Near-field patterns and geometric resonances in heterogeneous nanoparticle arrays

Comunicación presentada en la 12th International conference on Near-field Optics, Nanophotonics and Related Techniques (NFO-12), celebrada en San Sebastián del 3 al 7de septiembre de 2012.Geometric resonances in nanostructure arrays sustaining localized surface plasmons have received increasing attention in the last few years [1]. These geometric resonances exhibit narrower peaks than the purely plasmonic ones, therefore these systems are promising candidates for sensing applications [2]. Moreover, they provide new features in the near-field distribution patterns. Heterogeneous nanoparticle arrays offer a way to tune both the near and far-field response by using nanoparticles of different sizes, shapes or materials [3], but there is a lack of experimental studies in this context. We have investigated a two dimensional array of gold nanodisks with four different diameters and located 3º off-axis (see Fig. 1(a)), which also al lows studying the effect of a small alteration in the geometrical arrangement [4]. We present 0-order extinction spectra acquired in far-field, as well as measurements of the near field distribution using SNOM technique in collection mode with two orthogonal polarizations and different wavelengths. Far and near-field characterizations are understood with the help of FDTD numerical results. The far-field spectra (Fig. 1(b)) corroborate the existence of geometric resonances in our heterogeneous system, as predicted theoretically [3]. We show that the collective geometrical response induces a complex redistribution of the near-field (see an example in Fig. 1(c)), arising from the phase contribution of the field scattered from each kind of nanodisk. The main consequence is a dramatic break of the axial symmetry of the field pattern typically generated by nanodisks, and more generally, the suppression or the enhancement of the field surrounding the nanostructures. Our results offer a deeper insight on the interplay between individual and collective resonances.Peer Reviewe