Fig. 1: Concept of broken-symmetry 3R-MoS₂ metasurface.

Schematic of the wet etching process which forms atomically sharp equilateral triangular holes from the pre-etched circular holes of smaller size obtained via reactive ion etching of 3R-MoS₂ multilayer flake. Wet-etching provides atomically sharp edges, clean surface, and broken C₂ and C₄ symmetry, enabling the realization of quasi-bound-state-in-the-continuum (q-BIC) concept. The square insets are SEM images of the typical circular and triangular hole unit cells. Note that since wet etching consumes part of the material, it is necessary to start with a smaller initial circular hole diameter to achieve the desired size of the final triangular hole. Therefore, to achieve a comparable filling factor (ff) between circular and triangular metasurfaces, the initial circular hole size in the latter must be smaller in comparison to that of the former (see the dotted line representing the size of the original circular hole, as shown in the scanning electron microscopy image of the triangular nanohole). Scale bars of the SEM insets are 200 nm.