Polarization-insensitive terahertz spoof localized surface plasmon-induced transparency based on lattice rotational symmetry

In recent years, metasurfaces enabling a slow light effect in the terahertz band have seen considerable achievement. However, most of these advances demonstrated so far are polarization sensitive. In this work, we demonstrate polarization-insensitive terahertz slow light at spoof surface plasmon-induced transparency windows. Two types of metasurfaces based on different lattice layouts, a C2 and a C4 lattice symmetry, are compared. On the one hand, the metasurface with C2 lattice symmetry displayed a 5 ps slow light effect in a transparency window around 0.3 THz. On the other hand, the metasurface with a C4 lattice layout achieves a maximum of 28 ps slow light at 0.3 THz. The coupling coefficient and the damping ratio in the transparency window in the metasurface with C4 lattice symmetry are 5 times higher than in the metasurface with C2 lattice layout. Two eigenmode mode constructive interference introduces a positive group delay in the transparency window in the metasurface with C4 lattice symmetry, whereas the superposition of two eigenmodes in the metasurface with C2 lattice symmetry forms the transparency window without distinct coupling. Our results show that the point group symmetry or lattice structure of a metasurface has a huge impact on the group velocity of terahertz pulses and therefore introduces flexibility in the design of polarization-insensitive slow light devices for terahertz telecommunication application.