Selective bright and dark mode excitation in coupled nanoantennas.

Coupled nanoantennas as metamaterial unit elements possess peculiar spectral and radiational behaviors. We show that nanoantennas made of two identical plasmonic slot resonators can greatly enhance the quality factors of resonance spectra and control radiation patterns through the selective excitation of bright and dark coupled modes. We confirm experimentally the enhanced quality factor of a bright mode in coupled nanoantennas. Adding phase modulators to the coupled microwave antennas, we demonstrate the "dark mode only" excitation of coupled microwave antennas with an incident plane wave. We also show that the bright-to-dark mode conversion and the related changes in radiation patterns can be controlled by the polarization of incident waves. In particular, we achieve leftward or rightward uni-directional radiation upon the injection of left or right circularly polarized waves.