Flexible terahertz phase modulation profile via all-silicon metasurfaces

Metasurfaces represent a group of artificially designed structures capable of efficiently manipulating the transmission properties of incident waves, have attracted considerable attention in terahertz (THz) radiation, and gradually overcome the complexity of bulky optical systems. Although the introduction of meta-optics has made great progress in full-span phase modulation schemes within orthogonal polarization channels, flexible phase functions can still effectively alleviate the design complexity of wavefront engineering. Here, we experimentally demonstrate several designs of general-purpose all-silicon THz metasurfaces applied in different scenarios. By combining geometric phase and propagation phase, switchable multiple phase modulation in orthogonal circularly polarized (CP) channels can be realized by switching the handedness of the incident wave. The advantage of the proposed scheme is that not only a locked phase modulation profile can be generated within the cross-polarized channels with opposite handedness, but also the phase distribution within each channel can be arbitrarily tailored to break the locking, demonstrating the flexible wavefront manipulation capability of the spin-decoupled phase control. Two different metalens are experimentally demonstrated to verify the feasibility of the multiple phase modulation scheme, mainly exploiting the topological charge and type of the focused beams. This work allows the metasurface to show a higher degree of freedom in wavefront manipulation by using a simpler phase-composite algorithm.