Excite Spoof Surface Plasmons with Tailored Wavefronts Using High-Efficiency Terahertz Metasurfaces

Spoof surface plasmons (SSPs) play crucial roles in terahertz (THz) near-field photonics. However, both high-efficiency excitation and wavefront engineering of SSPs remain great challenges, which hinder their wide applications in practice. Here, a scheme is proposed to simultaneously achieve these two goals efficiently using a single ultracompact device. First, it is shown that a gradient meta-coupler constructed by high-efficiency Pancharatnam-Berry (PB) meta-atoms can convert circularly polarized (CP) THz beams into SSPs with absolute efficiency up to 60%. Encoding a parabolic phase profile into the meta-coupler based on the PB mechanism, it is demonstrated that the device can covert CP beams into SSPs with focusing or defocusing wavefronts, dictated by the chirality of the incident wave. Finally, two distinct chirality-dependent phase distributions are encoded into the meta-coupler design by combining the PB and resonance phase mechanisms, and it is demonstrated that the resulting meta-device can achieve SSP excitations with chirality-delinked bifunctional wavefront engineering. THz near-field experiments are performed to characterize all three devices, in excellent agreement with full-wave simulations. The results pave the road to realize ultracompact devices integrating different functionalities on near-field manipulations, which can find many applications (e.g., optical sensing, imaging, on-chip photonics, etc.) in different frequency domains.