Stimulated Hyper-Raman Scattering of Light in Water, Enhanced by Plasmon Resonance, under Optical Breakdown in the Field of Picosecond Laser Pulses

In this paper, we investigate the nonlinear scattering regime in water with different gas contents. It is found that the optical breakdown threshold at pump wavelength λ = 1064 nm is determined by the bulk number density of dissolved gas molecules. In degassed water samples, four-photon scattering occurs at frequency 2ω0 – Ω0, where ω0 is the pump frequency and Ω0 is a combination of the frequencies of symmetric and antisymmetric stretching vibrations of water molecules. We associate this scattering with the four-photon process that develops on the cubic nonlinearity of the medium. At the same time, in water saturated with dissolved air, the excitation threshold of optical breakdown is significantly lower than the excitation thresholds of four-photon scattering and optical breakdown in degassed water. At the same time, in samples of water saturated with dissolved air, nonlinear scattering near frequency 2ω0 occurs simultaneously with the breakdown. We attribute this phenomenon to hyper-Raman scattering that occurs on a quadratic nonlinearity inside liquid shells of nanometer thickness surrounding gas nanobubbles. The possibility of recording hyper-Raman scattering near the second harmonic is due to the enhancement because of the excitation of plasmon resonance during optical breakdown inside gas nanobubbles. Plasmon resonance is excited as a result of beats of a wave at frequency 2ω0 and Stokes/anti-Stokes waves at frequency 2ω0 – Ω0 during the interaction on a quadratic nonlinearity inside several liquid monolayers surrounding a gas nanobubble.