Tunable Terahertz Hybrid Metal–Graphene Plasmons

TLDR

Graphene supports terahertz surface plasmons whose resonant frequency can be tuned by gate voltage, but existing devices struggle to achieve near‑total modulation with electrical contacts, limiting practical applications. This work introduces a hybrid graphene‑metal structure that supports a new class of plasmon resonance. The design uses sub‑wavelength metal contacts as a capacitive grid and narrow graphene channels as a tunable inductive medium, creating a resonantly matched structure for incident terahertz waves. Experimentally the structure achieves near‑maximum resonant absorption in large‑area graphene, and simulations predict that high‑mobility graphene can yield almost 100 % transmission, enabling tunable THz detectors, absorbers, filters, and modulators while overcoming the contact‑integration challenge.

Abstract

Among its many outstanding properties, graphene supports terahertz surface plasma waves -- sub-wavelength charge density oscillations connected with electromagnetic fields that are tightly localized near the surface[1,2]. When these waves are confined to finite-sized graphene, plasmon resonances emerge that are characterized by alternating charge accumulation at the opposing edges of the graphene. The resonant frequency of such a structure depends on both the size and the surface charge density, and can be electrically tuned throughout the terahertz range by applying a gate voltage[3,4]. The promise of tunable graphene THz plasmonics has yet to be fulfilled, however, because most proposed optoelectronic devices including detectors, filters, and modulators[5-10] desire near total modulation of the absorption or transmission, and require electrical contacts to the graphene -- constraints that are difficult to meet using existing plasmonic structures. We report here a new class of plasmon resonance that occurs in a hybrid graphene-metal structure. The sub-wavelength metal contacts form a capacitive grid for accumulating charge, while the narrow interleaved graphene channels, to first order, serves as a tunable inductive medium, thereby forming a structure that is resonantly-matched to an incident terahertz wave. We experimentally demonstrate resonant absorption near the theoretical maximum in readily-available, large-area graphene, ideal for THz detectors and tunable absorbers. We further predict that the use of high mobility graphene will allow resonant THz transmission near 100%, realizing a tunable THz filter or modulator. The structure is strongly coupled to incident THz radiation, and solves a fundamental problem of how to incorporate a tunable plasmonic channel into a device with electrical contacts.