A team of researchers has developed a novel microscopy technique that uses a microscopic diamond probe to measure electric fields with extraordinary precision. The new method achieves resolutions on the scale of nanometers for space and femtoseconds for time, opening a new window into the ultrafast processes that govern the behavior of next-generation materials and technologies. This breakthrough allows scientists to directly observe the fleeting electrical dynamics on surfaces, which could accelerate the development of faster and more efficient semiconductors, optoelectronics, and other nanoscale devices.
The core of the technology lies in a carefully engineered defect within a diamond crystal, known as a nitrogen-vacancy (NV) center. By integrating this diamond probe into an atomic force microscope and using lasers to read its state, scientists can now map the strength and behavior of electric fields with a combination of speed and accuracy that was previously unattainable. The successful demonstration on a two-dimensional material, tungsten diselenide, confirms the probe’s capability to investigate the fundamental properties of advanced materials, paving the way for innovations in quantum sensing and ultrafast electronics.
A New Standard in High-Resolution Microscopy
Scientists at the University of Tsukuba successfully combined the principles of atomic force microscopy with the unique quantum properties of diamond to create their powerful new instrument. Atomic force microscopy (AFM) is a well-established technique that uses a physical probe to scan a material’s surface to create a map of its topography with nanoscale resolution. However, a standard AFM cannot directly measure electric fields with high time resolution. The innovation here was to create a specialized probe tip made from a diamond containing specific impurities.
By using this “diamond NV probe” as the tip of the AFM, the researchers could measure local electric field dynamics with both femtosecond temporal and nanometer spatial resolution. This dual capability represents a significant leap forward. While previous quantum sensing methods were often limited to microsecond timescales, this new approach improves the time resolution to the sub-picosecond domain. This allows for the direct observation of phenomena that occur on the natural timescale of electron motion, such as the generation and propagation of electrical signals within a material after being struck by light.
Harnessing a Flaw in the Diamond
The key to the probe’s sensitivity is a specific type of crystal defect called a nitrogen-vacancy center. An NV center is formed when two adjacent carbon atoms in the diamond’s rigid lattice are replaced by a single nitrogen atom and an empty space, or vacancy. This seemingly minor imperfection creates an isolated quantum system with unique properties that are highly sensitive to its external environment, including electric fields, magnetic fields, and temperature.
The measurement relies on a phenomenon known as the electro-optic (EO) effect, where the presence of an electric field alters the refractive index of a material. Pure diamond does not exhibit a strong EO effect. However, the research team discovered in previous work that the presence of NV centers within the diamond crystal makes it an effective electro-optic material. When the diamond probe is brought near a sample’s surface, the local electric field from the sample alters the diamond’s optical properties. A femtosecond laser can then be used to detect these minute changes with extreme sensitivity, effectively allowing the probe to “see” the ultrafast electric fields.
Achieving Unprecedented Measurement Scales
In their experiments, the researchers used the diamond probe to measure electric fields near the surface of a tungsten diselenide (WSe₂) sample. WSe₂ is a two-dimensional layered material, part of a class of materials being heavily investigated for future electronic and photonic applications. The team successfully detected electric field dynamics with a temporal resolution better than 100 femtoseconds and a spatial resolution of less than 500 nanometers. A femtosecond is one quadrillionth of a second, an incredibly short timescale during which light travels only a fraction of the width of a human hair. This level of temporal detail is crucial for understanding the primary steps of charge carrier dynamics in semiconductors.
The ability to resolve these ultrafast processes at the nanoscale is essential for verifying theoretical models and for engineering new devices. For example, in solar cells and photodetectors, the efficiency of the device depends on how quickly electrons and holes (positive charge carriers) can be separated after being created by light. The diamond probe allows scientists to directly visualize these processes as they happen, providing critical feedback for designing more efficient materials and device architectures.
Expanding the Quantum Sensing Toolkit
This work builds upon a growing field of research that uses NV centers in diamond for a wide range of sensing applications. The quantum spin state of an NV center is exceptionally sensitive to external magnetic fields, a property that has been harnessed to create high-resolution magnetic sensors. Researchers in Japan, for instance, previously developed a diamond-based sensor to map the subtle magnetic fields produced by electrical currents in the hearts of living rats with unprecedented millimeter-scale resolution. That device, operating at room temperature, offered a significant advantage over conventional sensors that require cryogenic cooling.
The new technique extends this sensing capability from magnetic fields to electric fields, while pushing the time resolution to a new frontier. The sensitivity of the NV center is not limited to electric and magnetic fields. Because its properties are also affected by thermal fluctuations, the same fundamental technology holds promise for nanoscale thermal sensing. This versatility makes the diamond NV probe a powerful, multi-modal tool for investigating a wide range of phenomena at the quantum level.
Future Directions and Applications
The successful measurement of ultrafast electric fields opens the door to numerous applications in materials science, physics, and engineering. The ability to probe the dynamics of 2D materials like WSe₂ is just the first step. This technique could be used to investigate the performance of high-speed transistors, study the behavior of exotic quantum materials, and characterize the operation of novel optoelectronic devices that convert light to electricity or vice versa. By providing a direct, real-time view of electron behavior, the diamond probe can help researchers identify bottlenecks and inefficiencies in current technologies.
Further refinements to the technology could lead to even greater spatial and temporal resolution. The research was supported by several grants from the Japan Society for the Promotion of Science and the Japan Science and Technology Agency, indicating strong institutional backing for this promising area of research. As the technique matures, it could become a standard characterization tool in research laboratories, accelerating the development cycle for new materials and pushing the boundaries of what is possible in electronics and quantum computing.