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Metamaterials and recently metasurfaces have been a powerful tool to control and manipulate electromagnetic waves and their interaction with matter. Active control of metamaterials is an expanding new direction, which is promising for the realization of novel active devices, such as optical switches, transducers, modulators, filters, and phase shifters at different wavelengths. The integration of passive metamaterials with a variety of tuning mechanisms has been extensively examined to generate active metamaterials that have novel functionalities. In general, there are two major schemes to implement active plasmonic systems. One is based on the integration of active media, that is, phase-transition materials, graphene and carrier-modulated semiconductors, which can respond to thermal, electrical and optical stimuli. The other is based on geometrical reconfiguration, that is, structural tuning of metamaterials.
Although the demonstrated devices provide some degree of tunability, their performances are limited to narrow spectra with a small dynamic range due to the material and fabrication limitations. Therefore, these technologies would greatly benefit from a material that yields large tunability over broad spectra. None of the existing materials provides these challenging requirements. Furthermore, the requirement for electrically controlled tunability places another challenge for practical applications of metamaterials.
Integrating metamaterial (a split ring resonator, SRR, in this work) in close proximity to graphene surface yields a new type of hybrid metamaterial whose resonance amplitude can be tuned. Previous attempts to integrate graphene with metamaterials yielded very limited modulation in IR and terahertz frequencies. Here, to tune the electrical resonance of metamaterials, we varied the charge density on graphene layer via ionic gating. It should be emphasized here that the technical challenge for graphene-based microwave devices is the requirement of large-area devices owing to the centimeter scale wavelength. To overcome this challenge, large-area graphene by chemical vapor deposition (CVD) on copper foils is used, which enables the realization of the microwave metamaterials. At 0 V, the device yields a resonance at 11.82 GHz with a resonance transmittance of −60 dB. When we applied a bias voltage, electrons and holes accumulate on the graphene electrodes and yield significant damping that diminishes the resonant behavior. For example, at 1.5 V, the resonance transmittance is -12 dB. Figure 2 shows the voltage dependence of the amplitude of transmittance at resonance and the phase at 11.82 GHz. The phase of the transmitted signal varies from -30° to 70°.
These active metamaterials enable efficient control of both amplitude (>50 dB) and phase (>90°) of electromagnetic waves. The operation frequency of these metamaterials can be easily scaled up to the terahertz and higher frequencies. Large modulation depth, simple device architecture, and mechanical flexibility are the key attributes of the graphene-enabled active metamaterials. We anticipate that the presented approach could lead to new applications ranging from electrically switchable cloaking devices to adaptive camouflage systems in microwave and terahertz frequencies.
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