Researchers at the Institute for Molecular Science (NINS) and SOKENDAI in Japan have achieved an extraordinary enhancement in nonlinear optical responses, boosting light output by over 2,000% per volt. This breakthrough, reported on February 3, 2026, involves the use of angstrom-scale gaps formed between a metallic tip and a substrate in a scanning tunneling microscope (STM). This method allows for significant confinement and enhancement of light intensity through plasmon excitation.
The findings were published in the journal Nature Communications. The study revealed that when varying the voltage across the junction within ±1 V, the intensity of second-harmonic generation (SHG) exhibited a quadratic relationship with voltage. This resulted in a remarkable modulation depth of approximately 2000%/V, representing a more than two-orders-of-magnitude improvement compared to previous electroplasmonic systems.
Significance of the Findings
In addition to SHG, the researchers noted similar substantial 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 electrical modulation mechanism is versatile, applicable across a broad spectral range and not confined to a specific optical wavelength or process.
The underlying cause of this significant modulation effect lies in the intense electrostatic field generated within the angstrom-scale gap. Typically, applying voltage across two electrodes creates electrostatic fields between them. However, because the strength of the field scales inversely with the distance of the gap, a mere 1 V applied across a few angstroms generates electrostatic fields nearing 10^9 volts per meter. Such extreme fields can directly alter the electronic states of molecules confined within the gap, greatly enhancing their nonlinear optical responses.
Conventional plasmonic structures, which usually range from tens to hundreds of nanometers in size, have not been able to achieve this level of electrical control until now.
Future Directions for Research
“This work shows that angstrom-scale metal gaps serve as a powerful platform for electrically controlling nonlinear light generation processes with large modulation depth,” stated Dr. Shota Takahashi, Assistant Professor at NINS. He emphasized the potential for these developments to lead to next-generation ultra-compact electro-photonic devices, where electrical and optical signals can be processed and converted at an extremely small spatial scale.
Looking ahead, Dr. Toshiki Sugimoto, Associate Professor at NINS and the principal investigator for the project, expressed intentions to further explore nonlinear optical materials with enhanced electric-field responsiveness. He noted, “We aim to develop a more comprehensive theoretical framework to quantitatively describe the electrical modulation mechanisms at play in angstrom-scale environments.” This research trajectory is expected to accelerate advancements across various fields, including nonlinear optics, nanophotonics, condensed-matter physics, and electronic engineering.
The implications of this research extend beyond theoretical interest, potentially revolutionizing how light is manipulated at the nanoscale and paving the way for innovative applications in technology and materials science.
