The plasmonic metamaterials and metasurfaces play a critical role in manipulating lights in the mid-infrared spectral region.

Here, a research team led by Prof. Dr. LIU Hongjun from Xi'an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences (CAS) first propose a novel plasmonic chiral structure with the giant optical activity in the mid-infrared spectral region.

(a) X-z cross-sectional view of the simulated electric field E in the designed structure under RCP light illumination; (b) under LCP light illumination. (c) X-z cross-sectional view of the z component of electric fields. (d) under RCP light illumination; (d) under LCP light illumination. (Image by XIOPM)

The chiral structure consists of the moiré patterns, which are formed by stacking double-layer graphene nanoribbons with a relative in-plane rotation angle.

It is demonstrated that the graphene-based plasmonic structure with moiré patterns exhibits the strong circular dichroism. The giant chiroptical response can be precisely controlled by changing the rotation angle and Fermi level of graphene.

Furthermore, a dielectric interlayer is inserted between two layers of graphene to obtain the stronger circular dichroism. Impressively, the strongest circular dichroism can reach 5.94 deg at 13.6 μm when the thickness of dielectric interlayer is 20 nm.

The proposed structure with graphene-based moiré patterns can be superior to conventional graphene chiral metamaterials due to some advantage of rotation-dependent chirality, flexible tunability and cost-effective fabrication.

It will advance many essential mid-infrared applications, such as chiral sensors, thermal imaging and chiroptical detectors.

(Original research article “Optics Express” (2020) https://doi.org/10.1364/OE.385450)