Maximizing the chirality of bound states in the continuum by inverse design

Planar metasurfaces with both chirality and high quality ( Q ) factors have important applications in many fields. A chiral metasurface empowered by a bound state in the continuum (BIC) can provide a perfect solution to this problem. However, the metasurface design method based on physical intuition requires a substantial amount of computational resources, and the limited design parameters of meta-atoms restrict metasurfaces from achieving optimal optical performance. Here, we apply an inverse design method based on adjoint topological optimization to automatically alter the refractive index distribution of the metasurface, thereby maximizing the chirality of the BIC metasurface. Through this inverse design approach, chiral BIC metasurfaces with 3D intrinsic chirality at the target wavelength are designed and fabricated. To demonstrate the versatility of the proposed inverse design method, the metasurfaces with specific elliptic polarization states are designed. The inverse design method we propose provides an effective solution for the efficient design of chiral BIC metasurfaces.