Ugo Sassi

We investigate the evolution of the Raman spectrum of defected graphene as a function of doping. Polymer electrolyte gating allows us to move the Fermi level up to 0.7 eV, as directly monitored by in situ Hall-effect measurements. For a given number of defects, we find that the intensities of the D and D' peaks decrease with increasing doping. We assign this to an increased total scattering rate of the photoexcited electrons and holes, due to the doping-dependent strength of electron-electron scattering. We present a general relation between D peak intensity and defects valid for any doping level.

We report an on-chip integrated metal graphene–silicon plasmonic Schottky photodetector with 85 mA/W responsivity at 1.55 μm and 7% internal quantum efficiency. This is one order of magnitude higher than metal–silicon Schottky photodetectors operated in the same conditions. At a reverse bias of 3 V, we achieve avalanche multiplication, with 0.37A/W responsivity and avalanche photogain ∼2. This paves the way to graphene integrated silicon photonics.

We present flexible photodetectors (PDs) for visible wavelengths fabricated by stacking centimetre-scale chemical vapor deposited (CVD) single layer graphene (SLG) and single layer CVD MoS2, both wet transferred onto a flexible polyethylene terephthalate substrate.The operation mechanism relies on injection of photoexcited electrons from MoS2 to the SLG channel.The external responsivity is 45.5A/W and the internal 570A/W at 642nm.This is at least two orders of magnitude higher than bulk-semiconductor flexible membranes and other flexible PDs based on graphene and layered materials.The photoconductive gain is up to 4 × 10 5 .The photocurrent is in the 0.1-100µA range.The devices are semi-transparent, with just 8% absorption at 642nm and work stably upon bending to a curvature of 6cm.These capabilities and the low voltage operation (< 1V) make them attractive for wearable applications.

The combination of plasmonic nanoparticles and graphene enhances the responsivity and spectral selectivity of graphene-based photodetectors. However, the small area of the metal-graphene junction, where the induced electron-hole pairs separate, limits the photoactive region to submicron length scales. Here, we couple graphene with a plasmonic grating and exploit the resulting surface plasmon polaritons to deliver the collected photons to the junction region of a metal-graphene-metal photodetector. This gives a 400% enhancement of responsivity and a 1000% increase in photoactive length, combined with tunable spectral selectivity. The interference between surface plasmon polaritons and the incident wave introduces new functionalities, such as light flux attraction or repulsion from the contact edges, enabling the tailored design of the photodetector's spectral response. This architecture can also be used for surface plasmon biosensing with direct-electric-redout, eliminating the need of bulky optics.

Graphene is ideally suited for photonic and optoelectronic applications, with a variety of photodetectors (PDs) in the visible, near-infrared (NIR), and THz reported to date, as well as thermal detectors in the mid-infrared (MIR). Here, we present a room temperature-MIR-PD where the pyroelectric response of a LiNbO3 crystal is transduced with high gain (up to 200) into resistivity modulation for graphene, leading to a temperature coefficient of resistance up to 900%/K, two orders of magnitude higher than the state of the art, for a device area of 300x300um2. This is achieved by fabricating a floating metallic structure that concentrates the charge generated by the pyroelectric substrate on the top-gate capacitor of the graphene channel. This allows us to resolve temperature variations down to 15umK at 1 Hz, paving the way for a new generation of detectors for MIR imaging and spectroscopy

We report vertically-illuminated, resonant cavity enhanced, graphene-Si Schottky photodetectors (PDs) operating at 1550nm. These exploit internal photoemission at the graphene-Si interface. To obtain spectral selectivity and enhance responsivity, the PDs are integrated with an optical cavity, resulting in multiple reflections at resonance, and enhanced absorption in graphene. Our devices have wavelength-dependent photoresponse with external (internal) responsivity~20mA/W (0.25A/W). The spectral-selectivity may be further tuned by varying the cavity resonant wavelength. Our devices pave the way for developing high responsivity hybrid graphene-Si free-space illuminated PDs for free-space optical communications, coherence optical tomography and light-radars

In monolayer (1L) transition metal dichalcogenides (TMDs) the valence and conduction bands are spin-split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark excitons, consisting of electrons and holes with opposite spin orientation, have lower energy than A excitons. The transition from bright to dark excitons involves the scattering of electrons from the upper to the lower conduction band at the K point of the Brillouin zone, with detrimental effects for the optoelectronic response of 1L-TMDs, since this reduces their light emission efficiency. Here, we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics in 1L-WS2. This occurs on a sub-ps time scale, and it is significantly dependent on temperature, indicative of phonon-assisted relaxation. Time-dependent ab initio calculations show that intravalley spin-flip scattering occurs on significantly longer time scales only at the K point, while the occupation of states away from the minimum of the conduction band significantly reduces the scattering time. Our results shed light on the scattering processes determining the light emission efficiency in optoelectronic and photonic devices based on 1L-TMDs.

Electron pairing in the vast majority of superconductors follows the Bardeen-Cooper-Schrieffer theory of superconductivity, which describes the condensation of electrons into pairs with antiparallel spins in a singlet state with an s-wave symmetry. Unconventional superconductivity was predicted in single-layer graphene (SLG), with the electrons pairing with a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing SLG on an electron-doped (non-chiral) d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, here we show evidence for a p-wave triggered superconducting density of states in SLG. The realization of unconventional superconductivity in SLG offers an exciting new route for the development of p-wave superconductivity using two-dimensional materials with transition temperatures above 4.2 K.

This study reports a novel green chemistry approach to assemble copper-nanowires/reduced-graphene-oxide hybrid coatings onto inorganic and organic supports. Such films are robust and combine sheet resistances (<30 Ω sq-1 ) and transparencies in the visible region (transmittance > 70%) that are rivalling those of indium-tin oxide. These electrodes are suitable for flexible electronic applications as they show a sheet resistance change of <4% after 10 000 bending cycles at a bending radius of 1.0 cm, when supported on polyethylene terephthalate foils. Significantly, the wet-chemistry method involves the preparation of dispersions in environmentally friendly solvents and avoids the use of harmful reagents. Such inks are processed at room temperature on a wide variety of surfaces by spray coating. As a proof-of-concept, this study demonstrates the successful use of such coatings as electrodes in high-performance electrochromic devices. The robustness of the electrodes is demonstrated by performing several tens of thousands of cycles of device operation. These unique conducting coatings hold potential for being exploited as transparent electrodes in numerous optoelectronic applications such as solar cells, light-emitting diodes, and displays.

The equilibrium optical phonons of graphene are well characterized in terms of anharmonicity and electron-phonon interactions, however their non-equilibrium properties in the presence of hot charge carriers are still not fully explored. Here we study the Raman spectrum of graphene under ultrafast laser excitation with 3ps pulses, which trade off between impulsive stimulation and spectral resolution. We localize energy into hot carriers, generating non-equilibrium temperatures in the ~1700-3100K range, far exceeding that of the phonon bath, while simultaneously detecting the Raman response. The linewidth of both G and 2D peaks show an increase as function of the electronic temperature. We explain this as a result of the Dirac cones' broadening and electron-phonon scattering in the highly excited transient regime, important for the emerging field of graphene-based photonics and optoelectronics.

Abstract Terahertz time-domain spectroscopy (THz-TDS) can be used to map spatial variations in electrical properties such as sheet conductivity, carrier density, and carrier mobility in graphene. Here, we consider wafer-scale graphene grown on germanium by chemical vapor deposition with non-uniformities and small domains due to reconstructions of the substrate during growth. The THz conductivity spectrum matches the predictions of the phenomenological Drude–Smith model for conductors with non-isotropic scattering caused by backscattering from boundaries and line defects. We compare the charge carrier mean free path determined by THz-TDS with the average defect distance assessed by Raman spectroscopy, and the grain boundary dimensions as determined by transmission electron microscopy. The results indicate that even small angle orientation variations below 5° within graphene grains influence the scattering behavior, consistent with significant backscattering contributions from grain boundaries.

Interband optical transitions in graphene are subject to pseudospin selection rules. Impulsive excitation with linearly polarized light generates an anisotropic photocarrier occupation in momentum space that evolves at time scales shorter than 100 fs. Here, we investigate the evolution of nonequilibrium charges towards an isotropic distribution by means of fluence-dependent ultrafast spectroscopy and develop an analytical model able to quantify the isotropization process. In contrast to conventional semiconductors, the isotropization is governed by optical phonon emission, rather than electron-electron scattering, which nevertheless contributes in shaping the anisotropic photocarrier occupation within the first few femtoseconds.

Abstract Hard disk drives (HDDs) are used as secondary storage in digital electronic devices owing to low cost and large data storage capacity. Due to the exponentially increasing amount of data, there is a need to increase areal storage densities beyond ~1 Tb/in 2 . This requires the thickness of carbon overcoats (COCs) to be &lt;2 nm. However, friction, wear, corrosion, and thermal stability are critical concerns below 2 nm, limiting current technology, and restricting COC integration with heat assisted magnetic recording technology (HAMR). Here we show that graphene-based overcoats can overcome all these limitations, and achieve two-fold reduction in friction and provide better corrosion and wear resistance than state-of-the-art COCs, while withstanding HAMR conditions. Thus, we expect that graphene overcoats may enable the development of 4–10 Tb/in 2 areal density HDDs when employing suitable recording technologies, such as HAMR and HAMR+bit patterned media

Earables are now pervasive, and their established purpose, ergonomy, and noninvasive interaction uncover exciting opportunities for sensing and healthcare research. However, it is critical to understand and characterize sensory measurements’ accuracy in earables impacting healthcare decisions. We report a systematic characterization of in-ear photoplethysmography (PPG) in measuring vital signs: heart rate (HR), heart rate variability (HRV), blood oxygen saturation (SpO$_2$2), and respiration rate (RR). We explore in-ear PPG inaccuracies stemming from different sensor placements and motion-induced artifacts. We observe statistically significant differences across sensor placements and between artifact types, with ITC placement showing the lowest intersubject variability. However, our study shows the absolute error climbs up to 29.84, 24.09, 3.28, and 30.80%, respectively, for HR, HRV, SpO$_2$2, and RR during motion activities. Our preliminary results suggest that in-ear PPG is reasonably accurate in detecting vital signs but demands careful mechanical design and signal processing treatment.

Graphene is ideally suited for optoelectronics. It offers absorption at telecom wavelengths, high-frequency operation and CMOS-compatibility. We show how high speed optoelectronic mixing can be achieved with high frequency (~20 GHz bandwidth) graphene field effect transistors (GFETs). These devices mix an electrical signal injected into the GFET gate and a modulated optical signal onto a single layer graphene (SLG) channel. The photodetection mechanism and the resulting photocurrent sign depend on the SLG Fermi level (EF). At low EF (<130 meV), a positive photocurrent is generated, while at large EF (>130 meV), a negative photobolometric current appears. This allows our devices to operate up to at least 67 GHz. Our results pave the way for GFETs optoelectronic mixers for mm-wave applications, such as telecommunications and radio/light detection and ranging (RADAR/LIDARs.).

Indoor environmental quality has been found to impact employees' productivity in the long run, yet it is unclear its meeting-level impact in the short term. We studied the relationship between sensorial pleasantness of a meeting's room and the meeting's productivity. By administering a 28-item questionnaire to 363 online participants, we indeed found that three factors captured 62% of people's experience of meetings: (a) productivity; (b) psychological safety; and (c) room pleasantness. To measure room pleasantness, we developed and deployed ComFeel, an indoor environmental sensing infrastructure, which captures light, temperature, and gas resistance readings through miniaturized and unobtrusive devices we built and named 'Geckos'. Across 29 real-world meetings, using ComFeel, we collected 1373 minutes of readings. For each of these meetings, we also collected whether each participant felt the meeting to have been productive, the setting to be psychologically safe, and the meeting room to be pleasant. As one expects, we found that, on average, the probability of a meeting being productive increased by 35% for each standard deviation increase in the psychological safety participants experienced. Importantly, that probability increased by as much as 25% for each increase in room pleasantness, confirming the significant short-term impact of the indoor environment on meetings' productivity.

Abstract The thermoacoustic effect of isolated single‐wall carbon nanotubes aligned between electrodes is experimentally observed for the first time by imaging the emitted acoustic wave using an atomic force microscopy‐based technique specifically developed for the task. The capability of such a technique for single‐point thermoacoustic measurements is first verified on carbon nanotubes layers with two electrodes for injecting alternate electric current. The technique is then demonstrated to allow the acquisition, simultaneously with the topography, of images reflecting the pressure of the acoustic wave at fixed distance from the sample. Such a capability is used to collect images reflecting the amplitude of acoustic waves generated by isolated nanotubes and nanotube bundles by the thermoacoustic effect.

Resistive-switching memories are alternative to Si-based ones, which face scaling and high power consumption issues. Tetrahedral amorphous carbon (ta-C) shows reversible, non-volatile resistive switching. Here we report polarity independent ta-C resistive memory devices with graphene-based electrodes. Our devices show ON/OFF resistance ratios$\sim$4x$10^5$, ten times higher than with metal electrodes, with no increase in switching power, and low power density$\sim$14$\mu$W/$\mu$m$^2$. We attribute this to a suppressed tunneling current due to the low density of states of graphene near the Dirac point, consistent with the current-voltage characteristics derived from a quantum point contact model. Our devices also have multiple resistive states. This allows storing more than one bit per cell. This can be exploited in a range of signal processing/computing-type operations, such as implementing logic, providing synaptic and neuron-like mimics, and performing analogue signal processing in non-von-Neumann architectures

Summary form only given. Two-dimensional Transition Metal Dichalcogenides (TMDs) have been widely studied because of the peculiar electronic band structure and the strong excitonic effects [1]. In these materials the large spin-orbit coupling lifts the spin degeneracy of the valence (VB) and the conduction band (CB) giving rise to the A and B interband excitonic transitions. In monolayer WS2, the spins of electrons in the lowest CB and in the highest VB at K/K' point of the Brillouin zone are antiparallel resulting in an intravalley dark exciton state at a lower energy than the bright exciton, see left panel of Fig.1. On the one hand, the presence of dark excitons has been revealed indirectly from the observation of anomalous quenching of the PL emission at low temperature in single-layer WS2 [2]; on the other hand, however, the intravalley spin-flip process is assumed to occur on a significantly long time scale, which is usually neglected in theoretical models describing exciton intra or inter valley scattering processes [3]. Here we use two-colour helicity-resolved pump-probe spectroscopy to directly resolve the intravalley spin-flip process of the photoexcited electrons in the CB of single -layer WS2 [4]. In our experiment, spin-polarized carriers are photo-injected by a circularly polarized pump beam resonant to the A exciton transition, while the co-circularly polarized probe pulse is tuned around B excitonic peak (see left panel in Fig.!). In this configuration, the scattering of electrons from the upper to the lower CB level is detected by measuring the build-up dynamics of the bleaching signal around the B exciton caused by a Pauli blocking effect (center panel in Fig.!). We find that at T=77 K this process occurs on a sub-ps time scale and it is significantly dependent on the temperature, strongly pointing to a phonon assisted relaxation process (right panel of Fig.!). Additional non-equilibrium quantum chemistry calculations confirm our findings showing that the upper CB states can be quickly depleted by efficient scattering processes mediated by phonons on a temporal scale close to our experimental results. Our results shed light on the intravalley spin relaxation process in single-layer WS2, determining the formation of the intravalley dark exciton, which we measure to occur on a sub-ps timescale. The study of dark excitons formation dynamics is important for designing TMD-based electronic/photonic devices.

In this paper an advance overview of our activity in the field of near-infrared silicon photodetectors, is presented. Proposed photodetectors are based on the internal photoemission effect through a Schottky junction and their fabrication results completely compatible with the silicon technology. Taking advantages by both new structures and new two-dimensional emerging materials a progressive increase in device performance has been demonstrated along the last years. Our insights show that silicon devices based on the internal photoemission effect are already suitable for power monitoring application and they could play a key role in the telecommunications opening new frontiers in the field of low-cost silicon photonic.

We present GFETs based on low contact resistance of 125 ohm·μm. High current gain was obtained with extrinsic ft of 35 GHz and an intrinsic ft of 100 GHz for device with 180 channel length and 12 μm channel width. The high RF performance together with our bottom-gate structure (graphene is on top and exposed to ambient), provide an opportunity for applications such as optoelectronic components and high frequency sensors.

Abstract Terahertz time-domain spectroscopy (THz-TDS) can be used to map spatial variations in electrical properties such as sheet conductivity, carrier density, and carrier mobility in graphene. Here, we consider wafer-scale graphene grown on germanium by chemical vapor deposition with non-uniformities and small domains due to reconstructions of the substrate during growth. The THz conductivity spectrum matches the predictions of the phenomenological Drude-Smith model for conductors with non-isotropic scattering caused by backscattering from boundaries and line defects. We compare the charge carrier mean free path determined by THz-TDS with the average defect distance assessed by Raman spectroscopy, and the grain boundary dimensions as determined by transmission electron microscopy. The results indicate that even small angle orientation variations below 5° within graphene grains influence the scattering behavior consistent with significant backscattering contributions from grain boundaries.

Abstract It is challenging for conventional top-down lithography to fabricate reproducible devices very close to atomic dimensions, whereas identical molecules and very similar nanoparticles can be made bottom-up in large quantities, and can be self-assembled on surfaces. The challenge is to fabricate electrical contacts to many such small objects at the same time, so that nanocrystals and molecules can be incorporated into conventional integrated circuits. Here, we report a scalable method for contacting a self-assembled monolayer of nanoparticles with a single layer of graphene. This produces single-electron effects, in the form of a Coulomb staircase, with a yield of 87 ± 13% in device areas ranging from &lt; 800 nm 2 to 16 μ m 2 , containing up to 650,000 nanoparticles. Our technique offers scalable assembly of ultra-high densities of functional particles or molecules that could be used in electronic integrated circuits, as memories, switches, sensors or thermoelectric generators.

Structural and topological features of graphene have been investigated widely in the (scanning) transmission electron microscope and include: grain structure [1], layer number and mis‐stacking [2], dislocations [3] and out of plane buckling [4, 5]. Here we explore new insights offered by scanning electron diffraction (SED), including quantitative analysis of crystal orientation and local strain. SED involves scanning the electron beam across a specimen and recording a diffraction pattern at each point. This provides a four‐dimensional (4d) dataset combining real and reciprocal space information with nanoscale spatial resolution [6]. SED can be performed over areas of a few square micrometres, and the rich 4d data can be analysed using a number of versatile schemes, as described below. This automated analysis enables numerous regions to be considered, an example of which is shown (Fig.1) from a graphene sample grown by chemical vapour deposition on copper [7]. ‘Diffraction images’ can be formed by plotting the intensity of a particular sub‐set of pixels in each diffraction pattern as a function of probe position to elucidate any variations in the diffraction condition. In Fig.1a, integration windows are selected around a particular set of first‐order ((1,0)‐type) and second‐order ((1,1)‐type) reflections to yield the ‘virtual’ dark field images in Fig.1b. These images reveal the local grain structure, in this case a grain in the lower right area of the map. They also show light/dark fringes associated with a small island (arrowed) as well as a fold (starred). This contrast can be attributed to deviation from perfect stacking between the island and the underlying graphene grain, or between layers in the fold. The contrast is understood in terms of variations in the interference condition for electrons scattered from atoms in each of the layers as their relative position varies spatially [5]. The most notable benefits of SED lie in further computational analysis. Orientation images can be produced by matching each diffraction pattern to a library of simulated patterns to automatically map the grain structure and determine the local orientation. All grains are then revealed, and the disorientation across grain boundaries can be determined (Fig. 1c). Strain and small orientation variations are also of considerable importance, and can be mapped with SED by comparing each pattern to an unstrained reference. Fig.1d shows up to 3% strain around the fold, as well as the rotation associated with the ~2º small angle grain boundary. Our approach can thus provide a comprehensive 'crystal cartography' of layered materials, paving the way for thorough understanding and exploitation of their unique structure and topology.

In this work an advanced overview in the field of near-infrared silicon photodetectors, is presented. Proposed photodetectors are based on the internal photoemission effect through a Schottky junction and their fabrication results completely compatible with the silicon technology. Taking advantage of both new structures and new two-dimensional emerging materials, a progressive increase in device performance has been demonstrated along the last years. Our insights show that silicon devices based on the internal photoemission effect are already suitable for power monitoring applications and they could play a key role in the telecommunications opening new frontiers in the field of low-cost silicon photonics.

Nature Communications 8: Article number: 14024 (2017); Published: 19 January 2017; Updated: 1 March 2017 The present address for U. Sassi is incorrect in this Article. This author does not have a present address. The correct full affiliation details for this author are given below: Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, UK.

In monolayer Transition Metal Dichalcogenides (TMDs) the valence and conduction bands are spin split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark exciton, consisting of an electron and a hole with opposite spin orientation, has lower energy than the A exciton. A possible mechanism leading to the transition from bright to dark excitons involves the scattering of the electrons from the upper to the lower conduction band state in K. Here we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics of electrons in the conduction band of single-layer WS$_2$. This process occurs on a sub-ps time scale and it is significantly dependent on the temperature, indicative of a phonon-assisted relaxation. These experimental results are supported by time-dependent ab-initio calculations which show that the intra-valley spin-flip scattering occurs on significantly longer time scales only exactly at the K point. In a realistic situation the occupation of states away from the minimum of the conduction band leads to a dramatic reduction of the scattering time.

Hard disk drives (HDDs) are used as secondary storage in a number of digital electronic devices owing to low cost ($ $10Tb/in$^2$.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text I. Goykhman, A. Eiden, D. De Fazio, U. Sassi, M. Barbone, and A. C. Ferrari, "High Responsivity Silicon-Graphene Schottky Avalanche Photodetectors for Visible and Telecom Wavelengths," in CLEO: 2015, OSA Technical Digest (online) (Optica Publishing Group, 2015), paper STh1I.4. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Measurements data collected in several institutions including the Racah Institute of Physics (STM data), at the Cambridge Graphene Centre (Raman spectroscopy data) and Department of Materials Science and Metallurgy (XRD and electronic transport data).

We investigate the evolution of the Raman spectrum of defected graphene as a function of doping. Polymer electrolyte gating allows us to move the Fermi level up to 0.7eV, as monitored by \textit{in-situ} Hall-effect measurements. For a given number of defects, we find that the intensities of the D and D' peaks decrease with increasing doping. We assign this to an increased total scattering rate of the photoexcited electrons and holes, due to the doping-dependent strength of electron-electron scattering. We present a general relation between D peak intensity and defects valid for any doping level

We report a hybrid graphene-microfiber odulator with low insertion loss ~0.4dB, and active, ~6.7%, control of absorption. This can be used for active control of ultrafast laser working in continuous wave, Q-switching and mode-locking operations.

Electron pairing in the vast majority of superconductors follows the Bardeen–Cooper–Schrieffer theory of superconductivity, which describes the condensation of electrons into pairs with antiparallel spins in a singlet state with an s-wave symmetry. Unconventional superconductivity was predicted in single-layer graphene (SLG), with the electrons pairing with a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing SLG on an electron-doped (non-chiral) d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, here we show evidence for a p-wave triggered superconducting density of states in SLG. The realization of unconventional superconductivity in SLG offers an exciting new route for the development of p-wave superconductivity using two-dimensional materials with transition temperatures above 4.2 K.

Active modulation of light with large optical bandwidths (∼100 nm [1]) is required in photonic devices, such as modulators for fiber communications [1] and saturable absorbers for ultrafast pulse generation [2,3]. Present modulators use expensive materials, (e.g. LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> [4], III-V semiconductors [5], and Ge on silicon-on-insulator (SOI) [6]), and are limited by narrowband operation (∼20 nm for quantum-confined Stark effect devices [7], or <0.1 nm for resonators [8]), or high loss (∼80% in Si Mach-Zehnder modulators [9]). Single layer graphene (SLG) can be transferred and integrated on a variety of waveguide systems (e.g. a-Si, Si <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> ) while offering a full-band operation [10]. In all-fiber platforms, including ultrafast lasers [2,3], high coupling losses (∼60% from fiber to SOI waveguide [6]) are driving development of fiber-based modulators. Ref. 3 reported an electrically controllable all-fiber optical modulator with a SLG transistor on a side-polished fiber. However, its gating speed was limited to∼300 Hz [11] due to the low mobility of the ionic liquid used for gating. Here, we report a graphene-microfiber modulator by integrating a microfiber onto a graphene (SLG plated) capacitor (GC). By varying the microfiber size and length we control the light interaction with the biased SLG plates. This can be used for active control of ultrafast laser working in continuous wave, Q-switching and mode-locking operations.

In this work we study the ultrafast spin/valley decay processes in a mechanically exfoliated WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> monolayer by Time-Resolved Faraday Rotation (TRFR) and Time-Resolved Circular Dichroism (TRCD). In both the experiments, due to the valley-dependent selection rules, a circular pump laser pulse injects spin-polarized electron-hole pairs only in the K or the K' valley. The intervalley scattering process are measured by detecting the angle of rotation of the probe polarization (TRFR) and the transient absorption of coand counter-circularly polarized beams.

Electron-phonon scattering and anharmonicity are the dominant mechanisms, that enable to describe the equilibrium phonon properties in graphene [1] and Raman scattering is the main tool for their characterization [2]. In the first tens fs after the photoexcitation, an out of equilibrium distribution of (hot) electron is induced with respect to the (cold) phonon bath. Within a few picoseconds, the fast electron-electron and electron-phonon non radiative recombination channels determine the equilibrium between the electronic distribution and the lattice. Therefore, on the laboratory timescale, continuous wave laser sources, commonly used for high resolution spontaneous Raman scattering, examine already equilibrated carrier-phonon distributions.

We use helicity-resolved transient absorption spectroscopy to track intravalley scattering dynamics in monolayer WS 2 . We find that spin-polarized carriers scatter from upper to lower conduction band by reversing their spin orientation on a sub-ps timescale.

The out-of-equilibrium Raman response of graphene is addressed by pulsed laser excitation. Phonon spectrum is rationalized by revisiting the electron-phonon picture in the light of a transient broadening of the Dirac cone.

Graphene is ideally suited for optoelectronic applications. It offers absorption at telecom wavelengths, high-frequency operation and CMOS-compatibility. We report optoelectronic mixing up to to 67GHz using a back-gated graphene field effect transistor (GFET). We also present a model to describe the resulting mixed current. These results pave the way for GETs optoelectronic mixers for mm-wave applications, such as telecommunications and RADAR/LIDAR systems.

Graphene is ideally suited for optoelectronic applications. It offers absorption at telecom wavelengths, high-frequency operation and CMOS-compatibility. We report optoelectronic mixing up to to 67GHz using a back-gated graphene field effect transistor (GFET). We also present a model to describe the resulting mixed current. These results pave the way for GETs optoelectronic mixers for mm-wave applications, such as telecommunications and RADAR/LIDAR systems.

Graphene is ideally suited for optoelectronics. It offers absorption at telecom wavelengths, high-frequency operation and CMOS-compatibility. We show how high speed optoelectronic mixing can be achieved with high frequency (~20 GHz bandwidth) graphene field effect transistors (GFETs).

It is challenging for conventional top-down lithography to fabricate reproducible devices very close to atomic dimensions, whereas identical molecules and very similar nanoparticles can be made bottom-up in large quantities, and can be self-assembled on surfaces. The challenge is to fabricate electrical contacts to many such small objects at the same time, so that nanocrystals and molecules can be incorporated into conventional integrated circuits. Here, we report a scalable method for contacting a self-assembled monolayer of nanoparticles with a single layer of graphene. This produces single-electron effects, in the form of a Coulomb staircase, with a yield of 87 ± 13% in device areas ranging from < 800 nm2 to 16 μm2, containing up to 650,000 nanoparticles. Our technique offers scalable assembly of ultra-high densities of functional particles or molecules that could be used in electronic integrated circuits, as memories, switches, sensors or thermoelectric generators. The integration of nano-molecules into microelectronic circuitry is challenging. Here, the authors provide a scalable method for contacting a self-assembled monolayer of nanoparticles with a single layer of graphene that produces single-electron effects, in the form of a Coulomb staircase, with a yield of at least 70%.