Broadband chirality and asymmetric transmission in ultrathin 90°-twisted Babinet-inverted metasurfaces

A broadband asymmetric transmission of linearly polarized waves with totally suppressed copolarization transmission is experimentally demonstrated in ultrathin 90°-twisted Babinet-inverted metasurfaces constructed by an array of asymmetrically split ring apertures. The only accessible direction-dependent cross-polarization transmission is allowed in this anisotropic chiral metamaterial. Through full-wave simulation and experiment results, the bilayered Babinet-inverted metasurface reveals broadband artificial chirality and asymmetric transmission, with a transmission contrast that is better than 17.7 dB within a 50% relative bandwidth for two opposite directions. In particular, we can modify polarization conversion efficiency and the bandwidth of asymmetric transmission via parametric study.

Figure 1

90°-twisted Babinet-inverted metasurface. The views of the (a) front and (b) back layers. Two ASRAs rotate with respect to each other by a twist angle of 90°. (c) The schematic of a unit cell of the stereo ASRA dimer array. Each ASRA resonator has different arc slits corresponding to angles α and β. Two ASRAs are coaxial. The forward propagation is along the −$z$ direction. (d) The top view of a fabricated Babinet-inverted metasurface sample, which was manufactured from F4B laminates using the photolithography technique.

Figure 2

The (a) and (c) reflection and (b) and (d) transmission spectra of the single-layer Babinet-inverted metasurface. The dotted, solid, and dash-dotted lines indicate geometric structures with β = 160°, β = 140°, and β = 100°, respectively, and the other open angle is α = 160°. The insets show polarizations of incident waves and $z$-component magnetic near-field distributions at frequencies marked by bold arrows.

Figure 3

Simulated and measured results of the 90°-twisted Babinet-inverted metasurface with α = 160° and β = 100°. (a) The simulated transmission spectra $t_{xx}$, $t_{yx}$, $t_{yy}$, and $t_{xy}$ and (b) the simulated polarization rotation angle θ and ellipticity η for a $y$-polarized forward-propagating wave. The insets indicate wave propagation directions. (c) The measured cross-polarization transmission spectra $t_{xy}$ and $t_{yx}$ for $y$- and $x$-polarized forward-propagating waves. (d) The simulated and measured asymmetric transmission parameter Δ${}^y$ for forward- and backward-propagating waves, where solid and dotted curves correspond to the measured and the simulated results, respectively.

Figure 4

(a) Simulated and (b) measured cross-polarization transmission $t_{xy}$ in the 90°-twisted Babinet-inverted metasurface as a function of the asymmetry of the ASRA. The asymmetry of the ASRA is controlled by the open angle β changing from 160° to 0° in 10° steps, and the other open angle, α, is always 160°, as shown in the inset. The three transmission peaks are denoted as first, second, and third. The nine samples with a constant spacer thickness of 1.5 mm were fabricated, where β = 0°, 40°, 60°, 80°, 100°, 120°, 130°, 140°, and 160°. (c) and (d) $Z$-component magnetic near-field distributions and charge distributions of the front and back layers at the first, second, and third transmission peaks for β = 0° and 100°. The fields in all maps are plotted in the same scale and recorded 0.2 mm away from the front and back layers. The symbols + and − indicate the surface charge distributions.

Figure 5

(a) Simulated cross-polarization transmission $t_{xy}$ in the 90°-twisted Babinet-inverted metasurface as a function of dielectric-layer thickness. (b) Measured transmission spectra of two bilayered metamaterials for $t$ = 1 and 1.5 mm, denoted by dotted and solid lines, respectively, in Fig. 5.