10.1117/2.1201402.005341 Nanoscale broadband terahertz communication Ian F. Akyildiz and Josep Miquel Jornet The peculiar propagation properties of electrons in graphene enable the creation of plasmonic transmitters, antennas, and receivers. Nanotechnology is providing a new set of tools to the engi- neering community to create nanoscale components that are able to perform simple specific tasks, such as computing, data storing, sensing, and actuation. Integrating several of these nanocomponents into a single device just a few cubic micro- meters in size will enable the development of more advanced nanodevices. By exchanging information, such nano-devices will be able to achieve complex tasks in a distributed manner. 1 The resulting nanonetworks will enable unique applications in the biomedical, industrial, and military fields, such as advanced health monitoring systems, nanosensor networks for biological and chemical attack prevention, and wireless network-on-chip systems for very large multicore computing architectures. For the time being, the communication options for nanode- vices are very limited. Miniaturizing a conventional metallic an- tenna to meet the size requirements of the nanodevices would require very high operating frequencies (hundreds of terahertz). The available transmission bandwidth increases with the an- tenna resonant frequency, but so does the propagation loss. With the likely very limited power of nanodevices, 2 this approach would compromise the feasibility of nanonetworks. In addition, it is not clear how a miniature transceiver could be engineered to operate at such very high frequencies. Moreover, intrinsic prop- erties of metals vary at the nanoscale, and common assumptions of antenna theory might no longer be valid. An alternative is to use novel nanomaterials such as graphene (i.e., a layer, one atom thick, of carbon atoms in a honey- comb crystal lattice) 3 to develop nanoantennas. In particular, the peculiar dynamic complex conductivity of graphene 4 en- ables the propagation of tightly confined electromagnetic modes at the interface between graphene and a dielectric material, which are commonly referred to as surface plasmon polari- ton (SPP) waves. Many metals and metamaterials support the propagation of SPP waves, but usually at very high frequencies Figure 1. Conceptual design of a graphene-based plasmonic nano- antenna. EM wave: Electromagnetic wave. SPP Wave: Surface plas- mon polariton wave. (e.g., IR and optical frequency bands). In contrast, SPP waves on graphene have been observed at frequencies as low as in the terahertz band and, moreover, can easily be tuned by material doping. We have recently proposed a graphene-based plasmonic nanoantenna for terahertz band communication among nano- devices 5, 6 (see Figure 1). The nanoantenna is composed of a graphene nanoribbon (GNR, the active element), mounted over a metallic flat surface (the ground plane), with a dielectric mate- rial layer in between (to support the GNR as well as to change its chemical potential by material doping). To analyze the per- formance of the nanoantenna, we have developed an analytical framework by starting from the dynamic complex conductivity of GNRs. Contrary to existing studies, which use the conductiv- ity of infinitely large graphene sheets, we take into account the impact of the lateral confinement of electrons in GNRs. 7 We first derived conditions for both transverse magnetic (TM) and transverse electric SPP wave modes to propagate on the GNR and obtained their complex propagation constant. The real part of the SPP propagation constant determines the propaga- tion length or decay of the SPP wave, whereas the imaginary part determines the SPP wave propagation speed. This can be up to two orders of magnitude below the speed of light in a vacuum and varies with the GNR width and chemical potential. Continued on next page