Tullio Rozzi

Scattering matrix approach to multichannel transport in many lead graphene nanoribbons

Multichannel analysis of graphene nanoribbons (GNR), often required for describing applications to practical devices, constitutes a heavy computational task, even in a simplified framework like that provided by discrete or nearest neighbour models. Scattering (S) matrix techniques, widely used for quantum transport in low dimensional systems and for the computation of electromagnetic fields, is shown here to provide a powerful formal platform for the analysis, and, in principle, the synthesis, of GNR multiport circuits. Periodic modes, solutions of GNR waveguides, are demonstrated to obey charge conservation and reciprocity constraints corresponding to unitary and symmetry properties of the S-matrix, under proper normalization conditions. We propose a systematic use of this approach to deal with problems such as scattering by lattice defects, the presence of external applied fields, crossing GNRs and T-junctions.

Broadband Scanning Microwave Microscopy investigation of graphene

In this work we describe the application of a dual-channel scanning probe microscope performing simultaneously Scanning Tunneling Microscopy (STM) and wide-band Near Field Scanning Microwave Microscopy (wide-band SMM) - developed by ourselves- to a graphene flake. In our system we introduce a conversion in Time-Domain to discriminate the desired information, achieving high quality microwave images with nanometric resolution. The graphene sample is deposited on a substrate of SiO2 with an additional deposition of gold (a contact finger). The preliminary measurements seem to show evidence of localized change of impedance near the edge of the flake.

A multichannel model for the self-consistent analysis of coherent transport in graphene nanoribbons.

In this contribution, we analyze the multichannel coherent transport in graphene nanoribbons (GNRs) by a scattering matrix approach. We consider the transport properties of GNR devices of a very general form, involving multiple bands and multiple leads. The 2D quantum transport over the whole GNR surface, described by the Schrödinger equation, is strongly nonlinear as it implies calculation of self-generated and externally applied electrostatic potentials, solutions of the 3D Poisson equation. The surface charge density is computed as a balance of carriers traveling through the channel at all of the allowed energies. Moreover, formation of bound charges corresponding to a discrete modal spectrum is observed and included in the model. We provide simulation examples by considering GNR configurations typical for transistor devices and GNR protrusions that find an interesting application as cold cathodes for X-ray generation. With reference to the latter case, a unified model is required in order to couple charge transport and charge emission. However, to a first approximation, these could be considered as independent problems, as in the example.

Full-wave analysis of photonic bandgap integrated optical components by the TLM-IE method

This paper presents the full-wave analysis of commonly encountered optical periodic structures by the novel transmission-line matrix/integral equation (TLM-IE) method. The TLM-IE is a three-dimensional full-wave hybrid technique that combines the advantages of the numerical TLM method in dense finite regions and those of the IE method in open regions where simple Green functions are available. The pre- and postprocessing tools of the TLM-IE solver provide visualization and physical insight into the dynamics of electromagnetic propagation in such devices. The aim of this effort is twofold: 1) to analyze the diffraction and reflection characteristics of photonic bandgap gratings and 2) to define optimization guidelines for the Q factor of integrated dielectric resonators.

Optical absorption of carbon nanotube diodes: Strength of the electronic transitions and sensitivity to the electric field polarization

Aim of this work is to model electrostatically doped carbon nanotubes (CNT), which have recently proved to perform as ideal PN diodes, also showing photovoltaic properties. The new model is able to predict the optical absorption of semiconducting CNT as function of size and chirality. We justify theoretically, for the first time, the experimentally observed capability of CNTs to detect and select not only a well defined set of frequencies, as resulting from their discrete band structure, but also the polarization of the incident radiation. The analysis develops from an approach proposed in a recent contribution. The periodic structure of CNTs is formally modeled as a photonic crystal, that is characterized by means of numerical simulators. Longitudinal and transverse components of the electric field are shown to excite distinct interband transitions between well defined energy levels. Equivalently, for a given energy of the incident radiation, absorption may show polarization ratios strongly exceeding unity.

Accurate calculation of the modes of the circular multiridge waveguide

Circular multiridge waveguides (CMRW) have been recently considered for their promise in application to tuningless dual mode filters. We propose an accurate method for characterizing such structures that is based on a combination of the Boundary Integral Method, in a form leading to a standard eigenvalue problem, with a proper choice of the current distributions on the ridges, which takes into account the edge effects. The resulting code permits to calculate very accurately several tens of modes by inverting only once a reasonably small matrix.

Design of a coplanar graphene-based nano-patch antenna for microwave application

We analyze a coplanar graphene-based nano-patch antenna. Besides the fact that graphene is a moderate conductor at those frequencies, the antenna can still exhibit sharp resonances. Although the poor graphene conductivity implies in general low-performance, a remarkable change of the scattering parameters, radiation pattern and antenna efficiency can be observed as the graphene surface impedance is changed by means of an external Bias.

Distance optical sensor for quantitative endoscopy

We present a novel optical sensor able to measure the distance between the tip of an endoscopic probe and the anatomical object under examination. In medical endoscopy, knowledge of the real distance from the endoscope to the anatomical wall provides the actual dimensions and areas of the anatomical objects. Currently, endoscopic examination is limited to a direct and qualitative observation of anatomical cavities. The major obstacle to quantitative imaging is the inability to calibrate the acquired images because of the magnification system. However, the possibility of monitoring the actual size of anatomical objects is a powerful tool both in research and in clinical investigation. To solve this problem in a satisfactory way we study and realize an absolute distance sensor based on fiber optic low-coherence interferometry (FOLCI). Until now the sensor has been tested on pig trachea, simulating the real humidity and temperature (37 degrees C) conditions. It showed high sensitivity, providing correct and repeatable distance measurements on biological samples even in case of very low reflected power (down to 2 to 3 nW), with an error lower than 0.1 mm.

Self-consistent simulation of multi-walled CNT nanotransistors

We present detailed results of the self-consistent analysis of carbon nanotube (CNT) field-effect transistors (FET), previously extended by us to the case of multi-walled/multi-band coherent carrier transport. The contribution to charge transport, due to different walls and sub-bands of a multi-walled CNT, is shown to be generally non-negligible. In order to prove the effectiveness of our simulation tool, we provide interesting examples about current–voltage characteristics of four-walled semi-conducting nanotubes, including details of numerical convergence and contribution of sub-bands to the calculation.

Size measurement in endoscopic images by low coherence interferometry

We present a novel optical sensor that is able to measure the real dimensions and areas of anatomical objects in endoscopic images by taking advantage of the knowledge of the real distance from the endoscope tip to the anatomical wall under examination. At present the major obstacle to quantitative endoscopy is the inability of calibrating the acquired images because of the magnification system. In order to solve this problem in a satisfactory way we have studied and realized a new system combining an imaging system, applied to a clinical endoscope, with an absolute distance sensor based on fiber optic low coherence interferometry (FOLCI). Up to now the sensor has been tested on pig trachea, simulating the real humidity and temperature (37 °C) conditions. The sensor show high sensitivity, providing correct measurements of the size of biological samples even in the case of very low reflected power (down to 2–3 nW).