which exhibit resonant toroidal dipole response. [10][11][12][13][14][15][16] Many successful designs were demonstrated in microwave frequency range, when the material losses are negligible. However, most of proposed designs are sensitive to the incident polarization and the toroidal dipole response has been observed in narrow frequency range. [11,16] In visible range the observation of toroidal dipole response is becoming quite challenging due to increasing dissipative losses of plasmonic materials.In this paper, we suggest a novel type of all-dielectric metalattice which supports dominant toroidal dipole response over a wide frequency range in the visible. The unit cell of the proposed metalattice consists of four rectangular bars with large aspect ratio as shown in Figure 1a. As soon as the size of one unit-cell is comparable to the wavelength of light in free space, we involve the term of metalattice which is not limited to subdiffraction regime and has been recently proposed in ref. [17]. In contrast to widely used high-index materials, such as, c-Si, Ge, and GaAs, we propose to use moderate index materials with the permittivity of about 10, such as, InP or GaP. [18] By performing the Cartesian multipole decomposition of the induced polarization currents inside the unit cell of the metalattice we demonstrate that such moderate index contrast structures exhibit strong toroidal dipole response, which is the dominant contribution over a wide spectrum range. We also found a correlation between the toroidal dipole response and longitudinal component of the electric near-field. To proof the concept we performed direct experimental characterization of far-and near-field properties of the designed metalattice in microwave frequency range, where the near-field mapping can be easily achieved.The optical response of a single dielectric bar with large aspect ratio has been previously discussed. [19] Due to the geometrical asymmetry of the single bar with large aspect ratio in x-y plane, the optical response of the metasurface made of periodically arranged bars depended on the incident wave polarization. [19] In fact, the polarization independent response is highly desirable for various applications.Motivated by these results, we explore an alternative material and structure, which should exhibit polarization independent response due to the C 4 rotational symmetry of its unit cell. It turns out that moderate index materials with refractive index around n ≈3 exhibit low dissipative losses. Particularly, in visible spectral range of 0.50-0.80 µm the refractive index A novel type of all-dielectric metalattice is proposed and experimentally demonstrated, which supports broadband dominant toroidal dipole response. The unit cell consists of four rectangular bars made of dielectric materials of moderate contrast. By performing multipole decomposition it is found that toroidal dipole response is dominant over a wide frequency range. The correlation between the toroidal dipole response and longitudinal electric field component is...

No abstract

No abstract

The ultrathin graphene metasurface is proposed as a mantle cloak to achieve wideband tunable scattering reduction around the spherical (three-dimensional) objects. The cloaking shell over the metallic or dielectric sphere is structured by a periodic array of graphene nanodisks that operate at infrared frequencies. By using the polarizability of the graphene nanodisks and equivalent conductivity method, the metasurface reactance is obtained. To achieve the cloaking shell for both dielectric and conducting spheres, the metasurface reactance as a function of nanodisks dimensions, graphene's Fermi energy, and permittivity of the surrounding areas can be tuned from the inductive to capacitive situation. Inhomogeneous metasurfaces including graphene nanodisks with different radii provide wideband invisibility due to extra resonances. We could significantly increase the 3-dB bandwidth more than the homogenous case by simpler realistic designs compared to the multi-layer structures. The analytical results are confirmed with full-wave numerical simulations.

Optical cloaking for cylindrical structures with arbitrary cross-section by using scattering cancellation method is presented. Nano-strips of silver in a dielectric host are used as a cloaking shell around the cylindrical object with arbitrary cross-section shapes. In this approach, the homogeneity effect on cloaking efficiency in the cloaking structure with circle cross-section is studied. The number of silver strips is the criteria of shell homogeneity. By increasing the number of strips for a constant filling factor, their widths become narrower and so the cloaking shell becomes more homogenous. It is shown that the cloaking efficiency is affected by the homogeneity of the cloaking shell and there is an optimum size of embedded particles to achieve required homogeneity in a cloaking structure.