A research group led by Professor Muneaki Hase from the Institute of Pure and Applied Sciences at the University of Tsukuba, Associate Professor Toshu An from the Nanomaterials and Devices Research Area at the Japan Advanced Institute of Science and Technology (JAIST), and Lecturer Paul Fons from the Faculty of Science and Technology at Keio University (at the time of the research, currently Professor in the Department of Electronics and Electrical Engineering) has successfully measured local electric field dynamics with femtosecond time resolution and nanoscale spatial resolution by integrating a quantum sensor into an atomic force microscopy (AFM). This achievement represents a significant step toward the social implementation of quantum sensors. The results were published in Nature Communications.
Schematic of ultrafast pump-probe electric field sensing measurements using a diamond NV probe. Measurements were performed in "pin-point mode," where the AFM probe was vertically approached and retracted at each designated points on the sample. The sample is scanned in the x-y direction using a piezo scanner.
Provided by the University of Tsukuba
A nitrogen-vacancy (NV) center functions as a quantum sensor capable of measuring magnetic fields, electric fields, temperature, and strain. When nitrogen is introduced into a diamond crystal, a vacancy forms adjacent to it where a carbon atom is missing, creating a quantum state.
When an electric field is applied to the NV center, an electro-optic (EO) effect occurs in which the refractive index changes. In this research, the EO effect was measured through irradiation with a femtosecond laser, and this was used as a probe to detect electric fields (diamond NV probe).
Hase explained: "Previously, attempts to investigate charge distribution on sample surfaces by combining STM (scanning tunneling microscopy) with femtosecond lasers were actively pursued in our field. When I was thinking up a new challenging theme, I conceived the idea of combining AFM with femtosecond lasers, which had been unexplored until then. Furthermore, in the AFM probe, rather than using commercially available silicon, I thought of using the diamond NV center, which is the subject of active research as a quantum sensor, in an unprecedented way through the EO effect. When we started the research, we had no idea whether it would succeed or not."
In this study, by fusing the NV center with AFM, the researchers aimed to achieve local electric field measurement with ultrafast time resolution exceeding previous detection limits, in addition to spatial resolution beyond the diffraction limit of light.
NV centers with controlled density were introduced near the surface (at a depth of 40 nanometers) of a high-quality diamond crystal with extremely few impurities, and this diamond crystal was processed into a diamond NV probe with a tip diameter of 500 nanometers or less by utilizing laser cutting and focused ion beam (FIB) technology. This diamond NV probe was attached to the cantilever of a self-sensing AFM based on the piezoresistive effect, which allows the incorporation of a femtosecond ultra-short laser.
Hase continued, "Since it is difficult to introduce a femtosecond laser in existing systematized AFMs due to space constraints, after much trial and error, we fabricated a custom-designed self-sensing AFM that is the only one of its kind in the world. Although research began in 2019, we have spent a great deal of time and effort on the design and fabrication of this AFM."
Using this system, they succeeded in detecting the surface electric field of n-type GaAs (gallium arsenide). Although the magnitude of the EO signal decreased to approximately 1/42 with the introduction of the diamond NV probe, local measurement was successful.
Furthermore, experiments were conducted using a sample in which a tungsten diselenide (WSe2) single crystal, a two-dimensional layered material, was transferred onto a silicon substrate. In this sample, the crystal thickness varied depending on the location, but attention was focused on the interface between the monolayer region in contact with the bulk crystal.
When local surface electric field measurements were performed using interfaces with different thicknesses, the researchers successfully sensed surface electric field signals reflecting the carrier characteristics of the monolayer and bulk regions with spatiotemporal resolution below 500 nanometers and 100 femtoseconds.
Additionally, when the decay of the time-resolved EO signal was modeled using an exponential function, only a component relaxing in approximately 200 femtoseconds was observed in the monolayer region. On the other hand, in the bulk region, it was found that in addition to this component, there was a contribution from a slow component relaxing in approximately 2 picoseconds. This indicates that in the monolayer region, the electric field only relaxes rapidly due to interaction with the substrate, whereas in the bulk region, intraband relaxation and intervalley relaxation of carriers bonded to the surface electric field contribute.
Hase stated: "When we were working on enhancing the sensitivity of the EO effect, after trial and error with experiments introducing NV centers at various densities, we discovered an NV center density that increases signal intensity 13-fold and reported this in a 2024 paper. In the future, to further increase detection sensitivity, we are considering methods such as incorporating plasmon enhancement through metal thin film coating used in surface-enhanced Raman spectroscopy (SERS), and methods to control the charge state of the NV center, as it is expected that the 13-fold enhancement of the EO effect is likely due to the charge state of the NV center."
The spatiotemporal-limit-sensing technology developed in this research is expected to become a foundational technology for basic physics and chemistry, such as for local electric field detection in power semiconductors and fuel cell materials, and local electric field detection in topological insulators.
Hase added, "Among the co-authors of the paper, there are researchers who have already entered the quantum life sciences field. Professor Hidemi Shigekawa of the University of Tsukuba is conducting research on photodynamic therapy using cell samples, and we plan to further advance our collaboration in the future."
Journal Information
Publication: Nature Communications
Title: An ultrafast diamond nonlinear photonic sensor
DOI: 10.1038/s41467-025-63936-8
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.