Scattering of Gaussian beams by disordered particulate media

•Electromagnetic scattering by a disordered particulate medium is studied.•The superposition T-matrix solver of the Maxwell equations is used.•Finite width of an incident Gaussian beam does not extinguish interference speckles.•Ensemble or spectral averaging is required to generate diffuse speckle-free patterns.

A frequently observed characteristic of electromagnetic scattering by a disordered particulate medium is the absence of pronounced speckles in angular patterns of the scattered light. It is known that such diffuse speckle-free scattering patterns can be caused by averaging over randomly changing particle positions and/or over a finite spectral range. To get further insight into the possible physical causes of the absence of speckles, we use the numerically exact superposition T-matrix solver of the Maxwell equations and analyze the scattering of plane-wave and Gaussian beams by representative multi-sphere groups. We show that phase and amplitude variations across an incident Gaussian beam do not serve to extinguish the pronounced speckle pattern typical of plane-wave illumination of a fixed multi-particle group. Averaging over random particle positions and/or over a finite spectral range is still required to generate the classical diffuse speckle-free regime.