Active control of polarization state in terahertz metalens

Abstract

Metalens shows unprecendented capability in manipulating the propagation of electromagnetic wave, and has much more potential in light focusing, imaging, etc. than the conventional bulky lens. However, since the light refraction of metalens is caused by the phase discontinuity over the spatial phase responses, numerous efforts should be devoted into the design of every metastructure to meet the phase demand, which makes it challenging to achieve tunable metalenses. Here, we propose several terahertz (THz) metalenses with polarization states continuously tunable at their focal points as the external optical pump varies. Every metastructure of these metalenses contains two hybrid circular split ring resonators (CSRRs) composed of Si and Au, respectively. It is shown that the electromagnetic response of the hybrid CSRRs can be split into two orthogonal Pancharatnam-Berry phase modes, which are controlled by the rotation angles of Si and Au-CSRRs. Under this theory, THz metalenses with single or multiple reconfigurable linear polarized (LP) focal points are designed and simulated. By varying the Si conductivity, the polarization angle of LP THz wave at the focal point can continuously rotate from the initial angle to the designed one. Our design of metalens paves a new way for tunable metasurfaces.