Nonlinear light scattering spectra in water: Dependence on the content of dissolved gas and effects of non‐stationarity

Here, we study stimulated nonlinear scattering in the spectral range near the second optical harmonic of a laser pump in water samples containing gas nanobubbles. It was found that the spectral and spatial characteristics of nonlinear scattering radiation undergo significant changes upon settling of initially degassed water samples in a flask, open to the atmospheric air. It turned out that the contact with the atmospheric air results in the growth of the volume number density of gas nanobubbles. It appeared that the physical mechanism of nonlinear scattering changes upon increasing the nanobubble content. In fact, four‐photon parametric scattering in degassed water, which occurs due to the cubic nonlinear susceptibility χ(3) of medium, transforms upon increasing the content of nanobubbles into stimulated hyper‐Raman scattering controlled by the constant (∂χ(2)/∂q), where χ(2) is the quadratic nonlinear susceptibility and q is coordinate of a nucleus. The physical nature of the observed phenomenon is the generation of a surface plasmon arising due to the optical breakdown inside a gas nanobubble, which leads to an effective buildup of vibrations of molecules/ions localized in the surface layer of nanobubble. The plasmon bandwidth Δω was theoretically estimated. It turned out that Δω ≈ ω0, where ω0 is the resonant plasmon wavenumber, that is, in our case, the quality factor of the plasmon oscillator is very low. Spectrum of nonlinear scattering for degassed water (left) and the same spectrum in the case where initially degassed water was in contact with the atmosphere for t = 12 h (right).