  We designed a cut-wire-pair metasurface that works as a transparent terahertz
half-wave plate, by matching the electric and magnetic resonances of the
structure. Due to the impedance matching nature of the resonances, a large
transmission phase shift between the orthogonal polarizations was achieved,
while permitting a high transmission. The electric and magnetic responses of
the proposed structure were confirmed by evaluating the electric admittance and
magnetic impedance. The structure was fabricated on a flexible film and its
helicity-conversion function in the terahertz frequency range was
experimentally demonstrated. The thickness of the device is less than 1/10 of
the working vacuum wavelength, and a high amplitude helicity conversion rate
over 80% was achieved. Finally, using simulations, we demonstrate the
feasibility of the gradually rotating cut-wire-pair array in terahertz
wave-front control.
