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Analysis of the magneto-optical activity for the magnetic- and electric-dipolar modes in magnetoplasmonic nanodiscs

AutorAmelles, G.; Cebollada, Alfonso ; García-Pérez, Fernando ; García-Martín, Antonio ; González Sagardoy, María Ujué ; Meneses-Rodríguez, David ; Sousa N. da; Froufe-Pérez, Luis S.
Fecha de publicación2013
EditorUniversity of Ottawa
Citación6th International Conference on Surface Plasmon Photonics (SPP6), May 26 to June 1st, 2013, in Ottawa, Canada.
ResumenMetal/dielectric/metal plasmonic nanodiscs present a rich optical behaviour with the appearance of a bonding and an anti-bonding configuration that result in an electric- and a magnetic-dipole, respectively [1]. Due to symmetry considerations, the coupling of each configu¬ration with the incident light differs, and the modes show a bright (electric dipole) or a dark (magnetic one) nature. The insertion of a ferromagnetic component in the structure introduces magneto-optical (MO) activity, and the presence of the MO response in both modes has been recently established, as well as the influence of the position of the ferromagnetic component [2]. So far, no attention has been paid to the differences in the MO response attending to the magnetic- or electricdipolar character of each mode, neither to the influence of the coupling between the two metallic disks. Here, we will present a detailed study on these aspects, obtain¬ed by the analysis of a pure Au nanodisc separated by a SiO2 spacer of variable thickness from a magneto¬plasmonic one composed by a 4nmAu/2nmCo superlat¬tice. Figure 1 shows the extinction and polar MO activity spectra for the structure with 10 nm of SiO2, for which the two disks couple strongly. The extinction data exhibit a strong peak around 600 nm, corresponding to the electric dipole (bright mode) and a very weak one around 950 nm, associated with the magnetic dipole (dark mode). The MO spectrum, on the other hand, presents clear peaks for both modes but of different intensities, whose origin can be understood as follows. For the electric dipole, p, an external magnetic field perpendicular to the discs induces a rotation in the discs plane and therefore the appearance of a perpendicular plane dipolar component, pmo. As p results from parallel electric dipoles at each individual nanodisc, pmo can also be decomposed in two parallel components in the top and bottom disk, contributing to the MO activity with the same sign. For the magnetic dipole, m, the applied magnetic field gives rise to Larmor precession and hence to the rotation of m in the disks plane. Since m originates from antiparallel electric dipoles at each nanodisc, a rotated m should come from rotated individual electric dipoles with antiparallel pmo projections. These antiparallel pmo contribute to the total MO response with opposed sign, so the resulting peak is of lower intensity. As the coupling between the discs is modified by changing the SiO2 thickness, the MO contribution of each individual nanodisc to the total response varies and consequently the relative intensity between peaks too. References: [1] A. Dmitriev, T. Pakizeh, M. Käll, and D.S. Sutherland, Small 3, 294 (2007) [2] J.C. Banthí et al., Adv. Mater. 24, OP36 (2012)
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