Hybrid Image Graphene Polaritons with Extremely Confined Mode and Field Enhancement in the Terahertz Regime

Abstract

noble metals, are generally used to compress mode sizes and break the diffraction limit of light. However, the SPPs exhibit highly delocalized mode confinements because noble metals behave as approximately perfect electric conductors in the THz regime, [21][22][23] which leads to a wave vector of the SPPs perpendicular to the interface nearly equal to that of free space THz waves. Such SPPs with a dominant electric field component normal to the tangential interface is called Zenneck [24,25] or Sommerfeld [26,27] waves. Their penetration depths into dielectric regions ranged from several millimeters to several centimeters. To compress mode sizes to deep subwavelength at the THz band, in addition to considering perforating subwavelength textured metal surfaces to create spoof SPPs, [28][29][30][31] graphene, [32,33] a monolayer of carbon atoms bonded in a honeycomb lattice, with the nearly pure imaginary surface conductivity (i.e., metallic features) was considered as one of the promising candidates. The merits of the graphene plasmons (GPs) [34][35][36][37][38] include the atomically thin sheet, reduction of GP wavelength, and tunable GP properties by electric gating and chemical doping in the graphene sheet. Therefore, several graphene-based devices operating in the THz regime, such as absorbers, [39] modulators, [40,41] switch, [42] imaging, [43] and plasmonic waveguides, [44][45][46] use the unique optical properties of graphene.Currently, the THz mode confinements of graphene-based waveguides are limited to the orders from 10 −2 to 10 −4 [44][45][46] of the mode area A 0 = λ 2 /4, where λ is the free space wavelength. To further enhance the mode confinements, a new waveguide configuration [47][48] consisting of a graphene sheet separated from a noble metal substrate by a nanometer dielectric spacer is proposed to support a new kind of GP mode with a highly confined mode size and a linear dispersion [49,50] called acoustic GP (AGP) or image GP (IGP), [51][52][53] which is similar to a symmetric GP of a double-layer graphene structure. [54,55] On the basis of the charge carrier viewpoint, [47][48][49][50][51][52][53] an IGP with an antisymmetric distribution of charge carriers is the hybridization of plasmons in a graphene sheet and its mirror image with out-of-phase charge oscillations inside the metal substrate. On the basis of field coupling, an IGP is explained by coupling a GP field in the graphene with a Zenneck wave in the metal. Therefore, the resultant field is significantly enhanced within the nanoscale spacer between the graphene and metal to provide high-confined electromagnetic fields and high field enhancement.Guiding terahertz waves below the diffraction limit of light typically excite graphene plasmons (GPs). However, the mode confinements of conventional GPs using graphene-dielectric interfaces are limited. To further squeeze the optical mode sizes, a graphene-dielectric-metal (GDM) configuration supporting an image graphene plasmon (IGP) has been reported recently through the plasmon interactio...