The control of spin electromagnetic (EM) waves is of great significance in optical communications. Although geometric metasurfaces have shown unprecedented capability to manipulate the wavefronts of spin EM waves, it is still challenging to independently manipulate each spin state and intensity distribution, which inevitably degrades metasurface-based devices for further applications. Here we propose and experimentally demonstrate an approach to designing spin-decoupled metalenses based on pure geometric phase, i.e., geometric metasurfaces with predesigned phase modulation possessing functionalities of both convex lenses and concave lenses. Under the illumination of left-/right-handed circularly polarized (LCP or RCP) terahertz (THz) waves, these metalenses can generate transversely/longitudinally distributed RCP/LCP multiple focal points. Since the helicity-dependent multiple focal points are locked to the polarization state of incident THz waves, the relative intensity between two orthogonal components can be controlled with different weights of LCP and RCP THz waves, leading to the intensity-tunable functionality. This robust approach for simultaneously manipulating orthogonal spin states and energy distributions of spin EM waves will open a new avenue for designing multifunctional devices and integrated communication systems.

Asymmetric transmission, defined as the difference between the forward and backward transmission, enables a plethora of applications for on-chip integration and telecommunications. However, the traditional method for asymmetric transmission is to control the propagation direction of the waves, hindering further applications. Metasurfaces, a kind of two-dimensional metamaterials, have shown an unprecedented ability to manipulate the propagation direction, phase, and polarization of electromagnetic waves. Here we propose and experimentally demonstrate a metasurface-based directional device consisting of a geometric metasurface with spatially rotated microrods and metallic gratings, which can simultaneously control the phase, polarization, and propagation direction of waves, resulting in asymmetric focusing in the terahertz region. These dual-layered metasurfaces for asymmetric focusing can work in a wide bandwidth ranging from 0.6 to 1.1 THz. The flexible and robust approach for designing broadband asymmetric focusing may open a new avenue for compact devices with potential applications in encryption, information processing, and communication.