## Abstract

Noble metal nanoparticle clusters show unique light absorption and catalysis properties, which have been widely used in the application of photocatalysis, optoelectronics, biomedical optics and so on. The absorption cross section of densely packed nanoparticle clusters, which can be enhanced or restricted due to the near field effects needs to be studied thoroughly. In this work, focusing on Au nanoparticle at the localized plasmon resonance wavelength, the effects of monomer diameter *D*, monomer number *N*, particle volume fraction *F*_{v} and complex refractive index *m* on the nondimensional absorption cross section *η* = *C*_{abs,total}/(*N·C*_{abs}) (normalized by *N* and the absorption cross section *C*_{abs} of a single particle) of densely packed nanoparticle clusters are studied by using the superposition T-matrix method. It is found that the enhancement (*η* > 1) and restriction (*η* < 1) mechanisms of the absorption cross section of nanoparticle clusters are determined by two competing factors (i.e. the multiple scattering and shielding effect), and the extent of these two mechanisms is mainly dependent on the monomer size parameter and the monomer number. The effect of the particle volume fraction on the nondimensional absorption cross section of nanoparticle clusters is totally different in different mechanisms. Specifically, the nondimensional absorption cross section peaks at the particle volume fraction of about 50% in the enhancement mechanism (in our calculation: *D* < 14 nm, *N* = 100), while in the restriction mechanism it decreases monotonously with increasing particle volume fraction. Moreover, the absorption efficiency of nanoparticle clusters with more absorptive monomer decreases more sharply with increasing particle volume fraction. The complex refractive index of particle shows significant effects on the nondimensional absorption cross section of nanoparticle clusters, and the largest nondimensional absorption cross section of nanoparticle clusters (*N* = 100) is larger than 8.

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