Researchers at the Institute for Molecular Science (NINS, Japan) and SOKENDAI have demonstrated a more than 2000% 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. The paper is published in the journal Nature Communications.

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%/V. 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. 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.