At the heart of this breakthrough lies the manipulation of the plasmonic nanogap formed between a metallic gold (Au) tip and substrate within a scanning tunneling microscope (STM) system. The angstrom-scale dimension of this gap—on the order of one-tenth of a nanometer—permits extreme confinement of electromagnetic fields, which is unattainable in conventional plasmonic arrangements characterized by tens to hundreds of nanometers. The researchers utilized this ultra-small gap to generate stimulated second-harmonic generation (SHG) signals that could be modulated by varying the voltage across the junction by only ±1 volt, yielding a quadratic dependence of SHG intensity on the applied bias.

This extraordinary voltage-tunable response arises from the immense electrostatic fields established inside the gap, which are on the order of 10⁹ volts per meter due to the inverse scaling of field strength with gap distance. Such colossal fields have a profound effect on the electronic states of molecules situated within the gap, dynamically altering their nonlinear optical susceptibilities. Unlike traditional plasmonic architectures where the applied fields are significantly weaker, this angstrom-scale metal junction creates an environment conducive to highly efficient and tunable nonlinear light-matter interactions.

Further extending the versatility of this platform, the team observed similar giant electrical modulation in sum-frequency generation (SFG) processes, which facilitate the up-conversion of mid-infrared photons into visible or near-infrared light. This finding highlights the tunability’s broadband nature and its applicability beyond a single nonlinear optical phenomenon or wavelength regime. Such flexibility is critical for future applications that demand multispectral control over light emission and conversion at the nanoscale.

This research not only elucidates the mechanisms underlying the voltage-controlled enhancement of nonlinear optical processes but also establishes a new paradigm for miniaturization in electro-optical devices. The angstrom-scale plasmonic junction represents an unprecedented technological platform where electrical and optical signals can be interfaced and manipulated simultaneously within a spatial domain reduced to the ultimate atomic scale.

Key to the implementation of this technology is the precision employed in constructing and stabilizing the STM junction, involving a gold tip delicately positioned over a gold substrate. The femtosecond near-infrared laser irradiation at fundamental frequency ω excites plasmonic resonances within the nanogap, while the resulting SHG at 2ω is detected with high sensitivity. This experimental setup leverages the synergy between scanning probe microscopy and nonlinear optics, offering unprecedented spatial and spectral resolution.

The implications of such efficient voltage-driven modulation are far-reaching. By achieving extremely high modulation depths, the electromodulation of nonlinear optical signals can significantly reduce the energy consumption in electro-photonic circuits. This is especially crucial for the ongoing miniaturization trends in photonics, where device footprints and power requirements must simultaneously shrink while maintaining, or even enhancing, performance.

Dr. Shota Takahashi, the lead author and assistant professor at IMS, emphasizes the transformative potential of this discovery. He notes that the ability to electrically govern nonlinear light generation with such precision and depth at angstrom scales could catalyze the development of the next generation of ultracompact electro-photonic devices. These devices may seamlessly interconvert electrical and optical information at scales far beyond the reach of current technologies, heralding new avenues in data processing and communication.

Looking ahead, the research team plans to explore materials exhibiting even stronger electric-field responsiveness to push the boundaries of modulation depth further. Parallel efforts aim to develop robust theoretical frameworks capable of quantitatively predicting electrical modulation effects in angstrom-scale junctions. Such models are paramount for transferring this technological innovation into practical devices and for extending it across diverse scientific disciplines.

The synergy of nonlinear optics, nanophotonics, condensed matter physics, and electronic engineering realized in this work underscores the multidisciplinary essence of modern scientific innovation. By harnessing ultra-confined electrostatic fields, the researchers have successfully bridged the gap between atomic-scale electronic manipulation and macroscopic optical phenomena, demonstrating a paradigm shift in the control of light.

In conclusion, the angstrom-scale plasmonic junction pioneered by the IMS team exemplifies the profound impact that nanoscale engineering can have on the fundamental understanding and application of nonlinear optical processes. This innovation sets a new standard for voltage-induced modulation efficiencies and paves the way not only for ultra-efficient electrophotonic devices but also for a deeper understanding of light-matter interactions at the smallest conceivable scales.

Subject of Research: Not applicable

Article Title: Giant near-field nonlinear electrophotonic effects in an angstrom-scale plasmonic junction

News Publication Date: 24-Jan-2026

Web References: 10.1038/s41467-026-68823-4

Image Credits: Adapted from Takahashi et al. (2026), Nature Communications

Keywords

Nonlinear Optics, Electroplasmonics, Angstrom-Scale Gap, Scanning Tunneling Microscope, Second-Harmonic Generation, Sum-Frequency Generation, Ultrafast Laser, Electro-Optical Modulation, Nanophotonics, Atomic-Scale Electronics, Plasmonics, Electro-Photonic Devices

At the heart of this research lies tungsten ditelluride (WTe₂), a layered compound consisting of a lattice where a single tungsten layer is sandwiched between sheets of tellurium atoms. This material has captivated scientists due to its exotic quantum behaviors, presenting opportunities to explore phenomena not accessible in conventional substances. By employing a meticulously engineered scanning tunneling microscope (STM) capable of resolving individual atoms, the MSU team could physically observe how atoms respond dynamically when subjected to terahertz frequency laser pulses. These pulses, traveling at the staggering rate of hundreds of trillions per second, interact with the material in a way that ‘nudges’ the top atomic layer out of alignment with its underlying counterparts, akin to slightly skewing the uppermost page of a stacked book.

This controlled displacement is more than just a structural curiosity. When the tungsten ditelluride’s surface atoms are displaced by the amplified terahertz field concentrated at the STM tip, the material’s electronic behavior shifts dramatically. Essentially, the top layer’s misalignment activates a unique “on” state, distinct from the normal “off” without laser influence. Pioneering the concept of a nanoscale switch, this transient modulation can toggle the electronic properties of the material in real time. Crucially, the STM not only produces these atomic distortions but also images them with subatomic resolution, providing unprecedented insight into how such perturbations govern the material’s new quantum states.

The success of this endeavor hinged on a sophisticated collaboration between experimentalists led by Associate Professor Tyler Cocker and theorists under Assistant Professor Jose L. Mendoza-Cortes. While Cocker’s team engineered and performed intricate laser-STM experiments, Mendoza-Cortes harnessed advanced quantum computational models to simulate and predict the behavior of WTe₂ systems under terahertz perturbations. Their combined efforts validated that the layers shift by approximately seven picometers during atomic vibrations—motions subtle enough to challenge even the most sensitive microscopes but crucial to the underlying electronic transformations. Furthermore, the theoretical models precisely matched the experimental frequencies at which atoms oscillated, offering a comprehensive picture of atomic dynamics and their directional displacements.

By bridging experimental observations and quantum simulations, the researchers uncovered that the induced atomic motion is localized exclusively to the material’s topmost layer, emphasizing the potential for targeted control at the nanoscale. This localized modulation could be exploited to design ultra-compact and swift electronic switches, underpinning devices with exceptional speed and minimal power consumption. Graduate student Daniel Maldonado-Lopez explained that this level of atomic-scale control might lay the foundation for future electronics that operate well beyond current limitations, presenting new avenues for integrating quantum materials into everyday technology.

At a broader scientific level, this work signifies a critical leap in the study of quantum materials—the exotic substances whose internal electrons exhibit behaviors defying classical physics. WTe₂ itself has garnered attention for its unusual electronic phases, including topological and Weyl semimetal properties, which hold promise for next-generation quantum computing platforms. By demonstrating that femtosecond laser pulses can induce real-time structural and electronic switching in such materials, the MSU researchers have opened a new realm where quantum mechanical effects can be harnessed actively, rather than merely observed passively.

This research also exemplifies how advanced instrumentation pushes the envelope of what is technologically possible. The scanning tunneling microscope used is not a typical imaging tool but a custom-built apparatus where the concentration of terahertz electromagnetic energy at the tip dramatically amplifies local fields. This enables direct coupling between light and matter on scales where traditional optical techniques fail, thus ushering in a new paradigm of light-matter interaction studies. By “reading” the atomic landscape electrically instead of visually, the STM reveals intricate behaviors and enables environment control conducive to exploring transient quantum states.

Significantly, the temporal aspect of this switching is pivotal. The terahertz pulses act as an ultrafast trigger to change material properties on timescales far shorter than typical thermal fluctuations or electronic noise, which means devices based on such phenomena could operate at unprecedented speeds. This dynamical control, transient yet repeatable, also avoids permanent alterations to the material’s structure, suggesting a route to reversible and energy-conscious device engineering. The team’s ability to photograph “on” and “off” states at the atomic scale is a landmark achievement, marrying optical, electronic, and mechanical modalities in one seamless system.

Looking ahead, the MSU scientists envision their findings propelling forward the development of technologies crucial for quantum information processing and energy-efficient computation. As components in consumer electronics—from smartphones to laptops—are fundamentally composed of materials like WTe₂, being able to dynamically switch their electronic states through controlled atomic movement heralds a transformative leap. This shift could reduce manufacturing costs, decrease power consumption, and elevate device performance simultaneously. Stefanie Adams, a graduate researcher part of the experimental team, emphasized that the material choices embedded in technology often seem fixed but can be reimagined through such quantum-scale innovations.

The study’s publication in the prestigious journal Nature Photonics underscores its significance within the scientific community. By harnessing computational resources from MSU’s Institute for Cyber-Enabled Research and blending them with breakthrough experimental techniques, the research serves as a shining example of interdisciplinary cooperation driving quantum science forward. As Tesla-sized magnetic fields and ultrafast lasers converge, the frontier of physics is no longer just about observing the quantum world but actively engineering it for practical applications.

In essence, this work is a testament to how wiggling atoms in tailored quantum materials can unlock unexpected pathways to revolutionize electronics. The transient atomic distortions induced by terahertz fields transform the electronic landscape on demand, offering a glimpse into a future where quantum materials are not just passive substrates but active elements in next-generation devices. With further research and development, the synergy of optical control and atomic precision championed by the MSU team could herald a new era of technological innovation, reshaping how we interact with electronic systems at the most fundamental level.

Subject of Research: Manipulation of atomic structure and electronic properties in quantum materials using terahertz laser pulses.

Article Title: Terahertz field control of surface topology probed with subatomic resolution

Web References:

• Michigan State University News
• Nature Photonics Journal

Keywords

Physics, Quantum Materials, Terahertz Pulses, Scanning Tunneling Microscopy, Electronic Switching, Quantum Computing, Material Science, Nanotechnology, Applied Sciences and Engineering

Quantum computers promise computational capacities exponentially surpassing those of classical supercomputers, leveraging quantum phenomena to tackle problems previously deemed intractable. However, their advancement is hampered by the fragility of quantum states, which are notoriously susceptible to environmental interference—a phenomenon known as quantum decoherence. For decades, physicists have hunted for materials resistant to this decoherence, capable of sustaining quantum information stably, yet definitive experimental validation has remained elusive.

The Oxford team, led by the Davis Group, introduced a sophisticated scanning tunneling microscope (STM) technique, innovatively adapted to capture minute quantum signatures at the atomic scale without perturbing the sample with external disturbances. This Andreev STM mode, conceptualized by Professor Séamus Davis, allows the detection of electrons confined within unique quantum states localized on the surface of topological superconductors—materials theorized to support exotic quasiparticles called Majorana fermions.

Topological superconductors represent an extraordinary quantum phase of matter whose surface states are “topologically protected,” meaning the stored quantum information is encoded in the system’s global geometric features rather than in fragile local states. Majorana fermions, in this context, emerge as quasiparticles capable of encoding quantum information non-locally. This non-local encoding inherently shields the quantum information from local noise or disturbances, dramatically enhancing qubit stability and paving the way for fault-tolerant quantum computing.

Despite theoretical predictions, the direct experimental identification of intrinsic topological superconductors has been a formidable challenge. Uranium ditelluride (UTe₂), discovered in 2019, has long tantalized physicists as a promising candidate exhibiting spin-aligned electron pairs—a crucial indicator of intrinsic topological superconductivity. Yet, concrete experimental confirmation had remained out of reach until now.

Utilizing the Andreev STM, the research team obtained ultra-high-resolution spectroscopic data revealing zero-energy surface states on UTe₂ that are consistent with theoretical predictions for intrinsic topological superconductivity. These findings confirm that UTe₂ naturally hosts such exotic quantum states, substantiating its status as an intrinsic topological superconductor. However, intriguingly, the Majorana fermions detected in UTe₂ appear in inseparable pairs, contrary to earlier expectations that envisaged isolated Majorana modes crucial for topological quantum computing.

This limitation notwithstanding, the employed Andreev STM technique stands as a landmark advance, offering a direct and reliable experimental methodology for probing topological superconductivity in a broad array of materials. By enabling the precise discrimination of intrinsic properties from extrinsic artefacts, this approach equips physicists with the tools needed to methodically explore and identify new superconducting materials that may harbor unpaired Majorana modes essential for quantum devices.

The implications of this work extend far beyond fundamental physics. Previous attempts to harness topological qubits—such as Microsoft’s Majorana 1 Quantum Processing Unit, which relies on synthetically engineered superconducting heterostructures—underscore the complexity and expense inherent in current quantum computer hardware development. The capacity to discover simple, naturally occurring crystalline materials that intrinsically exhibit topological superconductivity could greatly streamline and economize the fabrication of robust quantum devices.

Professor Séamus Davis highlighted the transformative potential of the discovery: the confluence of developing a new measurement technique, direct observation of topological surface states, and unambiguous identification of intrinsic topological superconductivity collectively heralds a new era in quantum materials science. This foundation promises to accelerate the identification of suitable materials to realize the anticipated quantum computing revolution.

Lead author Dr. Shuqiu Wang expressed enthusiasm regarding the breakthrough, emphasizing the excitement of witnessing the first spectroscopic signature of intrinsic topological superconductivity. Dr. Wang anticipates that the Andreev STM technique will unlock further discoveries in this vibrant research frontier, revealing previously inaccessible and exotic quantum phenomena.

This collaborative study also involved notable contributions from leading institutions such as the University of California – Berkeley, Lawrence Berkeley National Laboratory, Cornell University, University of Bristol, University of Maryland, Washington University, University College Cork, and University of Notre Dame, reflecting a worldwide commitment to solving the quantum materials puzzle.

Intrinsic topological superconductors, while still largely theoretical, are rapidly gaining experimental traction thanks to advances such as the Andreev STM. These efforts collectively signify a paradigm shift towards practical quantum technologies capable of harnessing exotic quantum matter in scalable, cost-effective platforms.

As quantum computing edges closer to practical realization, the innovations emerging from Oxford’s Davis Group have illuminated a promising corridor through the intricate landscape of quantum materials. The identification and characterization of intrinsic topological superconductors like UTe₂ enhance the roadmap toward fault-tolerant quantum machines, potentially ushering in an era where the full power of quantum mechanics can be harnessed to revolutionize computation, cryptography, and materials science across the globe.

Subject of Research: Identification and characterization of intrinsic topological superconductivity in uranium ditelluride (UTe₂) using the novel Andreev scanning tunneling microscope technique.

Article Title: Pair wave function symmetry in UTe2 from zero-energy surface-state visualization

News Publication Date: 29-May-2025

Web References:

• Davis Group, Oxford University – http://davis-group-quantum-matter-research.ie/
• DOI link to article – http://dx.doi.org/10.1126/science.adk7219

Image Credits: Catherine Dawson, Davis Group

Keywords: Quantum Computing, Topological Superconductors, Majorana Fermions, Andreev STM, Quantum Decoherence, UTe₂, Scanning Tunneling Microscope, Topological Quantum Computing, Quantum Materials, Intrinsic Topological Superconductivity