Anomalous polarization conversion in arrays of ultrathin ferromagnetic nanowires

Abstract

We study optical properties of arrays of ultrathin nanowires by means of the Brillouin scattering of light on magnons. We employ the Stokes/anti-Stokes scattering asymmetry to probe the circular polarization of a local electric field induced inside nanowires by linearly polarized light waves. We observe the anomalous polarization conversion of the opposite sign than that in a bulk medium or thick nanowires with a great enhancement of the degree of circular polarization attributed to an unconventional refraction in the nanowire medium.The study of magneto-optical response of tailored nanostructures is in the focus of active research of nanostructured materials [1][2][3]. Nonmagnetic metallic nanowires are well known in optics, and they are employed as building blocks of the so-called wire metama- Here, we study magneto-optical (MO) properties of ultrathin ferromagnetic Co nanowire arrays (see Fig. 1) by means of polarization-resolved Brillouin light scattering on magnons [11][12][13]. In a sharp contrast to the previous studies of ferromagnetic nanowires with the diameter D of 20 nm [14], here we study arrays of thin nanowires with diameter D ∼ 4.8 nm and lattice spacing ∼ 17 nm (Fig. 1). We reveal a striking anomaly of Stokes/antiStokes pattern in the spectra of Brillouin Light Scattering (BLS) from magnons in such structures. First, its asymmetry is inverted, which is impossible in principle in continuous non-structured metal layers; secondly, it is unconventionally large. Although the latter effect can be predicted formally for continuous films, such a huge asymmetry can only be calculated for materials with unrealistic optical parameters. This is explained by a strong modification of optical and magneto-optical properties for thin nanowires. While the coherent propagation of photons in the sample is weakly affected by thin wires due to their small volume concentration, the photon interaction with magnons is confined to the volume inside the wires where the wave polarization is strongly modified due to a huge mismatch in the dielectric constants of metallic nanowires and dielectric media. As a result, we observe that a linearly polarized light obliquely incident upon a metacrystal composed of ultrathin Co nanowires acquires ellipticity inside the wires that is enhanced by an order of magnitude being of the opposite sign as that in the case of continuous Co films or thicker Co wires. We visualize this effect by measuring the asymmetry of Stokes and anti-Stokes peaks in the Brillouin scattering spectra of light by the spin-wave modes of the wires, since the scattering can probe local electric fields [15]. We expect that our results will be instrumental for the emerging field of nonlinear spectroscopy of metamaterials [16] as well as for a design of novel structures with strong chiral responses [17] and polarization-sensitive light rooting [18][19][20].An array of ultrathin Co nanowires has been grown by sequential pulsed laser deposition of Co and CeO 2 in reductive conditions (P growth = 10 −5 mbar), leading to...

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