## 1 Introduction  
The terahertz range is a fascinating and promising frontier of research for a wide range of technological applications, from medical imaging to security, from astrophysics to consumer electronics [1; 2; 3]. The quickly increasing interest in THz range technology is driven by the possibility to fabricate reliable and portable coherent sources and sensitive detectors, in order to exploit the features of the field in such a range (0.1-10 THz), also known as the submillimeter wave range (1-0.03 mm). The THz range has begun to be explored thoroughly only over the last decade given the difficulties to approach such a range, either by extending the paradigms of the RF/microwave technology (hence, coming from lower frequencies) or those of the infrared (IR), or far infrared (FIR) technology (hence, coming from higher frequencies), e.g, [4, 5]. The THz range is, indeed, a sort of an unexplored \"boundary\" region between electronics and photonics, which was usually referred to as the \"THz gap\", to underline the difficulty to access this range by using the classical electronics or photonics technologies. In fact, classical microwave devices exhibit a dramatic drop of performance for frequencies above 100 GHz, due to the degradation of the transport properties. In addition, classical photonics devices cannot be efficiently used for frequencies below 10 THz, due to the steep increase of the conversion of the photon energy to the thermal energy. Using classical materials and classical design approaches it is extremely difficult to realize a compact THz circuit: it is indeed difficult to design lumped elements whose characteristic dimensions become comparable to, or higher than, the wavelength. For the same reason, the electrical interconnects no longer behave as transmission lines carrying only the fundamental mode, but rather as multimode channels, which makes frequency tuning difficult, limits the range of operating frequencies, and introduces high sensitivity to errors of their setting. Furthermore, low-loss, hollow, single-mode waveguides and resonators would require a costly surface purity. As a consequence, there is a lack of efficient sources of submillimeter-wave power sources, detectors, and low-loss interconnects in such a range, compared to the well-assessed RF/microwave or IR/FIR technology. The recent advances in nanotechnology suggested an alternative route to the THz technology, based on fascinating mechanical, electronic, and optical properties of the different nano-materials proposed in the last decade. Single-mode components (photonic crystals, transmission lines, microcavities, antennas) and lumped elements (capacitors, inductors, resistors, interconnects) have been fabricated by means of nano-sized elements, exploiting their novel electromagnetic properties. In general, they are related to their discrete spectrum of energy states, which is a consequence of the spatial confinement of the charge carrier's motion to sizes comparable with the de Broglie wavelength. These nanostructures are characterized by novel dispersion providing their unusual electromagnetic response. Carbon-based materials, like carbon nanotubes (CNTs) or graphene nanoribbons (GNRs), are definitely the most promising nanostructured materials for realizing the building blocks of high-frequency devices [5, 6, 7], given their outstanding electrical, thermal, and mechanical properties [8, 9, 10], which make them of great interest for THz applications [11, 12, 13, 14]. Indeed, carbon-based materials are proposed for several THz devices, like ballistic transistors [15], or waveguides and nanoantennas [16, 17]. Carbon materials are suitable for realizing THz sources and emitters [18, 19]. To this end, two main mechanisms have been proposed to excite resonances in the THz range. The most popular one is based on plasmon resonances.