All-Optical Technique Maps Magnetic Fields Without Microwaves

Feifei Zhou and colleagues at College of Metrology Measurement and Instrument, in collaboration with Nanyang Technological University and Southern University of Science and Technology, present an all-optical method using van der Waals quantum sensors based on negatively charged boron vacancy centres in hexagonal boron nitride. The method offers a microwave-free strategy for wide-field magnetometry by using the magnetically sensitive ground-state level anti-crossing of these centres, enabling precise determination of external magnetic fields through monitoring level shifts. The team achieved a photon shot-noise-limited sensitivity of 67.1 μT/√Hz, a roughly threefold improvement over existing optical detection methods, alongside a spatial resolution of about 1μm per pixel. They successfully imaged near-field DC magnetic field distributions over a 42 × 21 μm² area, potentially enabling key magnetometry in challenging environments.

Boron vacancy centres enable threefold enhancement in magnetic sensitivity

A sensitivity of 67.1 μT/√Hz was achieved, representing a threefold improvement over existing optically detected magnetic resonance methods. This surpasses the previously attainable limit of approximately 200 μT/√Hz, a significant advancement in the field of nanoscale magnetometry. The enhanced sensitivity unlocks the potential to detect weaker magnetic signals, previously obscured by noise, and opens new avenues for high-resolution magnetic imaging with applications spanning materials science, biological imaging, and data storage. The fundamental principle behind this improvement lies in the unique properties of negatively charged boron vacancy (V_B^-) centres within the hexagonal boron nitride ($h$-BN) lattice. These centres act as nanoscale sensors, their energy levels being highly susceptible to external magnetic fields. Specifically, the researchers exploited the magnetically sensitive ground-state level anti-crossing (GSLAC) phenomenon. GSLAC occurs when two energy levels, which would normally be separated, come into proximity due to the influence of a magnetic field, leading to a measurable shift in the optical transition frequency. This shift is directly proportional to the magnetic field strength, providing a means for precise magnetic field determination. Utilising negatively charged boron vacancy centres in hexagonal boron nitride, the all-optical technique circumvents the need for complex microwave equipment, simplifying experimental setups and reducing potential sources of interference commonly associated with traditional optically detected magnetic resonance (ODMR) techniques.

A 42 × 21 μm² area was used for wide-field imaging of direct-current magnetic fields generated by a current-carrying circuit, confirming the technique’s capability for spatial mapping. This demonstration of spatial resolution, approximately 1μm per pixel, is crucial for applications requiring detailed magnetic field mapping at the microscale. Raman spectroscopy and photoluminescence analysis confirmed the successful creation of these V_B^- centres within the hexagonal boron nitride ($h$-BN) material, exhibiting zero-field splitting parameters of around 3.48GHz and 50MHz respectively. These parameters are intrinsic to the V_B^- centre and characterise the energy level structure in the absence of an external magnetic field, providing a baseline for accurate magnetic field measurements. The 3.48GHz value relates to the splitting between the spin sublevels, while the 50MHz value represents a hyperfine interaction. The creation of these centres involves careful control of the $h$-BN flake during fabrication, often utilising electron beam irradiation followed by annealing to generate the vacancies and introduce the desired negative charge state. Although this 67.1 μT/√Hz sensitivity represents a strong advance, the current demonstration remains limited to a single pixel; scaling to a full-area scan while maintaining performance will be vital for practical applications. Maintaining this sensitivity across a larger area presents significant challenges related to variations in V_B^- centre density and optical alignment.

Simplified magnetic field imaging via all-optical sensing requires further noise validation

This approach offers a pathway towards more convenient magnetometry, particularly in environments where bulky microwave equipment presents a practical obstacle, offering a clear advantage for applications like materials science and potentially even biomedical imaging. The elimination of microwave components not only simplifies the experimental setup but also reduces radio frequency interference, which can be a significant limitation in sensitive magnetic field measurements. For instance, in materials science, this technique could be used to characterise the magnetic properties of nanoscale devices or to study the magnetic domain structure of thin films. In biomedical imaging, it could potentially be used to detect weak magnetic fields generated by neuronal activity or to track magnetically labelled cells. However, the current demonstration relies on an estimated sensitivity value, rather than a fully noise-characterised measurement. This raises an important question: can this sensitivity be reliably reproduced and improved upon with rigorous noise analysis across a larger imaging area, or does the reported performance represent an optimistic upper limit. A comprehensive noise analysis would involve identifying and quantifying all sources of noise, including photon shot noise, dark counts in the detector, and fluctuations in the laser intensity.

Quantifying noise sources and optimising the signal-to-noise ratio will be the focus of further investigation to ensure strong and reproducible measurements. Techniques such as lock-in amplification and careful shielding of the experiment from external electromagnetic interference will be crucial in minimising noise. The team at College of Metrology Measurement and Instrument, in collaboration with Nanyang Technological University, Southern University of Science and Technology, and Southern University of Science, established a new method for mapping magnetic fields. Negatively charged boron vacancy (V_B^-) centres, tiny imperfections within hexagonal boron nitride, are utilised, and the technique exploits a characteristic energy level shift, known as ground-state level anti-crossing, to precisely determine magnetic field strength. Reliance on optical detection, rather than microwave frequencies, allows for a more compact and potentially portable sensing system. Future work will likely focus on improving the density and coherence of the V_B^- centres, as well as developing more sophisticated data processing algorithms to extract the magnetic field information from the optical signal with greater accuracy and precision. The long-term goal is to create a practical, high-resolution, and portable magnetic imaging system based on this all-optical technique.

The research demonstrated all-optical, wide-field imaging of magnetic fields using negatively charged boron vacancy (V_B^-) centres in hexagonal boron nitride. This new technique measures magnetic fields by monitoring shifts in an energy level, achieving a sensitivity of 67.1 $μ$T/$\sqrt{\text{Hz}}$ and a spatial resolution of approximately 1 $μ$m per pixel over an area of 42 × 21 $μ$m². It represents an improvement over existing optical detection magnetic resonance methods and offers a pathway towards more robust quantum sensing. Researchers plan to further refine the technique by optimising signal-to-noise ratios and improving the properties of the V_B^- centres themselves.