Materials Science and Engineering B 149 (2008) 247–250 Local-field enhancement in metallic nanoplanets G. Pellegrini ∗ , G. Mattei, V. Bello, P. Mazzoldi CNISM, Dipartimento di Fisica, Universit` a di Padova, via Marzolo 8, I-35131 Padova, Italy Received 27 June 2007; accepted 4 September 2007 Abstract Generalized Multiparticle Mie theory is applied in order to study in detail local-field properties of metallic nanoplanets (i.e., a central cluster surrounded by small “satellite” clusters very close to its surface), obtained by ion beam techniques. Strongly asymmetric dimers are chosen as model systems in order to establish the influence of topological parameters such as satellite dimension and distance from the central cluster surface on the local-field enhancement, with calculated enhancement factors as high as ∼180 in the case of silver. Similar topological configurations are shown to be present in typical nanoplanet systems, with comparable local-field enhancement factors. Simulations for silver and silver–gold alloy in silica, and for gold in titania matrix are reported as examples of possible experimental systems. © 2007 Elsevier B.V. All rights reserved. Keywords: Interacting nanoparticles; Coupled plasmons; Local-field enhancement 1. Introduction The interaction of light with ensemble of coupled metal nanoclusters (NCs) has attracted much interest in the last few years, for the peculiar linear and non-linear optical properties of these systems. Extinction spectra and local-field enhancement are influenced by parameters such as particle size, number and relative positions as well as by incident light polarization state [1–3]. Given their far- and local-field properties strongly cou- pled clusters are promising for molecular sensors and miniature non-linear optical elements [4–8]. Strong electrodynamic cou- pling allows a polarization-mediated control over the plasmon resonance shape and position, and the possibility of three- dimensional subwavelength confinement along with plasmon waveguiding [9–16]. A number of different techniques have been adopted to model the optical properties of coupled clus- ters, herein included finite difference time domain (FDTD), T-matrix, discrete dipole approximation (DDA) and Generalized Multiparticle Mie (GMM)approaches [17–20]. In the present paper we describe by the GMM theory the local- field enhancement properties of peculiar structures of interacting nanoclusters called nanoplanets (NPs, i.e., a central cluster sur- rounded by small “satellite” clusters very close to its surface), ∗ Corresponding author. E-mail address: pellegrini@padova.infm.it (G. Pellegrini). synthesized by ion beam processing techniques [21–25].A detailed descriptions of the GMM formalism may be found in Ref. [26,27]. We first adopt strongly asymmetric dimers as model systems, in order to determine the influence on the local- field enhancement factor of parameters like satellite dimension and distance from the central cluster surface, as well as incident light polarization state. Subsequently GMM theory is applied to the NPs topology in three test cases: (i) Au x Ag 1-x NPs in SiO 2 matrix (i.e., an already synthesized experimental system [21–25]), (ii) Ag NPs in SiO 2 , and (iii) Au NPs in TiO 2 (the last two cases being of interest for possible new experimental systems). 2. Local field: asymmetric dimers The first investigated structures are strongly asymmetric sil- ver dimers in silica matrix (n = 1.45), which are chosen as model systems in order to study the direct coupling mech- anism between central and satellite clusters. Dimensions are R = 15 nm for the mother cluster, and a varying radius from 1 to 3 nm for the satellite ones, in close relation to sizes obtained in experimental systems [21–25]. Interparticle spacing is varied between 1 and 15 nm, in order to evaluate the influence of surface distances on the field enhancement. The electric field is com- puted on the x–y dimer equatorial plane, with a unitary amplitude monochromatic plane wave as an incident field, moving along the z axis in the positive direction. Field is calculated for three 0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.09.060