Geometry phase for generating multiple focal points with different polarization states.

Conventional lenses are always large and bulky to achieve desired wave-manipulating functions, hindering the development of integrated and miniaturized optical systems. Metasurfaces, two-dimensional counterparts of metamaterials, can accurately tailor the wavefront of electromagnetic waves at subwavelength scale, providing a flexible platform for designing ultra-compact and ultra-flat lenses, namely as metalenses. However, the previous geometry-phase-based metalenses usually generate focal point(s) with only one special polarization state, i.e., either linearly-polarized (LP) state or circularly-polarized (CP) state, which inevitably degrades further applications. Here, we propose and experimentally demonstrate an approach for designing terahertz (THz) metalenses based on geometry phase that can generate multiple focal points with different polarization states. Under the illumination of LP THz waves, three focal points with left-hand CP (LCP), right-hand CP (RCP) and LP states are observed. Furthermore, the position of each focal point can be flexibly manipulated in free space. Geometry metasurfaces consisting of micro-rods with the same shape but different in-plane orientations are fabricated to demonstrate these properties. This unique approach may enable an unprecedented capability in designing multifunctional THz devices with potential applications in imaging, detecting and communications.