Researchers at the Institute for Molecular Science (NINS) in Japan and SOKENDAI have reported a significant advancement in nonlinear optics. Their findings, published in the journal Nature Communications on February 3, 2026, reveal a more than 2,000% enhancement of voltage-induced nonlinear optical responses. This breakthrough was achieved by utilizing an angstrom-scale gap created between a metallic tip and substrate within a scanning tunneling microscope (STM).
The team discovered that by varying the voltage across the junction within a range of ±1 V, the intensity of second-harmonic generation (SHG) exhibited a quadratic relationship with the applied voltage. This resulted in a modulation depth of approximately 2000%/V, representing a remarkable improvement over existing electroplasmonic systems, which have not reached similar levels of efficacy.
The researchers also observed comparable levels of electrical modulation in sum-frequency generation, a nonlinear optical process that converts mid-infrared light into visible or near-infrared light. This indicates that the newly identified modulation mechanism is versatile, applicable across a broad spectral range rather than being confined to specific wavelengths or processes.
The core of this remarkable effect lies in the extreme electrostatic fields generated in the angstrom-scale gap. When voltage is applied across separated electrodes, it creates electrostatic fields between them. Given that field strength diminishes with increasing distance, applying just 1 V across a few-angstrom gap produces intense electrostatic fields on the order of 10^9 volts per meter. Such intense fields can significantly alter the electronic states of molecules trapped in the gap, thereby enhancing their nonlinear optical responses dramatically.
Conventional plasmonic structures, which typically range from tens to hundreds of nanometers, are unable to achieve such extreme conditions. This limitation has hindered access to similar levels of electrical control until now.
Dr. Shota Takahashi, an Assistant Professor at the Institute for Molecular Science, emphasized the significance of their findings. “This work shows that angstrom-scale metal gaps serve as a powerful platform for electrically controlling nonlinear light generation processes with large modulation depth,” he stated. “Such developments could pave the way for next-generation ultra-compact electro-photonic devices, where electrical and optical signals are processed and interconverted at the ultrasmall spatial scale.”
Looking ahead, Dr. Toshiki Sugimoto, Associate Professor and the principal investigator of the project, expressed ambitions to further enhance the depth of electrical modulation. “We plan to explore nonlinear optical materials that exhibit stronger electric-field responsiveness,” he noted. “We also aim to develop a more rigorous theoretical framework capable of quantitatively describing electrical modulation mechanisms operating in angstrom-scale spaces.”
The implications of this research extend across multiple fields, including nonlinear optics, nanophotonics, condensed-matter physics, and electronic engineering, highlighting the potential for advancements in technology and application.
For further details, refer to the publication: Shota Takahashi et al, Giant near-field nonlinear electrophotonic effects in an angstrom-scale plasmonic junction, Nature Communications (2026). DOI: 10.1038/s41467-026-68823-4.
