All‐Dielectric Metalattice with Enhanced Toroidal Dipole Response

Abstract

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...

Nonradiating photonics with resonant dielectric nanostructures

“…Other designs of all-dielectric metasurfaces with strong toroidal response for the visible range were proposed in Refs. [138,139].…”

Controlling high- Q trapped modes in polarization-insensitive all-dielectric metasurfaces

“…42 The most straightforward way to construct a polarization-insensitive metasurface based on the same particles is to rearrange them within the 2 × 2 supercells, similarly to those proposed for the split ring based metasurfaces. 31,[33][34][35] For the normal wave incidence, symmetry of the super-cell in the x − y plane is described by the group C 4 which consists of the four-fold axis for rotation around the z-axis ( Fig. 2(b)).…”

All-dielectric metasurfaces with trapped modes: Group-theoretical description

“…The physics of all-dielectric resonant nanophotonics can be broadly characterized by two major phenomena: (i) sharp spatial location of electric and magnetic fields at the nanoscale, which enhances a number of nonlinear effects, such as the harmonic generation, [6][7][8] alloptical switching, 9 and (ii) the multimodal interference via hybrid modes excitation. Many novel effects were discovered, such as directional light scattering, 10 optical magnetism, 11,12 bound states in the continuum, 13,14 toroidal and nonradiating optical anapole states, [15][16][17][18][19] etc.…”