Strong Magnetochiral Dichroism in Suspensions of Magnetoplasmonic Nanohelices

We show theoretically that self-assembled metamaterial suspensions of magnetoplasmonic entities in the form of magnetic-nanoparticle helices coiled around plasmonic (gold) nanorods exhibit very strong magnetochiral dichroism that is orders of magnitude higher than in conventional materials exhibiting this phenomenon (liquid molecular systems, anisotropic crystals, and chiral ferromagnets). The large values of the calculated magnetochirality are accompanied by an enhancement of the magneto-optical Faraday effect due to the excitation of surface plasmons at the nanorods as well as by an increase of the structural dichroism imparted from the helicoidal arrangement of the magnetic nanoparticles. However, the reported enhancement of the magnetochiral dichroism does not appear to be a cascade phenomenon of the structural and magnetic circular dichroism as, for example, in solutions of magnetic compounds, but appears as a possible manifestation of the emergence of a surface-plasmon-induced toroidal moment within the nanoparticle helices. To the best of our knowledge, it is the first time that the presence of a toroidal moment in a material can exhibit significant reciprocity-violating anisotropy in the optical regime.