The bound states in a continuum (BICs) are objective physical phenomena that defy conventional wisdom, and they exist in the radiating continuous spectrum but remain perfectly localized with non-radiation, which is different from the traditional bound states. In this paper, we report in theory and experiment the high thickness all-silicon terahertz (THz) metasurfaces supporting BIC and quasi-BIC, which is realized by simple pairs of elliptical pillars. Meanwhile, we used an extra optical pump to modulate the transmission amplitude difference between BIC and quasi-BIC metasurfaces, to complete the active control from “On” state to “Off” state. We utilize the abundant amplitude gradation of metasurfaces with different asymmetric degrees to develop the polarization-dependent THz near-field displaying application that can make the grayscale characteristics of a photograph reappear and also be dynamically controlled at On state to Off state.

A spin-selective focusing metasurface composed of chiral meta-atoms.

Conventionally, the realization of polarization transformation and wavefront manipulation in metasurfaces relies on the Pancharatnam–Berry (PB) phase together with the dynamic phase. However, the reported polarization transformation and wavefront manipulation were limited to spin-dependent wavefront manipulation for circular polarization (CP). To obtain more abundant functions, we propose a novel technology that relies on the dynamic phase with a spatial interleaving unit arrangement. With the functions of a quarter wave plate, the metasurfaces we designed can achieve multiple wavefront manipulations which are not only for the spin polarization transformation but also for the linear polarization transformation. Specifically, we design a bifocal metasurface, which can focus on one circularly polarized component as a point and spin-opposite component as a vortex under the linearly polarized (LP) incidence. With the further adjustment of the unit arrangement, the left-hand circularly polarized (LCP) and right-hand circularly polarized (RCP) components under the LP incidence can be refocused on the same point and then composited, resulting in a new LP exit wave. Furthermore, we prove theoretically that the desired x-LP component and y-LP component under the arbitrary CP incidence can also be manipulated independently. We believe that the versatility of this method will provide a novel platform for the development of terahertz integrated photonics.

Dual-functional polarization manipulation for orthogonal states of polarization in the terahertz frequency range.