Efficient generation of vectorial terahertz beams using surface-wave excited metasurfaces

On-chip devices for generating pre-designed vectorial optical fields (VOFs) under surface wave (SW) excitations are highly desired in integrated photonics. However, conventional devices are usually of large footprints, low efficiencies, and limited wave-control capabilities. Here, we present a generic approach to design ultra-compact on-chip devices that can efficiently generate pre-designed VOFs under SW excitations, and experimentally verify the concept in terahertz (THz) regime. We first describe how to design SW-excitation metasurfaces for generating circularly polarized complex beams, and experimentally demonstrate two meta-devices to realize directional emission and focusing of THz waves with opposite circular polarizations, respectively. We then establish a systematic approach to construct an integrated device via merging two carefully designed metasurfaces, which, under SW excitations, can separately produce pre-designed far-field patterns with different circular polarizations and generate target VOF based on their interference. As a proof of concept, we demonstrate experimentally a meta-device that can generate a radially polarized Bessel beam under SW excitation at ~0.4 THz. Experimental results agree well with full-wave simulations, collectively verifying the performance of our device. Our study paves the road to realizing highly integrated on-chip functional THz devices, which may find many applications in biological sensing, communications, displays, image multiplexing, and beyond.