Tuesday, December 8, 2015

Terahertz wave manipulation using graphene cut-wires


Integrated phase modulation devices are critical to the application of terahertz technology, such as imaging and communication. Reporting in Nanotechnology, researchers propose a graphene metasurface, consisting of nano-patterned graphene cut-wires. This metasurface can dynamically modulate the wave front of incident infrared and terahertz waves to enable new functionality, such as anomalously reflecting and focusing with a relatively high efficiency of about 60%.

When circularly polarised light impinges on graphene cut-wires supported by metal-dielectric dual layers, the scattered electromagnetic field will acquire a geometric phase shift related to the orientations of cut-wires. So by presetting the distribution profiles of cut-wires direction angle Θ, the wave front of scatterred light can be regulated into any shape desired. Discrete micro- or nanoscale graphene patches can also be fabricated by one-step (electron beam) lithography and O2 plasma etching process, once transferred on the metal-dielectric substrate. All the patches are doped by a thin layer of high-capacitance ion-gel that can inject carriers up to Fermi-level 0.8 eV.

Graphene focusing mirror

With this mechanism, flat focusing mirror made by graphene can be designed. When tuning the Fermi-level to 0.8 eV, this one-atom-thick graphene pattern layer can converge the energy of incident terahertz wave (58 μm) into the focal spot (F = 290 μm) with about 5 times intensity enchancement, even if the numerical aperture is very small.

Potential applications

Despite metasurfaces anomalously reflecting light that does not obey the traditional Snell laws, more functional components for infrared and terahertz waves can be achieved based on the metal-dielectric-graphene metasurface. This includes vortex beam generation for terahertz communications.
More information about this research can be found in the journal Nanotechnology 26 505203.

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