Direction-Dependent Janus Metasurface Supported by Waveguide Structure with Spoof Surface Plasmon Polariton Modes

In this work, a novel direction-dependent Janus metasurface (DDJM) supported by waveguide structure (WGS) with surface plasmon polariton (SSPP) modes is proposed. In -z-direction, linear-to-linear (LTL) polarization conversion (PC) can be implemented at the low frequencies by the reflection characteristics of the WGS. And in the +z-direction, the ultrawideband absorption takes shape due to the SSPP modes, in which strongly localized oscillation of free electrons are confined to the metal-dielectric interface assisted by the WGS. The combination of the WGS and the PC structure finally enables the low-frequency LTL PC with the high polarization conversion ratio (over 0.9) at 0.87-1.35 THz in -z-direction, corresponding to the relative bandwidth of 43.24%, as well as the absorption in +z-direction with an efficiency of over 0.9 in the frequency range of 0.8-1.2 THz. Also the fixed direction-independent transparent window brought by WGS runs from 1.57 to 1.90 THz when the co-polarized transmission coefficient exceeds 0.85, occupying a staggered band compared to that of PC or absorption. The favorable properties of the DDJM promise a potential practical application in full-space electromagnetic wave control and radar stealth technology fields.