We have shown within the quasistatic approximation that the giant fluctuations of local electromagnetic field in random fractal aggregates of silver nanospheres are strongly correlated with a local anisotropy factor S which is defined in this paper. The latter is a purely geometrical parameter which characterizes the deviation of local environment of a given nanosphere in an aggregate from spherical symmetry. Therefore, it is possible to predict the sites with anomalously large local fields in an aggregate without explicitly solving the electromagnetic problem. We have also demonstrated that the average (over nanospheres) value of S does not depend noticeably on the fractal dimension D, except when D approaches the trivial limit D = 3. In this case, as one can expect, the average local environment becomes spherically symmetrical and S approaches zero. This corresponds to the well-known fact that in trivial aggregates fluctuations of local electromagnetic fields are much weaker than in fractal aggregates. Thus, we find that, within the quasistatics, the large-scale geometry does not have a significant impact on local electromagnetic responses in nanoaggregates in a wide range of fractal dimensions. However, this prediction is expected to be not correct in aggregates which are sufficiently large for the intermediate-and radiation-zone interaction of individual nanospheres to become important.

We calculate the quasistatic electromagnetic density of states for aggregates of touching spheres, in particular, linear chains and computer-generated random fractal aggregates. Multipole moments with orders of up to L = 64 are taken into account for random aggregates with the number of particles of up to N = 100 and up to L = 8000 for linear chains. Extensive comparisons with the dipole approximation and geometrical cluster renormalization method are performed. Extinction spectra are calculated for several metals and black carbon. Long wavelength electromagnetic properties of fractal aggregates are considered in details.

We investigate the effect of particle polydispersity on the optical extinction spectra of colloidal aggregates of spherical metallic (silver) nanoparticles, taking into account the realistic interparticle gaps caused by layers of stabilizing polymer adsorbed on the metal surface (adlayers). The spectra of computer-generated aggregates are computed using two different methods. The coupled-multipole method is used in the quasistatic approximation and the coupled-dipole method beyond the quasistatics. The latter approach is applicable if the interparticle gaps are sufficiently wide relative to the particle radii. Simulations are performed for two different particle size distribution functions (bimodal and Gaussian), varying the number of particles per aggregate, and different distribution functions of the interparticle gap width. The strong influence of the latter factor on the spectra is demonstrated and investigated in detail. We investigate the effect of particle polydispersity on the optical extinction spectra of colloidal aggregates of spherical metallic (silver) nanoparticles, taking into account the realistic interparticle gaps caused by layers of stabilizing polymer adsorbed on the metal surface (adlayers). The spectra of computer-generated aggregates are computed using two different methods. The coupled-multipole method is used in the quasistatic approximation and the coupled-dipole method beyond the quasistatics. The latter approach is applicable if the interparticle gaps are sufficiently wide relative to the particle radii. Simulations are performed for two different particle size distribution functions (bimodal and Gaussian), varying the number of particles per aggregate, and different distribution functions of the interparticle gap width. The strong influence of the latter factor on the spectra is demonstrated and investigated in detail. Disciplines Biomedical Engineering and Bioengineering

We present an experimental spectroscopic study of large random colloidal aggregates of silver nanoparticles undergoing local restructuring. We argue that such well-known phenomena as strong fluctuation of local electromagnetic fields, appearance of "hot spots" and enhancement of nonlinear optical responses depend on the local structure on the scales of several nanosphere diameters, rather that the large-scale fractal geometry of the sample.Physical properties of surface plasmons (SPs) in disordered nanosystems have been the subject of intense studies in the past decade [1,2,3,4]. The two objects that attracted most attention are disordered two-component composites [5,6,7], including two-dimensional percolation films [8], and aggregates of nanometer-sized noble metal spheres formed in colloidal solutions (colloidal aggregates) [9]. The latter are subject of this paper. Optical and, more generally, electromagnetic effects that were discovered in colloidal aggregates include giant enhancement of nonlinear-optical responses [9, 10], inhomogeneous localization of electromagnetic eigenmodes [11,12], and optical memory [13,14].

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