Breakthrough in water-based light generation: 1,000-fold enhancement of white-light output using non-harmonic two-color femtosecond lasers

Scientists at Japan's Institute for Molecular Science have achieved a 1,000-fold enhancement in white-light generation inside water by using non-harmonic two-color femtosecond laser excitation. This previously unexplored approach in liquids unlocks new nonlinear optical pathways, enabling a dramatic boost in supercontinuum generation. The breakthrough lays a foundation for next-generation bioimaging, aqueous-phase spectroscopy, and attosecond science in water.

Researchers at the Institute for Molecular Science (NINS, Japan) and SOKENDAI have discovered a new optical principle that enables dramatically stronger light generation in water, achieving a 1,000-fold enhancement in broadband white-light output compared to conventional methods. The team used non-harmonic two-color femtosecond laser excitation, where the two laser wavelengths do not share an integer frequency ratio. While harmonic combinations (such as fundamental and second-harmonic light) are widely employed in nonlinear optics, this is the first demonstration that non-harmonic excitation in water can unlock a powerful regime of light-matter interaction.

By focusing two ultrashort pulses--1036 nm and a non-integer-related seed wavelength (e.g., 1300 nm)--into water, the researchers significantly amplified nonlinear optical effects including soliton compression, dispersive-wave emission, four-wave mixing, and cross-phase modulation. These cooperative effects produce an exceptionally bright supercontinuum, a rainbow-like white-light source crucial for ultrafast spectroscopy and imaging. Control experiments in heavy water (D₂O) showed no comparable enhancement, revealing that the effect is driven by water-specific dispersion and resonance conditions.

"By deliberately breaking the usual harmonic laser condition, we discovered a new way to amplify light inside water," says Dr. Tsuneto Kanai, lead researcher. "This opens an entirely new direction for ultrafast optics in liquids." Associate Professor Toshiki Sugimoto, the project's principal investigator, notes that "our finding offers a powerful approach to uncover phenomena of fundamental scientific and technological importance." The findings could accelerate breakthroughs in:

• deep-tissue biophotonics
• aqueous-phase and interfacial spectroscopy
• attosecond electron-dynamics studies in water
• optical sensing and nonlinear photonic technologies

This study establishes a new frontier in liquid photonics, using the world's most universal medium--water--as a platform for next-generation ultrafast optical science.

This work was released online as an Early Posting in Optics Letters on October 27, 2025. The study was conducted by the Institute for Molecular Science and SOKENDAI, Japan.