | Wavelength | Continuous wave optical pumping. |
| Laser Power (Operating) | ~200 mW | Power | Used for excitation. |
| Zero-Field Splitting (D) | ~2.87 GHz | Frequency | NV center ground state 3A2. |
| Zero-Field ODMR Linewidth | ~7.8 MHz | Frequency | Sample-dependent intrinsic linewidth. |
| Maximum Magnetic Gradient | ~10.5 T/m | Gradient | Used for spatial frequency encoding. |
| Microwave Source Range | 9 kHz - 6 GHz | Frequency | Source capability. |
| Microwave Power (Max Tested) | 30 dBm | Power | High power causes ODMR line broadening. |
| Camera Pixel Size | 3.45 ”m x 3.45 ”m | Dimension | CMOS camera resolution. |
Key Methodologies
Section titled âKey MethodologiesâThe experimental methodology integrates material preparation, precise magnetic alignment, wideband spatial encoding, and high-resolution heterodyne detection.
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NV Center Creation:
- A single-crystal CVD diamond (110) sample was used, starting with a low nitrogen concentration (< 1x10-4).
- The sample was irradiated using a 10 ± 0.5 MeV electron beam for 4 hours to create vacancies.
- The sample underwent two stages of vacuum annealing: 600 °C for 1 hour (plus 30 min hold) and 800 °C for 4 hours, to mobilize vacancies and form NV centers.
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Magnetic Field Alignment and Gradient Control:
- The diamond was mounted on a three-axis translation stage. A 13 mm spherical magnet was mounted separately.
- The magnet position was precisely adjusted to align the magnetic field (B) vector parallel to one of the NV crystal axes, maximizing the Zeeman shift for that axis and minimizing the number of observed ODMR peaks (from four pairs to two pairs, with three overlapping).
- The distance between the magnet and the diamond was fine-tuned to achieve the desired magnetic field gradient (up to 10.5 T/m) necessary for wideband spatial encoding.
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Wideband Spectrum Imaging (Spatial Encoding):
- A 532 nm laser (200 mW) was used for continuous optical pumping.
- Microwave signals (900 MHz to 6.0 GHz, 1 MHz step) were applied via a 2 mm ring antenna.
- A CMOS camera captured the fluorescence images, which were synchronized with the microwave frequency steps and averaged 200 times per data point.
- Image processing involved fitting the fluorescence intensity (PL) versus frequency for each pixel using a Lorentzian function to map the resonant frequency (f0) spatially, generating the wideband spectrum image.
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High-Resolution Measurement (Continuous Wave-Mixing):
- Two independent microwave sources were used: MW1 (resonant frequency f1) and MW2 (auxiliary frequency f1 + Îf).
- The two signals were applied simultaneously to the NV centers. The interference between the resonant and slightly detuned signals disrupts the spin polarization balance, generating an AC fluorescence signal oscillating at the difference frequency (Îf).
- By controlling Îf down to 0.1 Hz and measuring the resulting AC signal in the time domain, the system achieved a frequency resolution of 1 Hz after Fourier transformation.
Commercial Applications
Section titled âCommercial ApplicationsâThe ability to perform wideband microwave spectrum analysis with sub-hertz resolution opens up critical opportunities across several high-tech sectors.
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Quantum Sensing and Metrology:
- High-precision quantum spectrum analyzers for characterizing complex spin systems.
- Development of next-generation quantum magnetometers with enhanced sensitivity to weak, high-frequency signals.
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Wireless Communication (RF/Microwave):
- Real-time, high-fidelity monitoring and analysis of dense electromagnetic environments (e.g., 5G, 6G, and future wireless standards).
- Detection and identification of low-power or transient microwave signals in cluttered spectrums.
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Astronomy and Astrophysics:
- Ultra-sensitive detection and spectral analysis of faint, wideband radio frequency signals from cosmic sources.
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Medical and Biological Imaging:
- High-resolution mapping of localized microwave fields for Electron Paramagnetic Resonance (EPR) or Magnetic Resonance Imaging (MRI) applications, especially in small volumes.
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Defense and Security:
- Advanced signal intelligence (SIGINT) for wideband threat detection and electronic warfare analysis requiring fine spectral discrimination.
View Original Abstract
High-resolution wide-spectrum measurement techniques have important applications in fields such as astronomy, wireless communication, and medical imaging. Nitrogen-vacancy (NV) center in diamond is well known for its high stability, high sensitivity, real-time monitoring, single-point detection, and suitability for long-term measurement, and has an outstanding choice for spectrum analyzers. Currently, spectrum analyzers based on NV centers as detectors can perform real-time spectrum analysis in the range of several tens of gigahertz, but their frequency resolution is limited to a MHz level. In this study, we construct a quantum diamond microwave spectrum imaging system by combining continuous wave-mixing techniques. According to the spin-related properties of the NV center in diamond, we implement optical pumping by 532 nm green laser light illuminating the diamond NV center. A spherical magnet is used to produce a magnetic field gradient along the direction of the diamond crystal. By adjusting the size and direction of the magnetic field gradient, spatial encoding of the resonance frequency of the NV center is achieved. The magnetic field gradient induces the Zeeman effect on the diamond surface at different positions, generating corresponding ODMR signals. Through accurate programming, we coordinate the frequency scanning step size of the microwave source with the camera exposure and image storage time, and synchronize them circularly according to the order of image acquisition. Ultimately, after algorithmic processing, we successfully obtain comprehensive spectrum data in a range from 900 MHz to 6.0 GHz. Within the measurable spectrum range, the system employs continuous wave-mixing, simultaneously applying resonant microwaves and slightly detuning auxiliary microwaves to effectively excite the NV center. This method triggers off microwave interference effects, disrupting the balance between laser-induced polarization and microwave-induced spontaneous relaxation. Specifically, microwave interference causes the phase and amplitude of the fluorescence signal to change, leading to the generation of alternating current fluorescence signals. This further enhances the response of the NV magnetometer to weak microwave signals. The method enables the system to achieve a frequency resolution of 1 Hz in the measurable spectrum range, and it can separately measure the frequency resolution of multiple frequency points with a frequency step size of 1 MHz. The research results indicate that the wide-spectrum measurement based on NV centers can achieve sub-hertz frequency resolution, providing robust technical support for future spectrum analysis and applications.