Employing couplers to convert guided waves into free-space modes and flexibly control their wavefront is one of the key technologies in chip-integrated displays and communications. Traditional couplers are mainly composed of gratings, which have limitations in footprint, bandwidth, as well as controllability. Though the resonant/geometric metasurface newly emerges as a promising interface for bridging guided waves with free-space ones, it either relies on complex optimizations of multiple parameters, or is subject to the locked phase response of opposite spins, both of which hinder the functional diversity and practical multiplexing capability. Here, we propose and experimentally demonstrate an alternative with a spin-decoupled meta-coupler, simultaneously integrating triple functions of guided wave radiation, polarization demultiplexing, and dual-channel wavefront manipulation into a single device. By endowing polarization-dependent functionalities into a pure geometric metasurface, the out-coupled left-handed and right-handed circular polarization guided waves intelligently identify the predesigned phase modulation and reconstruct desired wavefronts, like bifocal focusing and holography multiplexing, with a polarization extinction ratio over 13.4 dB in experiments. We envision that the robust, broadband, and multifunctional meta-coupler could pave a way for the development of versatile multiplexed waveguide-based devices.

We propose and experimentally demonstrate a high quality (Q)-factor all-silicon bound state in the continuum (BIC) metasurface with an imperforated air-hole array. The metasurface supports two polarization-insensitive BICs originated from guided mode resonances (GMRs) in the frequency range of 0.4 to 0.6 THz, and the measured Q-factors of the two GMRs are as high as 334 and 152, respectively. In addition, the influence of the thickness of the silicon substrate on the two resonances is analyzed in detail. The proposed all-silicon THz metasurface has great potential in the design and application of high-Q metasurfaces.

High-Q metasurfaces have important applications in high-sensitivity sensing, low-threshold lasers, and nonlinear optics due to the strong local electromagnetic field enhancements. Although ultra-high-Q resonances of bound states in the continuum (BIC) metasurfaces have been rapidly developed in the optical regime, it is still a challenging task in the terahertz band for long years because of absorption loss of dielectric materials, design, and fabrication of nanostructures, and the need for high-signal-to-noise ratio and high-resolution spectral measurements. Here, a polarization-insensitive quasi-BIC resonance with a high-Q factor of 1049 in a terahertz all-silicon metasurface is experimentally achieved, exceeding the current highest record by 3 times of magnitude. And by using this ultra-high-Q metasurface, a terahertz intensity modulation with very low optical pump power is demonstrated. The proposed all-silicon metasurface can pave the way for the research and development of high-Q terahertz metasurfaces.

Anapole metamaterials have attracted growing attention in recent years due to their unique nonradiating and nontrivial properties. Although anapole modes have been demonstrated in metamaterials with three-dimensional structures, the design and realization of planar anapole metamaterials in a wide frequency range is still a big challenge. Here we propose and experimentally demonstrate a planar anapole metamaterial consisting of dumbbell-shaped apertures on a stainless-steel sheet at terahertz frequencies. The planar metamaterial can generate a resonant transparency in the terahertz spectrum due to the excitation of the anapole mode. Particularly, the frequency of anapole-induced resonant transparency can be tuned easily in the range of 0.15–0.93 THz by simply varying one geometric parameter of the dumbbell apertures. We anticipate that the resonant transparency in planar anapole metamaterials can be potentially used in filters, sensors, or other photonic devices.