D. Mencarelli

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.

Graphene-based electronically tunable microstrip attenuator

This paper presents the design of a graphene-based electronically tunable microstrip attenuator operating at the frequency of 5 GHz. The use of graphene as a variable resistor is discussed, and the modeling of its electromagnetic properties at microwave frequencies is fully addressed. The design of the graphene-based attenuator is described. The structure integrates a patch of graphene, whose characteristics can range from fairly good conductor to highly lossy material, depending on the applied voltage. By applying the proper voltage through two high-impedance bias lines, the surface resistivity of graphene can be modified, thus changing the insertion loss of the microstrip attenuator.

Graphene-Based Electronically Tuneable Microstrip Attenuator

This paper presents the design of a graphene-based electronically tuneable microstrip attenuator operating at a frequency of 5 GHz. The use of graphene as a variable resistor is discussed and the modelling of its electromagnetic properties at microwave frequencies is fully addressed. The design of the graphene-based attenuator is described. The structure integrates a patch of graphene, whose characteristics can range from being a fairly good conductor to a highly lossy material, depending on the applied voltage. By applying the proper voltage through two high-impedance bias lines, the surface resistivity of graphene can be modified, thereby changing the insertion loss of the microstrip attenuator.

Graphene as a tunable resistor

We present the design of a graphene-based electronically tuneable microstrip attenuator operating at a frequency of 5 GHz. The use of graphene as a variable resistor is discussed and the modelling of its electromagnetic properties at microwave frequencies is fully addressed. The design of the graphene-based attenuator is described. The structure integrates a patch of graphene, whose characteristics can range from being a fairly good conductor to a highly lossy material, depending on the applied voltage. By applying the proper voltage through two high-impedance bias lines, the surface resistivity of graphene can be modified, thereby changing the insertion loss of the microstrip attenuator.

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.

Efficient and versatile graphene-based multilayers for EM field absorption

We thoroughly investigate the possibility to absorb most (i.e., up to more than 90%) of the incident electro-magnetic radiations in thin multilayered PMMA/graphene structures, thus proposing the technical realization of a device with an operational frequency range in the millimeter-wave domain, i.e., 30u2009GHz–300u2009GHz. Our simulations demonstrate the concrete possibility to enhance the field absorption by means of a selective removal and proper micro-pattering within the graphene nmaterial, enabling a complete and efficient control of the graphene sheet conductance. This method is applied to design and engineer a class of devices, endowed with a wideband operation capability, showing almost no fluctuations throughout the whole range of mm-wave frequencies.

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.).

Rigorous simulation of nonlinear optomechanical coupling in micro- and nano-structured resonant cavities

Abstract A numerical method aimed to predict the optomechanical dynamics in micro- and nano-structured resonant cavities is introduced here. The rigorousness of it is ensured by exploiting the harmonic version of the transformation optics (TO) technique and by considering all the energy-transduction contributions of electrostriction, radiation pressure, photoelasticity and moving boundaries. Since our full-wave approach implements a multi-modal analysis and also considers material losses, from both a mechanical and an optical point of view, a considerable step further has been made in respect to the standard optomechanical perturbative theory. The efficiency and the versatility of the strategy are tested by analysing the optomechanical behaviour of a corrugated Si-based nanobeam and comparing numerical results to experimental ones from the literature.

Ballistic Ratchet effect on patterned graphene

ABSTRACT Charge scattering from spatially asymmetric antidots patterned on graphene has been investigated, in the ballistic scale. The Scattering Matrix (SM) approach, formally equivalent to the Non Equilibrium Greens Function method, but particularly suitable for large structures, has been applied in order to evaluate the electron/hole distribution in patterned graphene upon injection of charge from metal contacts. The mathematical formulation of the SM method has been reviewed and commented. The presented work can be the basis for a systematic study of the Ratchet effect in the above kind of structure. A completely deterministic characterization of this effect is proposed, based on the scattering of asymmetric defects under external electromagnetic excitation. The Ratchet effect could be usefully exploited to realize high frequency detectors, sensors, and harvesting devices.