G. Armelles

Highly sensitive detection of biomolecules with the magneto-optic surface-plasmon-resonance sensor

The characteristics of a novel magneto-optic surface-plasmon-resonance (MOSPR) sensor and its use for the detection of biomolecules are presented. This physical transduction principle is based on the combination of the magneto-optic activity of magnetic materials and a surface-plasmon resonance of metallic layers. Such a combination can produce a sharp enhancement of the magneto-optic effects that strongly depends on the optical properties of the surrounding medium, allowing its use for biosensing applications. Experimental characterizations of the MOSPR sensor have shown an increase in the limit of detection by a factor of 3 in changes of refractive index and in the adsorption of biomolecules compared with standard sensors. Optimization of the metallic layers and the experimental setup could result in an improvement of the limit of detection by as much as 1 order of magnitude.

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.

Magnetooptic effects in surface-plasmon-polaritons slab waveguides

This paper analyzes the magnetooptic (MO) effects in surface plasmon polaritons (SPP) propagating in thin metallic layers as a function of the metal thickness. Two different configurations are analyzed: a ferromagnetic metal surrounded by nonmagnetic dielectrics and a nonmagnetic metal bounded by ferromagnetic dielectrics. Depending on the configuration and the thickness of the metallic layer, waveguides with very low attenuation losses and high MO effects can be obtained. These configurations could be used to design MO active devices in SPP-based optics.

Au/Fe/Au multilayer transducers for magneto-optic surface plasmon resonance sensing

In this paper, we analyze the magnetoplasmonic MP features and sensing capabilities of Au/Fe/Au trilayer structures, as transducers of the magneto-optic surface plasmon resonance MOSPR biosensor. This biosensor, which can surpass the sensitivity of the standard SPR sensor, is based on a MP modulation technique generated by the simultaneous stimulation of the surface plasmon polaritons SPP and the transversal magneto-optical Kerr effect TMOKE. We study the magneto-optical activity of the trilayers as a function of the thickness and position of the Fe layer. We first demonstrate that this kind of structure allows modulating the SPP through an external magnetic field and moreover, induce a strong enhancement of the TMOKE effect. The modulation of the SPP is linearly proportional to the thickness of Fe layer and inversely proportional to the distance between the Fe layer and the external dielectric medium. Finally, we experimentally confirm a twofold increase in the MOSPR sensitivity with respect to the intensity-interrogated SPR biosensor in bulk refractive-index changes, keeping a similar chemical resistance and stability, unprecedented in other MP transducers, and biofunctionalization protocols.

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).

Magneto-optical properties of nickel nanowire arrays

We report on the magneto-optical properties of hexagonally arranged Ni nanowires embedded in anodic alumina templates. Due to the nanowire geometry, these samples show different response depending on the polarization orientation of the incident light, which leads to a high anisotropy of both their optical and magneto-optical properties. A strong increase of the magneto-optical activity is clearly observed with respect to the Ni bulk material. We associate this behavior to plasmon resonance of the Ni nanowires.

Radiative corrections to the polarizability tensor of an electrically small anisotropic dielectric particle

Radiative corrections to the polarizability tensor of isotropic particles are fundamental to understand the energy balance between absorption and scattering processes. Equivalent radiative corrections for anisotropic particles are not well known. Assuming that the polarization within the particle is uniform, we derived a closed-form expression for the polarizability tensor which includes radiative corrections. In the absence of absorption, this expression of the polarizability tensor is consistent with the optical theorem. An analogous result for infinitely long cylinders was also derived. Magneto optical Kerr effects in non-absorbing nanoparticles with magneto-optical activity arise as a consequence of radiative corrections to the electrostatic polarizability tensor.

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.