Elefterios Lidorikis

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

Rayleigh Imaging of Graphene and Graphene Layers

We investigate graphene and graphene layers on different substrates by monochromatic and white-light confocal Rayleigh scattering microscopy. The image contrast depends sensitively on the dielectric properties of the sample as well as the substrate geometry and can be described quantitatively using the complex refractive index of bulk graphite. For a few layers (<6), the monochromatic contrast increases linearly with thickness. The data can be adequately understood by considering the samples behaving as a superposition of single sheets that act as independent two-dimensional electron gases. Thus, Rayleigh imaging is a general, simple, and quick tool to identify graphene layers, which is readily combined with Raman scattering, that provides structural identification.

A three-dimensional optical photonic crystal with designed point defects

Photonic crystals offer unprecedented opportunities for miniaturization and integration of optical devices. They also exhibit a variety of new physical phenomena, including suppression or enhancement of spontaneous emission, low-threshold lasing, and quantum information processing. Various techniques for the fabrication of three-dimensional (3D) photonic crystals—such as silicon micromachining, wafer fusion bonding, holographic lithography, self-assembly, angled-etching, micromanipulation, glancing-angle deposition and auto-cloning—have been proposed and demonstrated with different levels of success. However, a critical step towards the fabrication of functional 3D devices, that is, the incorporation of microcavities or waveguides in a controllable way, has not been achieved at optical wavelengths. Here we present the fabrication of 3D photonic crystals that are particularly suited for optical device integration using a lithographic layer-by-layer approach. Point-defect microcavities are introduced during the fabrication process and optical measurements show they have resonant signatures around telecommunications wavelengths (1.3–1.5 µm). Measurements of reflectance and transmittance at near-infrared are in good agreement with numerical simulations.

Surface-Enhanced Raman Spectroscopy of Graphene

Surface-enhanced Raman scattering (SERS) exploits surface plasmons induced by the incident field in metallic nanostructures to significantly increase the Raman intensity. Graphene provides the ideal prototype two-dimensional (2d) test material to investigate SERS. Its Raman spectrum is well-known, graphene samples are entirely reproducible, height controllable down to the atomic scale, and can be made virtually defect-free. We report SERS from graphene, by depositing arrays of Au particles of well-defined dimensions on a graphene/SiO(2) (300 nm)/Si system. We detect significant enhancements at 633 nm. To elucidate the physics of SERS, we develop a quantitative analytical and numerical theory. The 2d nature of graphene allows for a closed-form description of the Raman enhancement, in agreement with experiments. We show that this scales with the nanoparticle cross section, the fourth power of the Mie enhancement, and is inversely proportional to the tenth power of the separation between graphene and the center of the nanoparticle. One important consequence is that metallic nanodisks are an ideal embodiment for SERS in 2d.

Tight-Binding Parametrization for Photonic Band Gap Materials

ever, there exist two important differences. First, Mie resonances’ states are not localized; in fact, they decay too slowly, as 1yr as r ! ‘, and this may lead to divergences in some matrix elements. However, in a lattice environment they may be taken as localized, with a localization length comparable to the interparticle dimension. Second, in the classical wave case, as opposed to the electronic case, the host medium supports propagating solutions for every frequency. For large wavelengths, this is the dominant propagation mode since no resonances have been excited yet, while for wavelengths comparable to the particle dimension, transmission is achieved mainly through transfer between neighboring localized resonances. Thus, we may assume that the lowest frequency band is plane wavelike, while the higher bands are TB-like. This picture is more easily justified in the case of wide gaps and narrow bands, but its validity seems to be much wider. Within the framework of the systems we studied, we verified this picture. Furthermore, we were able to show that the TB matrix elements, after an appropriate rescaling, are functions of the distance only. We will consider the scalar case of a 2D periodic array of N infinitely long dielectric cylinders in vacuum, with periodic boundary conditions and with the incident plane wave E polarized. We assume the normalized electric field for each band to be given by Ens

1.5 GHz picosecond pulse generation from a monolithic waveguide laser with a graphene-film saturable output coupler

A hybrid simulation approach is developed to study chemical reactions coupled with long-range mechanical phenomena in materials. The finite-element method for continuum mechanics is coupled with the molecular dynamics method for an atomic system that embeds a cluster of atoms described quantum-mechanically with the electronic density-functional method based on real-space multigrids. The hybrid simulation approach is implemented on parallel computers using both task and spatial decompositions. Additive hybridization and unified finite-element/molecular-dynamics schemes allow scalable parallel implementation and rapid code development, respectively. A hybrid simulation of oxidation of Si(111) surface demonstrates seamless coupling of the continuum region with the classical and the quantum atomic regions.

Multiscale simulation of nanosystems

We fabricate a saturable absorber mirror by coating a graphene- film on an output coupler mirror. This is then used to obtain Q-switched mode-locking from a diode-pumped linear cavity channel waveguide laser inscribed in Ytterbium-doped Bismuthate Glass. The laser produces 1.06 ps pulses at ~1039 nm, with a 1.5 GHz repetition rate, 48% slope efficiency and 202 mW average output power. This performance is due to the combination of the graphene saturable absorber and the high quality optical waveguides in the laser glass.

Photothermoelectric and Photoelectric Contributions to Light Detection in Metal−Graphene−Metal Photodetectors

The authors describe simulation approaches that seamlessly combine continuum mechanics with atomistic simulations and quantum mechanics. They also discuss computational and visualization issues associated with these simulations on massively parallel computers. Scientists are combining continuum mechanics and atomistic simulations through integrated multidisciplinary efforts so that a single simulation couples diverse length scales. However, the complexity of these hybrid schemes poses an unprecedented challenge, and developments in scalable parallel algorithms as well as interactive and immersive visualization are crucial for their success. This article describes such multiscale simulation approaches and associated computational issues using recent work as an example.

Photonics with Multiwall Carbon Nanotube Arrays

Graphenes high mobility and Fermi velocity, combined with its constant light absorption in the visible to far-infrared range, make it an ideal material to fabricate high-speed and ultrabroadband photodetectors. However, the precise mechanism of photodetection is still debated. Here, we report wavelength and polarization-dependent measurements of metal-graphene-metal photodetectors. This allows us to quantify and control the relative contributions of both photothermo- and photoelectric effects, both adding to the overall photoresponse. This paves the way for a more efficient photodetector design for ultrafast operating speeds.