broadband plasmon-induced transparency in plasmonic metasurfaces based on bright-dark-bright mode coupling

Plasmon-induced transparency (pit), an analog of electromagnetically induced transparency, originates from destructive interference of plasmonic resonators with different quality factors and brings about the extreme dispersion within the narrow transparency window, promising remarkable potential for slow light, nonlinear optics and biochemical sensors. however, sometimes a broad transmission frequency band is more desirable for other applications such as bandpass filters. in general, strong coupling between bright and dark plasmon modes in coupled resonant systems leads to wide transparency bandwidth at the pit resonance. based on multi-oscillator coupling theory, a metasurface structure consisting of three perpendicularly connected metallic nanobars is purposefully designed and numerically demonstrated to support broadband pit spectral response. the near-field patterns indicate that the broadening of the transparent band results from the constructive interference of dual excitations of the single non-radiative (dark) resonator by the two radiative (bright) antennas. these results show that this scheme of bright-dark-bright mode coupling is significantly beneficial for designing filters operating over a broad frequency range.