A comparative study of anti-Stokes shift under stimulated Rayleigh-Mie scattering in suspensions of Ag nanoparticles obtained in plasma discharge in liquid under ultrasonic cavitation

Abstract

Stimulated Rayleigh–Mie scattering (SRMS) in two-photon absorption liquids is realized by a Fourier transform-limited pulsed Nd-glass laser. For the first time, we have measured anti-Stokes spectral shifts of SRMS in toluene and hexane colloids of Ag nanoparticles, as well as in pure toluene. The suspensions are prepared in the plasma discharge excited in a liquid exposed to an intense ultrasonic field above the cavitation threshold. This novel technique has been developed for the synthesis of various nano-sized materials; it provides silver nanoparticles suspensions with controlled narrow distribution of the particle size. Ultrasonic cavitation results in a drastic change in the physical properties of the liquid, providing specific conditions in the electrical discharge plasma. Ag nanoparticles with 1-2 nm radii in toluene and hexane are obtained from silver electrodes under these specific conditions and acquire unique surface characteristics which prevent them from secondary agglomeration. The values of anti-Stokes spectral shifts of SRMS appreciably exceed the Rayleigh line width in those liquids. The four-wave mixing method is applied both experimentally and theoretically to display the process as Rayleigh-induced parametric generation. We show that the amplification effect is provided predominantly by thermally induced coherent polarization oscillations, while an interference-assisted thermal grating provides formation of a self-induced optical cavity inside the interaction region.