Broadband terahertz plasmon-induced transparency via asymmetric coupling inside meta-molecules

We reported a broadband terahertz (THz) plasmon-induced transparency (PIT) phenomenon owing to asymmetric coupling in between the bright-and-dark resonators in meta-molecules (MMs). Each MM contains a cut-wire resonator and a couple of identical size and gap opposite-directed U-shaped resonators in mirror symmetry. An upside displacement of cut-wire induces an asymmetric deviation of cut-wire away from the X-axis in the MMs. Then, the PIT effect occurs due to the asymmetric coupling of dark resonators. The width of the transparency window extends monotonically with the deviation increasing. A picosecondscale group delay of the THz wave is found at the transparent windows. The distribution of surface currents and electric energy reveals that the asymmetric coupling between cut-wire and U-shaped resonators results in an energy transfer from surface plasmon (SP) oscillations to the inductive-capacitive (LC) oscillation due to the local symmetry breaking in structures of MMs. A couple of counteract SPs cause the transparency window, while the LC resonance gives rise to the side modes in the THz frequency spectrum. Furthermore, the LC oscillations of side modes take place in between the cut-wire and the local area of the U-shaped resonators, which leads to a magnetic dipole momentum. The displacement of cut-wire leads to an asymmetric distribution of magnetic momentum in MMs, which extends the width of the transparency window. Our experimental findings present a new approach to develop broadband slow-light devices in the THz frequency range.