Sweeping Plasma Frequency of Terahertz Surface Plasmon Polaritons with Graphene

TL;DR

This paper demonstrates a novel method to dynamically sweep the plasma frequency of terahertz surface plasmon polaritons using a graphene-based structure, enabling tunable control over SPP propagation.

Contribution

The study introduces the first experimental demonstration of a graphene-based structure that dynamically controls the plasma frequency of THz SPPs through electrical gating.

Findings

Red shift of plasma frequency from 195 to 180 GHz with bias change

Effective control of SPP propagation on/off status in THz range

Both simulated and experimental validation of the tuning mechanism

Abstract

Plasma frequency is the spectral boundary for low-loss propagation and evanescent decay of surface plasmon polariton (SPP) waves, which corresponds to a high cut-off phenomenon and is typically utilized for identifying SPPs. At terahertz (THz) frequencies, a metal line with periodic metallic grooves can mimic the conventional optical SPPs, which is referred to as designer SPPs. Theoretically, the plasma frequency of THz SPPs decreases as the groove depth increases. Here, by replacing the metallic grooves with graphene sheets, dynamically sweeping SPP plasma frequency is demonstrated for the first time. The metal-graphene hybrid structure comprises a metal line with periodic graphene grooves, a thin-layer ion gel for gating graphene, and metallic tips for uniforming gate field. As the chemical potential changes, the average conductivity of graphene is modulated so that the effective depth of the graphene grooves changes, which sweeps the plasma frequency of THz SPPs consequently. Both simulated and experimental data demonstrate a red shift of plasma frequency from 195 to 180 GHz at a low bias from -0.5 to 0.5 V. The proposed structure reveals a novel approach to control the on/off status of SPP propagation in the THz range.