Broadband Magneto-Optical Response of Magnetoplasmonic Quasicrystals.

Resent research has demonstrated the enhancement of magneto-optical effects in structured media. In particular, the attention has been paid to plasmonic crystals which are periodic structures supporting surface plasmon polaritons. It was demonstrated that the transverse Kerr effect and the Faraday effect are resonantly enhanced in magnetoplasmonic crystals, and also novel promising effects arise, namely the longitudinal intensity effect [1–3]. As a rule, these effects are of resonant nature, due to their relation to the excitation of eigenmodes, which leads to narrow spectral range of magneto-optical response. In the present work, we propose and demonstrate an approach for forming a broadband magneto-optical response using one-dimensional magnetoplasmonic quasicrystals. Plasmonic quasicrystalline structures offer advances in their optical response over their periodic counterparts, such as broadband and polarization-independent optical transmittance [4]. The considered 1D magnetoplasmonic quasicrystalline structure is formed by a metallic quasicrystal grating on top of the smooth magnetic dielectric layer on a substrate. The sequence of metal stripes and air slits of the grating can be described by symbols ‘1’ and ‘0’. Our structure is based on the 1D binary Fibonacci sequence, where ‘0’ is substituted by ‘010’. The metal grating of the experimentally studied samples is made of 80-nm-thick gold layer. The air slit width corresponding to single ‘0’ in the binary sequence is 80 nm and the metal stripes width corresponding to single ‘1’ in sequence is 600 nm. The magnetic dielectric is bismuth substituted iron-garnet of composition Bi1.5Gd1.5Fe4.5Al0.5O12. The thickness of the magnetic film was made rather small, 80 nm, to exclude the waveguide modes excitation in the considered frequency range, so only surface plasmon polaritons (SPPs) can be excited. Spectrum of the SPPs excited by the incident light in a plasmonic grating structure is determined by the reciprocal lattice vectors which enter the phase matching condition. The numerical calculation of the Fourier transform of the quasicrystalline pattern reveals that the reciprocal lattice for the quasicrystal is discrete and it is far denser compared to the periodic-crystal’s one. In particular, it is non-equidistant. For example, the studied structure possesses reciprocal vectors equal to 15.39, 16.76 and 19.01 μm−1, while the corresponding periodic structure has only reciprocal vector of 18.48 μm−1 in this spectral range. The excitation of the SPPs in plasmonic structures with magnetic materials is accompanied by the resonant enhancement of the Transverse magneto-optical Kerr effect (TMOKE). Experimentally measured TMOKE spectrum for the magnetoplasmonic quasicrystal is shown in Fig. 1, together with calculated SPP dispersion curves. It is far richer than the one for the corresponding periodic structure. Apart from the first pair of resonances at around λ=820 nm (for small incidence angles) that is quite similar to the resonances for the periodic structure, two other additional pairs appear at around λ=890 nm and λ=950 nm corresponding to the reciprocal vectors of 16.76 and λ=15.39 μm−1. It demonstrates that the magneto-optical response of plasmonic quasicrystals is broadband, contrary to single narrow resonances in the case of periodic structures. It makes the proposed structures very promising for numerous nanophotonics applications including optical sensing, control of light, all-optical control of magnetization etc. Additionally, TMOKE spectroscopy is an efficient tool for investigation of the peculiarities of plasmonic quasicrystals. The multiplicity of the excited plasmonic modes attracts attention also because they possess different values of the penetration depth. Estimations show that for the plasmonic resonances shown in Fig. 3a the SPP penetration depth in the magnetic dielectric varies approximately from 70 to 100 nm. This fact opens new possibilities for manipulation of the optical near field, 3D sensing, control of the inverse magneto-optical effects and optically-induced magnetization. Quasicrystalline structures provide designable reciprocal lattice, i.e. the set of reciprocal vectors and therefore, dispersion of eigenmodes, by means of adjusting geometrical parameters. In particular, plasmonic quasicrystals offer designable spectrum of magneto-optical response for light modulation, which is prosperous for parallel light information processing at several frequencies. Furthermore, the plasmonic quasicrystals are prosperous for achieving other broadband magneto-optical effects related to the excitation of eigenmodes. If the structure supports waveguide modes then there are many resonances for TE and TM modes with the resonant wavelengths close to each other. This condition is favorable for the enhancement of the Faraday effect and the longitudinal intensity effect, as the TE-TM conversion is the most effective. The work is supported by the Russian Presidential Grant MK-2047.2017.2.