  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.
