  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 put forward the concept of non-Hermitian doping in metasurfaces,
showing that an ordinary retroreflector can be switched to asymmetric one by
introducing absorptive defects in local regions. The asymmetric absorption
performance begins with zero at the Hermitian state, gradually increases under
non-Hermitian doping, and reaches the maximum of unity at the exceptional
point. This effect is experimentally demonstrated at microwave frequencies via
the observation of asymmetric near-field distribution and far-field scattering
properties and from a planar metasurface. Furthermore, while importing local
gain constituents in conventional retroreflector, it can be tuned to an
extremely asymmetric one-side amplifier, extending unidirectional amplification
from one-dimensional waveguide to two-dimensional scattering systems. The
proposed methodology provides an alternative pathway for engineering
electromagnetic metadevices and systems with small perturbations. Introduction
