Non-harmonic two-color femtosecond lasers achieve 1,000-fold enhancement of white-light output in water
Sadie Harley
scientific editor
Robert Egan
associate editor
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.
This in Optics Letters.
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鈥�1,036 nm and a non-integer-related seed wavelength (e.g., 1,300 nm)鈥攊nto 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 (D鈧侽) showed no comparable enhancement, revealing that the effect is driven by water-specific dispersion and resonance conditions.
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Wavelength-dependent dispersion and phase-mismatch conditions for four-wave mixing in ordinary water (H鈧侽) and heavy water (D鈧侽). The figure illustrates how water's unique dispersion enables efficient nonlinear optical coupling under non-harmonic two-color excitation, while D鈧侽 does not satisfy the same resonance and group-velocity matching conditions. This difference explains why the dramatic enhancement in supercontinuum generation occurs only in H鈧侽. Credit: Institute for Molecular Science / Tsuneto Kanai -
Measured white-light spectra generated in H鈧侽 and D鈧侽 under non-harmonic two-color excitation. A dramatic broadband enhancement is observed only in H鈧侽, demonstrating that the effect arises from water-specific dispersion and ultrafast interaction pathways. The absence of enhancement in D鈧侽 confirms that subtle molecular and vibrational properties of water dictate the nonlinear optical response. Credit: Institute for Molecular Science / Tsuneto Kanai
"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鈥攚ater鈥攁s a platform for next-generation ultrafast optical science.
The study was conducted by the Institute for Molecular Science and SOKENDAI, Japan.
More information: Tsuneto Kanai et al, Dramatic Enhancement of Supercontinuum Generation in H鈧侽 by Non-Harmonic Two-Color Excitation, Optics Letters (2025).
Journal information: Optics Letters
Provided by National Institutes of Natural Sciences
