Theoretical description of bifacial optical nanomaterials

Abstract

In general, optical nanomaterials composed of noncentrosymmetric nanoscatterers are bifacial, meaning that two counter-propagating waves inside the material behave differently. Thus far a practical theory for the description of such materials has been missing. Herein, we present a theory that connects the design of the bifacial nanomaterial's "atoms" with the refractive index and wave impedance of the medium. We also introduce generalized Fresnel coefficients and investigate the role of electromagnetic multipoles on the bifaciality. We find that in any material two counter-propagating waves must experience the same refractive index, but their impedances can differ. The model is demonstrated in practice by the design of a nanomaterial slab with one of its facets being optically reflective, while the other being totally non-reflective.

Bifacial Metasurface with Quadrupole Optical Response

“…In particular, it also includes the toroidal multipoles that are missing in the classical decomposition (see, e.g., Sec. 4.1 of [41]). As an example, the toroidal dipole is a simple combination of the electric-current octupole elements (see Fig.…”

“…The disc-dimer geometry (see the inset of Fig. 1) is chosen because it is a generic asymmetric configuration for controlling the two most significant lowest-order multipole moments, the electric-current quadrupole and electric dipole moments, independently of the polarization of the incident field [19,[29][30][31]. Similar interaction effects, such as asymmetric optical scattering and reflection, have been reported in regard to asymmetric nanoscatterers [32][33][34][35] and metasurfaces in the infrared [36][37][38] and microwave [39] spectral ranges.…”

“…In this case, the normal-incidence reflection and transmission coefficients ρ 0 and τ 0 of the metasurface can, for a linearly polarized optical wave, be written as [19] …”

“…For example, optically magnetic metamaterials are, as a rule, spatially dispersive [18,19], which is unwanted for many proposed applications, such as a perfect lens and an optical cloak. Furthermore, often-used metal metamolecules show too-high optical absorption, and due to unavoidable fabrication errors, also scattering loss.…”

“…Recently demonstrated examples include holographic elements [23], lenses and axicons [24], antireflection coatings [25], mirrors [26], and polarization converters [27,28]. To our knowledge, however, a metasurface or a metamaterial with an electric-dipole-free optical response [29] and with a strong dependence of optical reflection on the illumination side [19] has not yet been demonstrated experimentally in the visible spectral range.…”