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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.

It is well known that localized surface plasmon resonances (LSPRs) greatly influence the optical properties of metallic nanostructures. The spectral location of the LSPR is sensitive to the shape, size, and composition of the nanostructure, as well as on the optical properties of the surrounding dielectric. [1] The latter effect has been used to develop different types of optical biosensors for which biological reactions near the surface of the nanostructure can be monitored through the changes in the frequency of the LSPR. [2][3][4][5][6] The induced electromagnetic field associated with the LSPR is greatly enhanced at the metal/dielectric interface, a phenomenon that is the basis for various types of surface-enhanced spectroscopy, such as surface-enhanced Raman scattering. [7] Furthermore, metallic nanoparticles have been shown to have light-guiding capabilities on the nanometer scale. This makes them suitable for the development of nano-optic devices. [8] The overwhelming majority of LSPR studies have focused on Au or Ag nanoparticles because these metals have suitable optical constants for application with visible wavelengths of light. However, once the morphology and composition of a nanostructure have been fixed, it is difficult to change or control the LSPR properties by external means, which would be desirable for the development of active nanoplasmonic devices. One way to overcome this problem could be to embed the metal nanostructure in an active medium, such as a liquid crystal, [9] which can be controlled by an external electrostatic field, or a ferromagnetic garnet, [10,11] which can be moderated by a magnetic field. An alternative approach could be to let the controlling field act directly on the metallic nanostructure, for instance, using nanoparticles made of ferromagnetic metals. Such metals have strong magneto-optical (MO) activity, that is, their optical properties change markedly even if the applied magnetic field is weak. Unfortunately, this high optical absorption results in a strong damping of any intrinsic LSPR that prevents the development of active plasmonic devices made solely of ferromagnetic metals. A promising route forward could be to combine ferromagnetic materials that would promote strong MO activity with noble metals that could induce plasmonic response. The large enhancement and spatial localization of the electromagnetic field associated with the LSPR suggest that a strong enhancement of the MO properties should be possible. [12] Several attempts to develop these kinds of structures have been carried out using different chemical synthesis methods to fabricate complex onion-like nanoparticles made of noble metals and ferromagnetic materials. [13][14][15][16] These systems do exhibit LSPRs, but so far no MO activity has been reported. On the other hand, continuous thin films made of Au/Co/Au trilayers were found to lead simultaneously to well-defined propagating surface plasmon polaritons and to strong MO activity at low magnetic fields. [17] Moreover, such composite structure...

In this Letter we show that nanostructures made out of pure noble metals can exhibit measurable magneto-optic activity at low magnetic fields. This phenomenon occurs when the localized surface plasmon resonance of the nanostructure is excited in the presence of a static magnetic field parallel to the propagation of incident light. The large magneto-optical response observed comes from an increase of the magnetic Lorentz force induced by the large collective movement of the conduction electrons in the nanostructures when the resonance is excited.

We demonstrate magnetic field control of surface plasmon excitations in noble-metal/ferromagnetic/noble metal trilayers, analogous to the effects previously observed in semiconductor structures. We show that the coupling of an external magnetic field to the surface plasmon-polariton wave vector is greatly enhanced in the metallic structure due to the ferromagnetic nature of one of its constituents. The observed coupling could be used to modulate the surface plasmon response in ultrasensitive spectroscopic applications. DOI: 10.1103/PhysRevB.76.153402 PACS number͑s͒: 78.20.Ls, 73.20.Mf, 78.66.Bz, 42.25.Bs Surface plasmon-polariton ͑SPP͒ modes are electromagnetic excitations localized at the interface between two media, one with positive and the other with negative dielectric constant. These modes may appear at the interface between a degenerate semiconductor and a dielectric or between a metal and a dielectric. In the former case, due to the low value of the plasma frequency of the semiconductor, the frequencies of the SPPs are restricted to the far infrared range, whereas in the second case the SPP modes can have frequencies varying from the far infrared to the visible range. The propagation characteristics of the SPPs and their EM field distribution depend strongly on the optical properties and interface morphology of the system. This dependence has been exploited in different optical contexts such as light guiding at the subwavelength scale, 1-3 optical switching, 4 biochemical sensing, 5 or nanometer resolved far-field optical microscopy. 6 To date SPPs are commonly considered as passive, i.e., insensitive to the magnetic field and just depending on the optical and geometrical properties of the system. In this work we demonstrate the control of SPP excitations in metallic trilayer structures by means of an external magnetic field. We show that the coupling of the magnetic field to the wave vector of the plasmon is greatly enhanced by the ferromagnetic nature of the trilayer structure. This effect was first studied theoretically in semiconductor-based SPPs 7-9 and in metals. 10 The effect of the magnetic field on the properties of the SPP modes depends on the relative orientation of the applied magnetic field with respect to the wave vector of the SPP. In particular, we will show that when the magnetic field is applied perpendicular to the direction of propagation of the SPP and parallel to the interface, it modifies the dispersion relation of the SPP mode in such a way that the dispersion relation depends on the k direction ͓i.e., w͑k͒ w͑−k͔͒. Experimentally this magnetic field induced nonreciprocity has been observed on semiconductor-based SPPs, 11 but not yet in metallic systems. This is due to the high magnetic field needed to observe magnetic field induced effects on metallic based SPPs.One way to reduce the required external magnetic field is to incorporate ferromagnetic metals. Due to the magnetooptical ͑MO͒ activity that many ferromagnetic materials exhibit at low magnetic fields, surface magnetopl...

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. Astrophys. J. 186, 705-714 (1973). 5. B. T. Draine, "The discrete dipole approximation and its application to interstellar graphite grains," Astrophys.J. 333, 848-872 (1988). 6. B. T. Draine and J. Goodman, "Beyond Clausius-Mossotti: Wave propagation on a polarizable point lattice and the discrete dipole approximation," Astrophys. J. 405, 685-697 (1993). 7. W. L. Barnes, A. Dereux, and T. W. Ebbesen, "Surface plasmon subwavelength optics," Nature 424, 824-830 (2003).

Metal‐dielectric Au‐Co‐SiO2 magnetoplasmonic nanodisks are found to exhibit large magneto‐optical activity and low optical losses. The internal architecture of the nanodisks is such that, in resonant conditions, the electromagnetic field undertakes a particular spatial distribution. This makes it possible to maximize the electromagnetic field at the magneto‐optically active layers and minimize it in the other, optically lossy ones.