J. B. González-Díaz

Plasmonic Au/Co/Au nanosandwiches with enhanced magneto-optical activity.

Financial support from the Spanish Ministry of Science and Education (NAN2004-09195-C04-01 and MAT2005-05524-C02-01), and UE (NoE-Phoremost) are acknowledged.

Magnetoplasmonic nanostructures: systems supporting both plasmonic and magnetic properties

We present in this work our current understanding on magnetoplasmonic structures, that is, systems whose constituents exhibit simultaneously magnetic and plasmonic properties. We analyze both the influence of the plasmon resonance on the magneto-optical properties of the system and the ability of the magnetic field to modulate the plasmon properties. In particular we show how, in magnetoplasmonic systems sustaining localized or propagating surface plasmons, the associated electromagnetic field enhancement gives rise to an enhancement of the magneto-optical activity. On the other hand, we have analyzed the modulation of the propagating surface plasmon polariton wavevector in noble metal/ferromagnet/noble metal trilayers by an external magnetic field. These phenomena can be addressed as new concepts for the development of active plasmonic devices.

Localized surface plasmon resonance effects on the magneto-optical activity of continuous Au/Co/Au trilayers

We study how the magneto-optical activity in polar configuration of continuous Au/Co/Au trilayers is affected by the excitation of localized plasmon resonances of an array of Au nanodiscs fabricated on top of them over a dielectric SiO(2) spacer. We show that the effect of the nanodiscs array is twofold. First, it optimizes the absorption of light at specific photon energies corresponding to the localized surface plasmon excitation of the array, modifying the reflectivity of the system (we define this effect as the purely optical contribution). Second, upon localized plasmon resonance excitation, the electromagnetic field in the whole system is redistributed, and an enhanced magneto-optical activity occurs at those energies where the electromagnetic field in the magnetic layer is increased (this effect is identified as the purely magneto-optical contribution of the nanodiscs array).

Enhancement of the magnetic modulation of surface plasmon polaritons in Au/Co/Au films

The deposition of a dielectric overlayer on top of Au/Co/Au multilayer films can significantly enhance the magnetic field induced modulation of the surface plasmon polariton (SPP) wave vector. This enhancement is analyzed as a function of the thickness of the dielectric overlayer and the associated SPP electromagnetic field confinement and redistribution. The decrease in SPP propagation distance is taken into account by analyzing an adequate figure of merit.

Cobalt dependence of the magneto-optical response in magnetoplasmonic nanodisks

In this work, we study the plasmon-mediated enhancement of the magneto-optical (MO) activity in the case of Au/Co/Au nanodisks as a function of the Co content. We show that both the spectral shape and amplitude of the complex polar Kerr rotation are modified by the relative amount of gold (diamagnetic) and cobalt (ferromagnetic). We demonstrate that such shape modification is dictated by the predominance of the absorptive or dispersive part of the MO constant in each material, which is intimately related to the origin of their MO activity.

Magneto-optical effects in interacting localized and propagating surface plasmon modes

We report that the effect of an external magnetic field on the propagation of surface plasmons can be effectively modified through the coupling between localized (LSP) and propagating (SPP) surface plasmons. When these plasmon modes do not interact, the main effect of the magnetic field is a modification of the wavevector of the SPP mode, leaving the LSP virtually unaffected. Once both modes start to interact, there is a strong variation of the magnetic field induced modification of the SPP dispersion curve and, simultaneously, the LSP mode becomes sensitive to the magnetic field.

Surface Plasmon Resonance Effects in the Magneto-Optical Activity of Ag–Co–Ag Trilayers

A complete magneto-optical characterization of Pt capped Ag-Co-Ag sputtered trilayers with varying Co thickness is presented. Polar Kerr rotation and ellipticity exhibit a gradual increase in signal as the Co layer gets thicker, with a sharp feature around 3.75 eV due to the bulk Ag plasmon resonance. The transverse Kerr signal of such trilayers is strongly affected by the surface plasmon polariton (SPP) resonance excitation when measured in the Kretschmann configuration, with a maximum transverse Kerr effect for around 2 nm Co thickness which corresponds to the optimum SPP excitation. This behavior is in contrast with the monotonous increase in transverse Kerr signal with Co thickness measured when the SPP is not excited. These results are in qualitative agreement with simulations using the transfer matrix formalism, the discrepancies mainly being due to the deviation from 2-D growth of the system and possible Ag and Co oxidation effects.

Polarizability and magnetoplasmonic properties of magnetic general nanoellipsoids

An approach to compute the polarizability tensor of magnetic nanoparticles having general ellipsoidal shape is presented. We find a surprisingly excellent quantitative agreement between calculated and experimental magneto-optical spectra measured in the polar Kerr configuration from nickel nanodisks of large size (exceeding 100 nm) with circular and elliptical shape. In spite of its approximations and simplicity, the formalism presented here captures the essential physics of the interplay between magneto-optical activity and the plasmonic resonance of the individual particle. The results highlight the key role of the dynamic depolarization effects to account for the magneto-optical properties of plasmonic nanostructures.

Tunable magneto-photonic response of nickel nanostructures

In this letter, we present both experimental and numerical studies of the magneto-optical (MO) properties of nickel infiltrated opals. Ni can show interesting MO properties that can be controlled by nanostructuration through colloidal crystals templating. Nanostructuration allows the coupling of light to surface plasmon modes of Ni, and a clear dependence of the MO response as a function of the structural parameters of the template has been observed. This dependence can be used in future tunable devices such as switchers or MO modulators.

Plasmon-enhanced magneto-optical activity in ferromagnetic membranes

Experimental and theoretical evidence of plasmon-enhanced Kerr rotation in purely ferromagnetic membranes with sufficiently small dimensions to be out of extraordinary optical transmission conditions (45 nm pore diameter, 90nm lattice constant), is reported in this work. It is shown that the spectral location of the enhanced Kerr rotation region varies as the refractive index of the material inside the pore is modified. A similar behavior is obtained if the pore radius changes while keeping the pore concentration unchanged. Those are clear signatures indicating that localized surface plasmon resonances propagating along the pores govern the magneto-optical response of the membrane.