Towards remotely directional transmission of terahertz wave in air: The concept of free-space photonic crystal waveguide

Abstract Remotely directional transmission of terahertz (THz) wave in free space is essential in fields of astronomy, space and ground based communications, as well as sensing for atmospheric pollutants, etc. In previous studies, large-aperture halfway mirrors were employed in order to fully collect and then focus the heavily naturally diffracted beam of THz wave to the final target. In this case, however, the broad THz beam not only made a large-size transmission system, but also led to a heavy energy loss due to water vapor absorption within the beam cross-section. Here, we theoretically proposed two index-guiding-based THz waveguide structures composed of plasma filament arrays in air, i.e., hollow cylindrical plasma (HCP) waveguide and free-space photonic crystal (FPC) waveguide (triangular lattice point matrix with a central point defect). Both structures have been demonstrated to well support THz propagation modes, which were confined inside the tiny central air core of the waveguide for an attenuation length up to hundreds of meters. Compared with each other, the FPC waveguide exhibited better performances, including smaller mode field diameter, longer attenuation length and better single-mode property for a broad THz band, making this scheme a promising candidate for long-distance guidance of THz waves in the atmosphere.