Fig. 4: Surface current distribution at resonant frequencies.

a Fano mode resonance, b dipole resonance, and c quadrupole resonance at 0.4, 0.58, and 0.69 THz, respectively, for asymmetry parameter α = 100% where the colourscale represents normalized current intensity. The red arrows on the surface of each unit cell indicate instantaneous directions of the current flow at certain phase shifts. The black arrows in the bottom left represent the directions of the electric field (E), magnetic field (H), and propagation (k) vectors.