Synergy and reciprocity between Brillouin and Mie scattering by the presence of silver nanoparticles in the stimulated Brillouin scattering cell

This study investigates simultaneous stimulated Mie scattering (SMS) in a Brillouin cell with low-concentration Ag nanoparticles (NPs) compared to a pure medium—a topic not previously addressed. A standing acoustic wave forms a stationary Bragg grating from NPs within the interaction length, redistributing particles to higher-density regions. We used Ag NPs with an absorption peak at 450 nm, far from the 1064 nm excitation wavelength, where plasmonic and two-photon absorption are negligible. Results show NPs produce a single hybrid process—stimulated Brillouin-Mie scattering (SBMS)—not two independent processes (SMS and SBS). NPs experience a dipole force proportional to the intensity gradient, move to standing-wave maxima, and form a Bragg grating, which enhances backscattered intensity via synergy with the SBS-induced acoustic grating. At high energies, two-photon absorption, thermal grating formation, and secondary nonlinear effects reduce SBS efficiency. However, the optimal energy for maximum reflectivity remains constant across concentrations, due to coherent field reinforcement by forward Mie scattering (no frequency shift). The colloidal medium acts as an effective material with tunable Brillouin gain and phonon lifetime, applicable to Brillouin microscopy for NP-injected biological treatments—previously studied only in pure media.