Investigation of Saturation Phenomena in Spatially Dispersive Graphene-Based Photoconductive Antennas Using Hot-Carriers Theory

摘要

Recently, graphene has attracted numerous attentions in current researches. Thanks to promising advantages of this meta-material, it is possible that the silicon devices will be replaced by graphene and carbon derivatives in the future. THz devices and especially graphene-based devices require both electronic and electromagnetic modeling. In this paper, a graphene-based photoconductive antenna (GPCA) is designed in 0.2-2.5 THz frequency range. Due to very low optical to THz conversion efficiency in a GPCA, usually the GPCA inputs increase to their maximum level. In these situations, some non-local phenomena such as velocity overshoot phenomenon (VOP) occur in a GPCA that its modeling becomes more complicated and therefore more comprehensive models than the drift-diffusion model are required to model the driving photocurrent in a GPCA.In this paper, a graphene-based photoconductive antenna (GPCA) is designed by considering its electronic and electromagnetic modeling. For the electronic modeling, energy balance transport model (EBTM) is utilized, because high DC and laser powers are applied to the GPCA, and moreover, it has sub-micron dimensions. On the other hand, for the electromagnetic modeling, an effective conductivity (non-local model) is developed for a graphene strip (GS)-based waveguide by using a semi-classical model to investigate propagation of TM-polarized surface plasmon polaritons (SPPs) by considering spatial dispersion (SD). Additionally, a per unit length circuit model is proposed to validate the non-local model. Then, the GPCA’s working frequency is selected based on the electromagnetic modeling. In the EBTM, by using hot-carriers theory, saturation of photocurrent and velocity overshoot phenomenon (VOP) with regards to the applied bias are investigated. Then, a time-dependent equivalent circuit is proposed to model the GPCA’s operational principles. This circuit enables us to study screening effects in first picoseconds after excitation, which cause saturation of radiated power with respect to the illuminating laser power. Finally, via a coherent detection, radiated THz spectrum is detected through introducing a transfer function for the GPCA.