This work appears 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—1,036 nm and a non-integer-related seed wavelength (e.g., 1,300 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.