Unilateral Asymmetric Radiation in Bilayer Metasurfaces

Abstract Asymmetric radiation (AR) achieved by breaking the spatial symmetry of the metasurface has been widely concerned for its rich physics and broad prospect in micro/nano‐optical devices. However, the AR on the same side of the metasurface is still hard to be realized limited by the spatial controllability of the nanostructures. Here, a bilayer metasurface composed of two layers of nanopillars with hexagonal lattice is engraved, and AR response is observed on the same side of the metasurface. By tailoring the shift between layers, the bound ‐ state in the continuum (BIC) supported by the metasurface becomes quasi‐BIC. Meanwhile, the far‐field polarization of the quasi‐BICs with in‐plane wave‐vectors k_∥ are different from those with −k_∥ wave‐vectors, meaning that the transmissions on the same side of the metasurface but of light with opposite incident angles are different (T(k_∥) ≠ T(−k_∥)), indicating a new type of AR and is called unilateral AR (UAR). The asymmetric transmission is verified by experiments of bilayer metasurface with shifts, and a maximum difference of 0.1 can be obtained. This work reveals a new type of AR and enables the design of a series of functional nano‐optical devices including on‐chip lasers and energy‐efficient couplers.