Flexible PT-Symmetric Optical Metasurfaces

The synthesis of ultrathin photonic structures in order to effectively redirect and mold optical wave fronts into arbitrary shapes is of crucial importance in modern beam steering, imaging, and sensing technologies. To this end, planar subwavelength systems such as optical metasurfaces have been intensely investigated in recent years. Such arrangements rely on abrupt, yet controllable, phase shifts imparted on the incident beam, by means of judiciously designed anisotropic nanoantennas. Here, we propose and demonstrate an altogether different methodology in order to manipulate the flow of light, by adopting a diatomic parity-time (PT)-symmetric Bravais-lattice topology, the unit cell of which involves only a transparent and a lossy optical component. In this respect, a honeycomblike configuration is employed, the principal symmetries of which are progressively broken through specific geometric transformations. The complex near-field coupling interactions between neighboring diffractive elements give rise to a discerning enhancement or attenuation along specific directions in the far field, over a broad range of wavelengths in the visible domain. In this work, we report the realization of an all-passive PT-symmetric optical metasurface on a flexible polyimide substrate, capable of demonstrating selective directional scattering. Our study draws a clear connection between surface topology and radiation directivity, which can be systematically utilized toward observing unconventional transport effects in flat- and curved-space PT lattices.