Giant magneto-optical/faraday effect in flexible non-magnetic/plasmonic polymer matrix composites

Abstract

In selected metals (Ag, Au), surface plasmon resonantes appear when monodisperse nanoparticles embedded into a transparent medium interact with visible radiation. One of the main difficulties for preparing plasmonic materials is the strong tendency of nanoparticles to aggregate. In this sense, a new approach is herewith presented in which, metallic nanoparticles were obtained on sepiolite (a hydrated magnesium silicate) microparticles. These microparticles which actas nanoparticle carriers were dispersed into transparent polymers (LDPE and PS) by conventional methods. As a result, tinted films (from 50 to 100 ?m thick) with very good mechanical stability were obtained. These films present a very good degree of transparency due to the index matching between sepiolite and polymeric matrices, which minimize the light scattering by defects. Additionally they present well defined surface plasmon resonance absorption peaks. Due to the excellent optical quality of these materials, it has been possible to measure a giant magneto-optical Faraday rotation. In fact, according to theoretical models, (Hui and Stroud, APL, 50, 950 (1987)) non-magnetic plasmonic dilutions may present a notable Faraday rotation. In this work, gold and silver sepiolites dispersed in 60 pm PS films present Faraday rotation of about 3.10(4) º/T.m in the neighbourhood of the SPR spectral region. More precisely, the Faraday rotation reaches the maximum at the long wavelength edge of the SPR absorption curve, being approximately O at the maximum of SPR absorption curve. The measured Faraday rotation value is of the same order of magnitude to that reported for transparent magnetic materials. Therefore, it is shown that gold and silver nanoparticles may be employed for fabricating magnetic field sensors in a simple procedure which can be easily scaled-up.