Chiral dichroism in resonant metasurfaces with monoclinic lattices

We reveal that chiral response can be achieved in resonant metasurfaces with a monoclinic lattice symmetry (the so-called Bravais oblique lattices) where the mirror symmetry is broken by the lattice asymmetry and by a substrate, while each individual meta-atom remains fully achiral. We describe the underlying mechanism by introducing the mode chirality parameter as a quantitative measure of the lattice chiral eigenmodes. We confirm experimentally selective linear and nonlinear chiral interaction of resonant silicon metasurfaces with circularly polarized light.