Angle enhanced circular dichroism in bilayer 90°-twisted metamaterial.

Intrinsic and extrinsic chiral responses have been widely investigated in metamaterials, however the relationship between them has been seldom discussed. We numerically and experimentally demonstrate angle enhanced chiral dichroism and study the separation between intrinsic and extrinsic chiral responses in metamaterial with asymmetrically split aperture dimers. The metamaterial exhibits triple-band resonant circular dichroism at normal incidence. The oblique incidence leads to giant enhancement of circular dichroism at two low-frequency resonances while yields an obvious resonance split of the circular dichroism in the vicinity of the high-frequency resonance. The whole circular dichroism response results from the balance between intrinsic and extrinsic chirality and the circular dichroism spectra at positive and negative angles of incidence exhibit an asymmetry due to the existence of intrinsic chirality. Importantly, the intrinsic chirality in the metamaterial may be individually investigated since extrinsic chiral response may be removed from the total circular dichroism by superimposing two circular dichroism spectra at positive and negative incident angles. The metamaterial will be promising to achieve enhanced chiral response and also separately utilize intrinsic and extrinsic chirality for manipulating the polarization state of light.

Tunable chiroptical response of graphene achiral metamaterials in mid-infrared regime.

We numerically investigate a tunable and extrinsic chiroptical response of a graphene achiral metamaterial in mid-infrared regime. The achiral metamaterial is composed of cascaded metallic split ring apertures and complementary graphene rings patterned on a dielectric layer. The strong extrinsic chiroptical responses of the metamaterial are allowed at oblique incidence and the integrated graphene can dynamically modulate extrinsic chirality by changing its Fermi level. The spectra of the chiroptical responses will show a blue shift with increasing the Fermi level of the patterned graphene. The maximal values of circular dichroism in the reflection and transmission modes can reach 80% and 50%, respectively. The maximal values of polarization rotation angle in the reflection and transmission modes can reach 80° and 60°, respectively. This graphene-based metamaterial design paves a way for a myriad of important terahertz (THz) and mid-infrared applications, such as optical modulators, absorbers and polarizers.