Theoretical description for nonlinear dynamic light scattering based on stimulated Raman effect (NLDLS‐SRS)

Abstract

Dynamic light scattering (DLS) is a method to analyze the diffusion constant and mesostructure of colloidal systems based on refractive index fluctuations. For example, DLS evaluates particle size distribution, particle drift velocity, and the typical length of the cooperative diffusion in gels. However, DLS inherently lacks chemical sensitivity since it detects the spatial Fourier component of the refractive index at the scattering vector (q). This work theoretically demonstrates the feasibility of nonlinear dynamic light scattering (NLDLS) of stimulated Raman effect that reflects molecular vibration. Specifically, we show that by interfering with two pump beams (in the pump‐probe method terminology), it is possible to extract the spatial Fourier component of the imaginary part of the third‐order nonlinear susceptibility. Here, the phase modulation of either of the two beams is effective for mode‐selective detection of the Fourier component. The difference in the wavevectors of these beams corresponds to q in DLS. The result is the same as DLS in mesostructural sensitivity but has a molecular vibration selectivity. The proposed assay in the present paper enables qualitative and chemical‐structure analysis of the colloidal systems without labeling and size differentiation in a mesopic regime.