Peña-Rodríguez and Pal Nanoscale Research Letters 2011, 6:279 http://www.nanoscalereslett.com/content/6/1/279 Figure 1a, where ri and ti (i = 0, ..., 3) are the radii and thicknesses of the ith layer (for the MDM structure r0 = t0 = 0). The extinction efficiencies for the different con- figurations were calculated by means of Scattnlay [28], a computer implementation of the algorithm developed by Yang [31] for the calculation of the scattering of EM radiation by a multilayered sphere. Moreover, in order to explain the observed shifts of the SPR, we have used a complementary, mainly qualitative method which has been developed recently: the theory of plasmon hybridi- zation [20]. In this approach the characteristics of the SPR are explained in terms of the interactions between Figure 1 Schematic representation of the MDM and DCN structures. (a) Schematic representation of the studied MDM and DCN structures (for the MDM r0 = t0 = 0) and (b) their corresponding energy diagrams, representing plasmon hybridization. Page 2 of 5 the plasmons of metallic nanostructures with simpler shapes. For instance, the SPR of metallic nanoshells can be understood from the coupling between the plasmons modes of a sphere (|ωs〉) and a cavity (|ωc〉), where two new plasmon oscillation modes are created: a higher energy (antibonding) mode (|ω+〉) and a lower energy (bonding) mode (|ω-〉), corresponding to the antisym- metric and symmetric interactions between the |ωs〉 and |ωc〉 modes, respectively. The energy level diagram for plasmon hybridization in the studied MDM (red) and DCN (blue) structures is depicted in Figure 1b. The plasmon resonance in a MDM [DCN] structure can be viewed as the interaction between the plasmon responses of the inner sphere [nanoshell] (|ωs〉 [ ω2 and ω+2 ]) and the outer ( ωo and − − ω+o ) nanoshell. Three hybridized modes are obtained for the MDM, the energy mode ω− corresponds to the − antisymmetric coupling between the symmetric plasmon resonance modes of the outer ( ωo ) nanoshell and the − sphere plasmon. The coupling between the higher- energy antibonding mode of the outer nanoshell and the nanosphere plasmon modes is very weak and only one hybridized mode is produced in this case ( ω++ ). In con- trast, four hybridized modes are produced in the DCN structure because its inner nanoshell has two energy modes. The energy mode ω−− ( ω+ ) corresponds to the − antisymmetric (symmetric) coupling between the sym- metric plasmon resonance modes of the inner ( ωi ) and − outer ( ωo ) nanoshells. On the other hand, the energy − mode ω+− ( ω++ ) corresponds to the symmetric (anti- symmetric) coupling between the antisymmetric plas- mon resonance modes of the inner ( ωi ) and outer − ( ω+o ) nanoshells (Figure 1c). Although, in principle, there exists also a coupling between the antisymmetric and symmetric plasmons of the separate nanoshells, it has only a small influence on the hybridized modes, due to the large energy separation between those two modes [20]. The characteristics of the surface plasmon resonance obtained for the multilayer structures studied in this work primarily depend on the properties of the SPR of the two constituents metal layers (their composition and thickness) and the strength of the coupling between them (defined by the thickness of the dielectric layer that separates them). Since the dependence of the opti- cal response in terms of geometrical parameters has been previously studied in detail for both structures [22,23], we focus only on the influence of the composi- tion. For this, we have selected a configuration where both the dielectric layer thickness and the total size of the particles are constant, while the thickness of the metal layers is simultaneously varied (inversely), in order