“…For example, eigenvectors of a non-Hermitian Hamiltonian do not require to be orthogonal, and at certain points in the parameter space called exceptional points (EPs), two eigenvectors, as well as their eigenvalues, coalesce. Metasurfaces provide an excellent arena for exploring the underlying physics of non-Hermitian systems because they can be engineered with precise control over the structural parameters that govern resonator properties and easily be measured by standard optical reflection or transmission. − Recently, PT-symmetric metasurfaces and their topological features of non-Hermitian matrices near EPs have been investigated; − − for example, Genevet et al demonstrated that by precisely designing the sizes of Al nanoantennas, a planar chiral plasmonic metasurface exhibits 2π topological phase accumulation distributed along any arbitrarily closed parameter loop encircling the EP in a reflection regime . Up to now, however, most of the works have focused on plasmonic metasurfaces, wherein the phenomena related to non-Hermitian properties such as anomalous transparency, unidirectional transmission, − power oscillation, , and so on mainly arise from electric dipolar scattering and decaying of metallic nanoantennas, completely irrespective of other orders of resonant modes.…”