A. Sindona

Plasmon Modes of Graphene Nanoribbons with Periodic Planar Arrangements

Among their amazing properties, graphene and related low-dimensional materials show quantized charge-density fluctuations-known as plasmons-when exposed to photons or electrons of suitable energies. Graphene nanoribbons offer an enhanced tunability of these resonant modes, due to their geometrically controllable band gaps. The formidable effort made over recent years in developing graphene-based technologies is however weakened by a lack of predictive modeling approaches that draw upon available abxa0initio methods. An example of such a framework is presented here, focusing on narrow-width graphene nanoribbons, organized in periodic planar arrays. Time-dependent density-functional calculations reveal unprecedented plasmon modes of different nature at visible to infrared energies. Specifically, semimetallic (zigzag) nanoribbons display an intraband plasmon following the energy-momentum dispersion of a two-dimensional electron gas. Semiconducting (armchair) nanoribbons are instead characterized by two distinct intraband and interband plasmons, whose fascinating interplay is extremely responsive to either injection of charge carriers or increase in electronic temperature. These oscillations share some common trends with recent nanoinfrared imaging of confined edge and surface plasmon modes detected in graphene nanoribbons of 100-500xa0nm width.

Spatial dispersion effects upon local excitation of extrinsic plasmons in a graphene micro-disk

Excitation of surface plasmon waves in extrinsic graphene is studied using a full-wave electromagnetic field solver as analysis engine. Particular emphasis is placed on the role played by spatial dispersion due to the finite size of the two-dimensional material at the micro-scale. A simple instructive set up is considered where the near field of a wire antenna is held at sub-micrometric distance from a disk-shaped graphene patch. The key-input of the simulation is the graphene conductivity tensor at terahertz frequencies, being modeled by the Boltzmann transport equation for the valence and conduction electrons at the Dirac points~(where a linear wave-vector dependence of the band energies is assumed). The conductivity equation is worked out in different levels of approximations, based on the relaxation time ansatz with an additional constraint for particle number conservation. Both drift and diffusion currents are shown to significantly contribute to the spatially dispersive anisotropic features of micro-scale graphene. More generally, spatial dispersion effects are predicted to influence not only plasmon propagation free of external sources, but also typical scanning probe microscopy configurations. The paper set the focus on plasmon excitation phenomena induced by near field probes, being a central issue for the design of optical devices and photonic circuits.

Innovative full wave modeling of plasmon propagation in graphene by dielectric permittivity simulations based on density functional theory

We report on an ab initio technique for modeling the electromagnetic response of graphene in the THz range. Quantum mechanical calculations are performed using linear response density functional theory, and compared with a semi-phenomenological model derived from the Kubo formula. We present a novel concept of dispersive conductivity, which goes beyond the Kubo-Drude model and results in a self-consistent constitutive relation for the analysis of plasmon propagation in complex nanosystems. The rigorous characterization of the constitutive relation may be inserted in electromagnetic full-wave solvers, providing a new paradigm for nanoelectronic computations at THz frequencies.

Plasmon Modes in Extrinsic Graphene: Ab initio Simulations vs Semi-classical Models

Excitation and propagation of surface plasmons in intrinsic and extrinsic graphene are analyzed from the fundamental point of view, using time-dependent density functional theory in linear response regime. Density functional calculations, being set up from first principles, do include anisotropic effects in the unique electronic structure of graphene that cause remarkable consequences even on the THz band. The main signature of this anisotropy is the occurrence of two distinct plasmon modes over a frequency range of 1 to 300 THz, where most photonic devices currently operate with large bandwidths and low losses. Further anisotropic features are inherent to the different electromagnetic response of graphene to positive and negative doping concentrations. The Dirac-cone approximation provides a simplified insight, assuming an isotropic graphene band structure near the Fermi level, which is found to be reliable at probing frequencies below (sim 20) THz and doping levels associated to Fermi energy shifts below/above ±0. 3 eV. In these limits, a continuous integral expression derived from the Kubo formula represents an easy-to-use tool capable of catching the main essence of the process.

Electrical conductivity of graphene: a time-dependent density functional theory study

Excitation and propagation of surfaces waves in graphene are analyzed within a frequency band of 1 to 300 THz, and a time domain of 1 to 10 ps. An ab initio approach, based on time dependent density functional theory in linear response regime is used. The key outputs of the simulation are the ab-initio conductance in time and frequency. This is shown to tend to a continuous integral relations in graphene, when the valence and conduction bands is treated within the conical approximation, in agreement with a widely used construction derived from the Kubo formula. Non-negligible differences are observed between the ab-initio and continuous methods at frequencies larger than a few tens of THz, i.e., at times shorter that 0.1ps, where the conical approximation reaches its limits of validity. The main conclusion of the study is that a novel conductivity concept is introduced, which represents a fundamental improvement with respect to some commonly used methods in electromagnetic simulations, working at THz frequencies. These tools may open the way to properly analyze graphene related materials, hethero-structures and interfaces.).

Comparison of rigorous vs approximate methods for accurate calculation of 2D-materials band structures and applications to THz nanoelectronics

We report on ab initio and semi-empirical techniques to investigate the electromagnetic response of 2D materials with honeycomb lattice. Band structure simulations, using density functional theory, are performed on pristine graphene and silicene. The predictions on the unique electronic features of these systems are compared to those obtained with some commonly used approaches, based on the tight-binding approximation and k.p perturbation theory. The analysis is extended to computing the surface conductivity of graphene. Our results confirm the good agreement between fundamental and approximated methods up to THz frequencies. At the same time, they show how the ab initio methods have the capability of predicting electronic properties and plasmon propagation in more realistic nano-devices, where the semi-empirical methods require further scrutiny. The above formulation will be inserted in electromagnetic full-wave solvers, for the investigation and design of 2D THz nanodevices.

Ab initio modelling of dielectric screening and plasmon resonances in extrinsic silicene

Propagation of surface waves in extrinsic silicene is scrutinized, using time dependent density functional theory in linear response regime. Collective charge density oscillations are predicted to occur at energies ranging from a few meV to some eV. These oscillations are analogous to the extrinsic plasmon modes of monolayer graphene, which are generated by two different types of charge carriers, i.e., Dirac electrons moving with distinct Fermi velocities. Our findings show how the striking plasmonic properties of silicene and monolayer graphene are independent on the chemistry of the group-IV element, buckling parameter, or hybridization state. These properties have the potential of being exploited for the design of next-generation nanodevices operating at THz frequencies.

Full-wave techniques for the electromagnetic-quantum transport modeling in nano-devices

We report on multiphysics full-wave techniques in the frequency (energy)-domain and time-domain, aimed at the investigation of the combined electromagnetic-coherent transport problem in nano-structured materials and devices, in particular carbon-based materials/devices. The quantum transport is modeled by i) discrete Hamiltonians at atomistic scale, ii) Schrödinger equation, and/or Dirac/Dirac-like eqs. at continuous level. In the frequency-domain, a rigorous Poisson-coherent transport equation system is provided. In the time-domain, Maxwell equations are self-consistently coupled to the Schrödinger/Dirac equations.