Spin-decoupled metalens with intensity-tunable multiple focal points

Abstract

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.