Invisibility by metamaterials is of great interest, where
optical properties are manipulated in the real permittivity–
permeability plane. However, the most effective approach
to achieving invisibility in various military applications is
to absorb the electromagnetic waves emitted from radar to
minimize the corresponding reflection and scattering, such that
no signal gets bounced back. Here, we show the experimental
realization of chip-scale unidirectional reflectionless optical
metamaterials near the spontaneous parity-time symmetry
phase transition point where reflection from one side is
significantly suppressed. This is enabled by engineering the
corresponding optical properties of the designed paritytime
metamaterial in the complex dielectric permittivity
plane. Numerical simulations and experimental verification
consistently exhibit asymmetric reflection with high contrast
ratios around a wavelength of of 1,550 nm. The demonstrated
unidirectional phenomenon at the corresponding parity-time
exceptional point on-a-chip confirms the feasibility of creating
complicated on-chip parity-time metamaterials and optical
devices based on their properties