Metamaterials comprise assemblies of multiple elements fashioned from composite materials that exhibit unusual optical properties not found in nature. The EU-funded GRAB-A-META (Graphene based active metamaterials) project used graphene metamaterial technology to develop active control of optical properties, including polarisation and the speed of light. The electromagnetic properties of these artificially constructed materials can be engineered by designing subwavelength-scale metallic structures, called meta-atoms, to give unexpected physical phenomena. These include negative index of refraction, gigantic chirality and analogue electromagnetically induced transparency (EIT). The metamaterial system featured a very thin, free-standing flexible platform capable of full electrical control. This not only enabled control of novel optical properties, it also overcame most of the limitations of current semiconductor-based metamaterials. Researchers studied chiral metamaterials combined with a gated single-layer graphene. Results showed that transmission of a terahertz (THz) wave with one circular polarisation can be electrically controlled without affecting that of the other circular polarisation. This led to remarkably large modulation depths with a low gate voltage. Scientists also showed that polarisation memory operations with 10-year retention time were possible. This was achieved by hybridising the graphene, a ferroelectric material, and the chiral metamaterials. By enabling the electric control of polarisation, graphene chiral metamaterials may lead to a range of applications in the field of THz technologies. These include ultra-compact active polarisation modulators for telecommunications and imaging devices and ultrasensitive sensors for identifying chirality and structures of macromolecules. Active graphene EIT metamaterials slow light devices were also developed that can control light speed through gate voltages. Theoretical analysis indicated that this active EIT switching is due to the suppression of dissipative loss of the radiative resonator by electrically tuning the conductivity of the integrated graphene layer. GRAB-A-META work on electrically active tuning of EIT metamaterials may pave the way for designing active slow light devices, ultrasensitive sensors and non-linear devices.
Graphene, metamaterials, chirality, GRAB-A-META, electromagnetically induced transparency