Metasurfaces Excited by an Evanescent Wave for Terahertz Beam Splitters with a Tunable Splitting Ratio

In terahertz (THz) photonics, a beam splitter is an important component. Although various THz beam-splitting devices using several principles have been proposed, the splitting ratio of existing designs is not adjustable. Here, a THz beam splitter with an adjustable splitting ratio is demonstrated using a metasurface integrated onto a prism. The metasurface excited by an evanescent wave can convert part of a linearly polarized incident wave into a cross-polarized wave and manipulate its phase simultaneously. As a result, the cross-polarized wave can pass through the interface, even if the incident angle is larger than the total internal reflection angle, while the co-polarized wave is reflected by the prism. The splitting ratio of the device can be adjusted from 4.56:1 to 0.63:1 by tuning the resonant response of the metasurface and varying the interval distance between the metasurface and the prism. The metasurface samples are fabricated using low-cost standard printed circuit technology, and the experimental results are consistent with the simulations. Therefore, the proposed beam splitter with a tunable splitting ratio is promising as a key component in the THz system.