Terahertz chiral sensing and polarization‐multiplexing communication demand subwavelength devices that dynamically invert polarization helicity. Metasurfaces can enhance anisotropy and fine tunability at subwavelength scales for this purpose. Although metasurfaces enabling deep modulation between orthogonal polarizations have been designed, they suffer from low conversion efficiencies. In this study, it is shown that the efficiency of conversion from linear to circular polarization by conventional single‐layer transmissive metasurfaces cannot exceed a fundamental limit. A dynamic reflective metasurface free from this limitation is then proposed. The device includes multilayer structures working as a terahertz quarter‐wave plate with switchable slow and fast axes. A phase transition of vanadium dioxide induces the necessary structural transformation of the metallic patterns. A practical fabrication method, based on the room‐temperature bonding technique of sapphires, is presented. Dynamic helicity inversion is demonstrated at 0.90 THz, with a conversion efficiency of over 80% that is beyond the fundamental limit of single‐layer transmissive metasurfaces (65%) and more than four times greater than that of previously reported devices.
The metasurface on the back cover (see article number 2101615 by Yosuke Nakata and co‐workers) efficiently converts an incident wave with linear polarization (green) into a reflected one with circular polarization (red/blue). Depending on the connection state (represented by red/blue) of the metallic pattern, the rotation direction of circular polarization is switched. The dynamic control of the connection state is realized by leveraging vanadium dioxide. The device is applicable to highly sensitive terahertz detection of chiral molecules that respond differently depending on the handedness of the circular polarization.
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