Diffraction Enhanced Transparency and Slow THz Light in Periodic Arrays of Detuned and Displaced Dipoles

We demonstrate that a periodic lattice of detuned resonators can suppress the THz extinction at the central resonant frequency, leading to an enhanced transparency due to diffraction. The system consists of metallic rods of two different sizes, each of them supporting a strong half-wavelength (λ/2) resonance, which are spatially displaced within the unit cell of the lattice. Using a coupled dipole model we show that the diffraction enhanced transparency (DET) window has its origin in the interference between two surface lattice resonances, arising from the diffractively enhanced radiative coupling of the λ/2 resonances in the lattice. Group-index measurements show that the THz field is strongly delayed by more than 4 orders of magnitude at the transparency window. Since DET does not involve the near-field coupling of resonators, the fabrication tolerance to imperfections is expected to be very high. This remarkable response renders these systems as very interesting components for THz communication.