Diffraction enhanced transparency in a hybrid gold-graphene THz metasurface

Terahertz (THz) near-field microscopy is used to investigate the underlying physics that leads to diffraction enhanced transparency (DET) in a periodic array of detuned metallic rods. At the transparency frequency, the system is highly dispersive and THz radiation is delayed for several tens of picoseconds before being re-emitted into the forward direction. Using polarization sensitive measurements of the electric THz near-field spectrum, we demonstrate that an out-of-phase field distribution is formed in the unit cell leading to a reduced coupling to the far-field. This quadrupolar field distribution originates from the excitation and interference of surface lattice modes produced by the enhanced radiative coupling through the lattice of localized resonances in the metallic rods. These results represent the first experimental near-field investigation of DET, shedding light onto this phenomenon and providing important information for its further development. Implementing DET in applications requires control over the transparency window. We demonstrate that adding a monolayer of graphene, absorbing at THz frequencies, is sufficient to fully suppress the DET despite its monoatomic thickness. This efficient suppression is due to the diffractive wave character of DET and a metal insulator metal resonance formed between the localized resonance of the gold resonator and the extended graphene layer. The possibility to exert control over the transparency window by changing the conductivity of graphene by altering the Fermi level opens the possibility of active THz devices.