Plasmonic Resonance of Low-Frequency Stimulated Raman Scattering during Water Optical Breakdown

Abstract—

Experiments on time-dependent stimulated Raman scattering (SRS) and optical breakdown in the field of a picosecond laser pulse were performed with water samples containing gas nanobubbles (bubstons) with different concentrations. It is found that an optical breakdown in water contacting with atmosphere for a long time leads to the occurrence of an anomalously strong low-frequency SRS line at a frequency of ≈700 cm^–1; its intensity significantly exceeds that of the SRS line at the frequency ≈3410 cm^–1, which is due to the O–H stretching vibration of water molecules. A hypothesis about generation of a resonant plasmon during bubston breakdown in water was proposed to explain the observed effect. It is shown that the resonant plasmon was excited due to the pump wave beating and low-frequency Stokes scattering wave, shifted by the frequency of librational vibration of water molecules.