We propose a type of hybrid structure metalens composed of a graphene-loaded metallic metasurface sandwiched by two mutually orthogonal gratings, which can work in transmission modes for dynamic terahertz wavefront manipulation with tunability and enhanced focusing efficiency. Experimental results show that due to the multi-reflection between the metasurface layer and the grating layers, the focusing efficiency is enhanced by 1.8 times, and the focal length of the metalens is increased by 0.61mm by increasing the applied gate voltage on the graphene from 0V to 1.4V. We hope the proposed structure may open a new avenue for reconfigurable THz metasurfaces with high efficiencies.
Control of terahertz (THz) wave polarization state is of great significance for imaging and communication. Dynamically control of THz wave polarization state is achieved by an electronically controlling composite metasurface consisting of the gold cross antennas and a monolayer graphene. The graphene composite metasurface acts as a quarter-wave plate when the external control voltage is 0 V, by which the polarization state of the incident THz wave is converted from linear polarization to circular polarization. When voltage is increased, the chemical potential of graphene is increased gradually, transmitted polarization state of the THz wave is changed from right circular polarization to right elliptical polarization, and to linear polarization. Furthermore, the polarization state of the THz wave is able to be changed from left circular polarization to left elliptical polarization, and to linear polarization if the device is clockwise rotated by 90°. Our work will offer a new avenue for tunable THz polarization modulation devices.
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