Conventional nanoscale electroplasmonic structures provide limited electrical tunability of nonlinear optical responses. Scientists at Japan's Institute for Molecular Science have demonstrated an angstrom-scale electroplasmonic platform enabling giant modulation (2000% V -1 ) of near-field nonlinear optical effects across a broad spectral range. The breakthrough provides a novel scheme of highly efficient electro-optical conversion in an infinitesimal spatial scale, laying the foundation for atomic-scale ultracompact electrophotonic information-processing technology.
Researchers at the Institute for Molecular Science (NINS, Japan) and SOKENDAI have demonstrated a more than 2000%-V −1 voltage-induced enhancement of near-field nonlinear optical responses. To achieve this giant modulation, they focused on an angstrom-scale gap formed between a metallic tip and substrate in a scanning tunneling microscope (STM), which can strongly confine and enhance light intensity through plasmon excitation (Figure 1). The researchers discovered that when the voltage across the junction was varied within ±1 V, the intensity of second-harmonic generation (SHG) changed quadratically with voltage and exhibited giant modulation with a depth of ~2000% (Figure 2). This represents a more than two-orders-of-magnitude improvement over previous electroplasmonic systems.
Moreover, similar giant electrical modulation was also observed for sum-frequency generation, a nonlinear optical process that upconverts mid-infrared light into visible or near-infrared light (Figure 3). This demonstrates that the newly discovered electrical modulation mechanism is applicable to the broad spectral range, not limited to a specific optical wavelength or nonlinear optical process.
Detailed analysis revealed that the origin of this modulation effect lies in the giant electrostatic field formed inside the angstrom-scale gap. Generally, applying the voltage across two separately placed electrodes generates electrostatic fields between them. Importantly, because the field strength scales inversely with the gap distance, applying just 1 V across the few-angstrom gap generates intense electrostatic fields on the order of 10⁹ volts per meter. Such extreme fields directly modify the electronic states of molecules confined in the gap, profoundly enhancing their nonlinear optical responses. Conventional plasmonic structures, typically tens to hundreds of nanometers in size, cannot reach this regime. This is why similar levels of electrical control have remained inaccessible until now.
"This work shows that angstrom-scale metal gaps serve as a powerful platform for electrically controlling nonlinear light generation processes with large modulation depth," says Dr. Shota Takahashi, Assistant Professor at the Institute for Molecular Science. "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."
"We plan to further push the limits of electrical modulation depth by exploring nonlinear optical materials that exhibit stronger electric-field responsiveness," says Dr. Toshiki Sugimoto, Associate Professor at the Institute for Molecular Science and the project's principal investigator. "We also aim to develop a more rigorous theoretical framework capable of quantitatively describing electrical modulation mechanisms operating in angstrom-scale spaces. Advances in these directions are expected to accelerate progress across a broad range of disciplines, including nonlinear optics, nanophotonics, condensed-matter physics, and electronic engineering."
Nature Communications
Experimental study
Not applicable
Giant near-field nonlinear electrophotonic effects in an angstrom-scale plasmonic junction
24-Jan-2026