Unusual magneto-optical behavior induced by local dielectric variations under localized surface plasmon excitations
© González-Díaz et al; licensee Springer. 2011
Received: 4 November 2010
Accepted: 2 June 2011
Published: 2 June 2011
We study the effect of global and local dielectric variations on the polarization conversion r ps response of ordered nickel nanowires embedded in an alumina matrix. When considering local changes, we observe a non-monotonous behavior of the r ps, its intensity unusually modified far beyond to what it is expected for a monotonous change of the whole refractive index of the embedding medium. This is related to the local redistribution of the electromagnetic field when a localized surface plasmon is excited. This finding may be employed to develop and improve new biosensing magnetoplasmonic devices.
During the last years, a great effort has been devoted to the study of metallic nanoparticles due to their distinct optical properties with respect to that of the bulk material . These differences arise mainly from their ability to uphold charge density oscillations known as localized surface plasmons (LSPs). These spatially localized modes may appear at a metal/dielectric interface, manifesting themselves as optical resonances in the transmission and reflection spectra, being their most significant feature the local enhancement of the electromagnetic (EM) field at the metal/dielectric interface . The spectral position, width, and intensity of the optical resonances are extremely dependent on the size, shape, particle inter-distance, embedding environment, or material components of the nanoparticles. In a number of works, the influence of such parameters has been thoroughly studied putting forward the possibility of tailoring their optical response through the morphology of the particles [3–6].
More recently, the optical response arising from the combination of both surface plasmon resonances and magneto-optical (MO) properties that takes place in ferromagnetic nanoparticles is under intensive study. Different theoretical and experimental works [7–11] have pointed out that LSPs affect the MO response, finding an enhancement of the signal that has been usually ascribed to a pure optical effect related to the plasmonic excitation [10, 12–14]. However, the MO activity defines in terms of the reflectivity coefficients as Φ = r ps/r pp, being r ps the polarization conversion and r pp the optical response (when the magnetic field is applied perpendicular to the sample plane). Therefore, the MO response may also be enhanced by modifying r ps. This was first shown in , where the authors suggested as a possible origin the strong localization of the EM field in the MO active material due to the LSP excitation. The scope of this work is to study more in detail the correspondence between the polarization conversion and the EM field under LSP excitations. To do so, we will theoretically analyze the r ps dependence to global and local dielectric changes of the surrounding media in periodic ferromagnetic nanowire arrays. We will show that the different dielectric environments affect the EM field distribution when the LSP is excited, consequently changing the spectral position and intensity of the r ps peak. Moreover, we will prove that variations of the refractive index in the close vicinity of the wires extremely affect the r ps, making its intensity much larger and/or smaller than that obtained if the whole embedding matrix is replaced. This is a consequence of the local redistribution of the EM field induced by the plasmon excitation at the metal/dielectric interface.
In summary, we have theoretically analyzed the relation between the LSP-induced enhancement of the EM field and the polarization conversion in hexagonally ordered ferromagnetic nanowires. We have shown that local variations of the refractive index extremely affect the |r ps| response, which is the consequence of the local EM field redistribution at the LSP resonance within the MO active material. We expect these results may find important applications in biosensing and novel magnetoplasmonic devices.
This work was supported by the EU (NMP3-SL-2008-214107-Nanomagma), the Spanish MICINN ("MAGPLAS" MAT2008-06765-C02-01/NAN and "FUNCOAT" CONSOLIDER INGENIO 2010 CSD2008-00023), the Comunidad de Madrid ("NANOBIOMAGNET" S2009/MAT-1726 and "MICROSERES-CM" S2009/TIC-1476), and CSIC ("CRIMAFOT" PIF08-016-4).
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