Spectra of light reflected by aggregate structures of submicron particles

We analyze the results of numerically exact computer modeling of spectral dependences of the intensity of light scattered by different aggregates of submicron spherical particles in the visible range and compare them to those of individual homogeneous spheres. The computations are performed using the Lorenz-Mie theory and the superposition T-matrix methods for spheres and aggregates, respectively. We show that, in the spectra of aggregates, along with the interference extrema peculiar of individual constituents, an additional maximum appears at longer wavelengths. The latter is considered as manifestation of collective effects in aggregate structures and can be explained by the interference of waves singly scattered by aggregate constituents that form groups along the incident radiation direction. In the spectra of randomly oriented fractal-like aggregates, this maximum becomes narrower with increasing the number of constituents and, starting from some number, its position becomes almost resistant to the further growth of aggregates and sensitive only to the refractive index and sizes of constituents. The number of constituents providing a stable collective maximum is higher for fluffier structures. In comparison with rather densely packed clusters, sparser ones exhibit the less expressed collective maximum with a slope weakly declining to the long-wavelength range. In the spectra of clusters containing particles with slightly varied sizes, the collective maximum survives, while the interference features induced by individual constituents naturally become smoother. (C) 2020 Elsevier Ltd. All rights reserved.