Generalized Kerker effect in PT-symmetric nanoantenna array

All-dielectric nanophotonics is a rapidly developing and practical alternative to plasmonics for nanoscale optics. The electric and magnetic Mie resonances in high-index low-loss dielectric nanoresonators can be engineered to exhibit unique scattering response. Recently, nanophotonic structures satisfying parity-time (PT) symmetry have been shown to exhibit novel scattering responses beyond what can be achieved from the conventional nanoresonators. The complex interference of the magnetic and electric Mie resonances and lattice modes excited in PT-symmetric nanoantenna arrays give rise to a scattering anomaly called lasing spectral singularity (SS), where the scattering coefficients tend to infinity. In our previous work [1], we demonstrated the existence of lasing spectral singularities in vertically stacked 2D GaInP PT-symmetric metasurface. In this paper, we analyze the direction-sensitive scattering response of the PT-symmetric GaInP metasurface by decomposing the total scattered field into the electric and magnetic multipoles. The far-field scattering response at the singularity is highly asymmetric for incidence from either the gain or loss side and can be tuned by changing the geometry. By analyzing the phase of even- and odd-parity higher order multipoles, we explain the observed scattering response over a broad parameter space in terms of generalized Kerker effect. The interference between the direction-dependent excitation of different order multipoles and the overall 2D-lattice resonance opens a route towards designing a special class of tunable sources exhibiting direction-sensitive emission properties.