A waveplate, well known as a phase retarder, manipulates the polarization state of incident light travelling through it and is a necessary component in many optical systems. Generally, waveplates are constructed out of birefringent materials which lead to the different phase retardation of incident light according to initial polarization state. For the terahertz (THz) spectral range, however, there are some limitations including thickness, transmission, and cost for the waveplate fabrication due to the lack of natural materials that possess strong birefringence in this region. Recently, THz waveplates based on metasurfaces have been proposed to overcome this limitation. Singlelayered metasurfaces have been utilized as THz quarter-waveplates (QWPs), for which a retarded phase of 90 degrees is sufficient. . To achieve phase retardation of 180 degrees and realize THz half-waveplates (HWP), stacked single-layered metasurfaces have been adopted , but this approach requires sophisticated fabrication processes and sacrifices structural simplicity.
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