Controlling Asymmetric Retroreflection of Metasurfaces via Localized Loss Engineering

Metasurfaces based on subwavelength resonators enable novel ways to manipulate the flow of light at optical interfaces. In pursuit of multifunctional or reconfigurable metadevices, efficient tuning of macroscopic performance with little structural/material variation remains a challenge. Here, we propose the concept of localized loss engineering in electromagnetic (EM) metasurfaces, showing that an ordinary symmetric retroreflector can be switched to an extraordinary asymmetric one by introducing absorptive defects in local regions. The asymmetric performance begins with zero at the Hermitian state, gradually increases under non-Hermitian localized loss modulation, and reaches the maximum at the exceptional point (EP). The metasurface at the EP exhibits extremely asymmetric performance without the need of deeply discretized subwavelength elements. As a proof of concept, the localized loss-assisted asymmetric reflection is experimentally demonstrated at microwave frequencies via the observation of near-field distributions and far-field scatterings from a planar metasurface. Our methodology opens new opportunities for engineering EM systems with small perturbations and designing reconfigurable or multifunctional metadevices.