One-Dimensional Chirality: Strong Optical Activity in Epsilon-Near-Zero Metamaterials

We suggest that electromagnetic chirality, generally displayed by 3D or 2D complex chiral structures, can occur in 1D patterned composites whose components are achiral. This feature is highly unexpected in a 1D system which is geometrically achiral since its mirror image can always be superposed onto it by a 180 deg rotation. We analytically evaluate from first principles the bianisotropic response of multilayered metamaterials and we show that the chiral tensor is not vanishing if the system is geometrically one-dimensional chiral; i.e., its mirror image cannot be superposed onto it by using translations without resorting to rotations. As a signature of 1D chirality, we show that 1D chiral metamaterials support optical activity and we prove that this phenomenon undergoes a dramatic nonresonant enhancement in the epsilon-near-zero regime where the magnetoelectric coupling can become dominant in the constitutive relations.

Figure 1

Sketch of metamaterial slab (with $N=3$ layers) and waves scattering geometry. The transmission amplitudes of the right-handed ($\mathrm{RCP},+$) and left-handed ($\mathrm{LCP},-$) circular polarized plane waves are not equal for $\theta \ne 0$ as manifestation of 1D chirality.

Figure 2

Frequency dispersion of effective electromagnetic parameters. (a) Real and (b) imaginary parts of the effective permittivities ${\epsilon }_{\perp }$ and ${\epsilon }_{||}$. (c) Real and imaginary part of the chiral parameter $\kappa$.

Figure 3

Optical activity of a slab whose permittivities and chiral parameter are reported in Fig. 2 and of thickness $L=0.4\mu \mathrm{m}$. (a) Circular dichroism [$\mathrm{\Delta }(\%)$] and (b) circular birefringence $\delta \mathrm{\Phi }$ as functions of wavelength $\lambda$ for $\theta =0$ deg (solid line), $\theta =30$ deg (dashed-dotted line), and $\theta =-30$ deg (dashed line). (c) Parameter $\rho$ measuring the magnitude of the magnetoelectric coupling term relative to the local dielectric contribution from the $y$ component of the first of Eq. (3). (d) $\mathrm{\Delta }(\%)$ (squares) and $\delta \mathrm{\Phi }$ (circles) as function of $\theta$ for $\lambda =0.5\mu \mathrm{m}$.

Figure 4

Comparison between effective medium theory (EMT) (solid lines) and full-waves analysis (dashed lines) predictions about the optical activity of a metamaterial structure whose material parameters are those used for obtaining Fig. 2 with $L=0.4\mu \mathrm{m}$ and $\theta =30$ deg. (a) Circular dichroism $\mathrm{\Delta }$ and (b) circular birefringence $\delta \mathrm{\Phi }$ as functions of the wavelength $\lambda$.