Active Metal-Graphene Hybrid Terahertz Surface Plasmon Polaritons

Surface plasmon polaritons (SPPs) are propagating electromagnetic surface waves with local
electric field enhancement and non-diffraction limit at optical frequencies. At terahertz (THz) frequencies, a
metal line with periodic grooves can mimic the optical SPPs with the same high cut-off response, which is
referred to as designer SPPs. Here, by replacing metal grooves with graphene sheets, a novel active metalgraphene
hybrid SPP device achieves significant phase modulation. Theoretically, the dispersion curves of
THz SPPs are determined by the dimensions and periodicity of the grooves. Changing the chemical potential
of graphene sweeps the effective groove depth, which correspondingly shifts the SPP cut-off frequency and
modulates the slow-wave phase. A prototype device is fabricated and characterized under varying bias applied
for graphene. The experiment demonstrates that the cut-off frequency red shifts from 200 to 177 GHz, and the
phase variation is as large as 112° at 195 GHz under a low bias from -0.5 to 0.5 V. Simultaneously, the SPP
transmittance is modulated by a factor of more than 3 dB from 140 to 177 GHz due to the graphene absorption.
The proposed structure reveals a novel approach to study the non-reciprocal and topological SPPs with active
modulation in the THz range.