Bifunctional Manipulation of Terahertz Waves with High-Efficiency Transmissive Dielectric Metasurfaces.

Multifunctional terahertz (THz) devices in transmission mode are highly desired in integration-optics applications, but conventional devices are bulky in size and inefficient. While ultra-thin multifunctional THz devices are recently demonstrated based on reflective metasurfaces, their transmissive counterparts suffer from severe limitations in efficiency and functionality. Here, based on high aspect-ratio silicon micropillars exhibiting wide transmission-phase tuning ranges with high transmission-amplitudes, a set of dielectric metasurfaces is designed and fabricated to achieve efficient spin-multiplexed wavefront controls on THz waves. As a benchmark test, the photonic-spin-Hall-effect is experimentally demonstrated with a record high absolute efficiency of 92% using a dielectric metasurface encoded with geometric phases only. Next, spin-multiplexed controls on circularly polarized THz beams (e.g., anomalous refraction and focusing) are experimentally demonstrated with experimental efficiency reaching 88%, based on a dielectric meta-device encoded with both spin-independent resonant phases and spin-dependent geometric phases. Finally, high-efficiency spin-multiplexed dual holographic images are experimentally realized with the third meta-device encoded with both resonant and geometric phases. Both near-field and far-field measurements are performed to characterize these devices, yielding results in agreement with full-wave simulations. The study paves the way to realize multifunctional, high-performance, and ultra-compact THz devices for applications in biology sensing, communications, and so on.