Achromatic terahertz quarter waveplate based on silicon grating

In this paper, an achromatic quarter waveplate scheme at terahertz frequency range is proposed. The waveplate is based on a silicon grating with large fill factor and period close to the wavelengths of the terahertz wave. Owing to the grating structure, the frequency dependence of waveplate is compensated. An achromatic QWP is fabricated and experimentally demonstrated to show an excellent wideband feature at the range of [0.47, 0.8] THz, which is more than 6 times wider than a conventional QWP working at single frequency 0.5 THz, by assuming ± 3% variance of phase retardation.