Negative refractive index due to chirality

We demonstrate experimentally and numerically that metamaterials based on bilayer cross wires give giant optical activity, circular dichroism, and negative refractive index. The presented chiral design offers a much simpler geometry and more efficient way to realize negative refractive index at any frequency. We also developed a retrieval procedure for chiral materials which works successfully for circularly polarized waves.

Figure 1

(Color online) (a) Schematic representation of one unit cell of the cross-wire structure. (b) Photograph of one side of a fabricated microwave-scale cross-wire sample. The geometry parameters are given by $a_x=a_y=15\text{mm}$, $l=14\text{mm}$, $w=0.8\text{mm}$, $s=1.6\text{mm}$, ${\varphi }_0=45°$, and $\varphi =30°$. The thickness of the copper is $t_m=36\mu \text{m}$.

Figure 2

(Color online) (a) and (b): Simulated and measured magnitude of the transmission coefficients for the right circularly polarized, $\left|T_{++}\right|$ (red solid), and the left circularly polarized, $\left|T_{--}\right|$ (blue/dark gray dashed), electromagnetic wave, respectively. (c) and (d): Simulated and measured polarization azimuth rotation, $\theta$, respectively. (e) and (f): Simulated and measured ellipticity angle, $\eta$, respectively.

Figure 3

(Color online) The retrieved effective refractive index for the right circularly polarized EM wave, $n_{+}$ (red/gray solid), and the left circularly polarized EM wave, $n_{-}$ (blue/dark gray dashed). The black dotted curve shows the refractive index calculated by the permittivity and the permeability, $n=\sqrt{ϵ\mu }$, and the green/light gray dash-dotted curve shows the chiral parameter, $\kappa$.

Figure 4

(Color online) The real part of the effective permittivity (red/gray solid) and the effective permeability (blue/dark gray dashed), extracted from the simulation data.

Figure 5

(Color online) The simulated current density distribution for the right circularly polarized EM wave at 6.5 GHz (a) and for the left circularly polarized EM wave at 7.5 GHz (b). The cones (in red) show the direction and magnitude of the current density. The magnitude of the current density is also shown by the color on the wires, with red and blue corresponding to largest and smallest values.