Efficient manipulations of circularly polarized terahertz waves with transmissive metasurfaces

The unrestricted control of circularly polarized (CP) terahertz (THz) waves is important in science and applications, but conventional THz devices suffer from issues of bulky size and low efficiency. Although Pancharatnam–Berry (PB) metasurfaces have shown strong capabilities to control CP waves, transmission-mode PB devices realized in the THz regime are less efficient, limiting their applications in practice. Here, based on Jones matrix analysis, we design a tri-layer structure (thickness of ~λ/5) and experimentally demonstrate that the structure can serve as a highly efficient transmissive meta-atom (relative efficiency of ~90%) to build PB metadevices for manipulating CP THz waves. Two ultrathin THz metadevices are fabricated and experimentally characterized with a z-scan THz imaging system. The first device can realize a photonic spin Hall effect with an experimentally demonstrated relative efficiency of ~90%, whereas the second device can generate a high-quality background-free CP Bessel beam with measured longitudinal and transverse field patterns that exhibit the nondiffracting characteristics of a Bessel beam. All the experimental results are in excellent agreement with full-wave simulations. Our results pave the way to freely manipulate CP THz beams, laying a solid basis for future applications such as biomolecular control and THz signal transportation.Metasurfaces: efficient terahertz controlUltrathin metasurfaces that can efficiently manipulate circularly polarized terahertz waves in transmission rather reflection have been demonstrated by scientists in China. The devices were fabricated by Min Jia and colleagues from Fudan University and Capital Normal University. The tri-layer structures operate at frequencies of around 0.6 THz and suit future on-chip applications for terahertz photonics. Various designs were tested resulting in a circularly polarized terahertz Bessel beam generator and a device exhibiting the photonic spin Hall effect. The metasurfaces consist of a periodic array of meta-atoms, each composed of three thin layers of metal in a U-shape that are separated and surrounded by polyimide. Changing the angular orientation of the meta-atoms allows different phase gradients to be programmed into the metasurfaces bringing the desired functionality.