Nonlinear nanophotonics leverages engineered nanostructures to funnel light into small volumes and intensify nonlinear optical processes with spectral and spatial control. Owing to its intrinsically large and electrically tunable nonlinear optical response, graphene is an especially promising nanomaterial for nonlinear optoelectronic applications. Here we report on exceptionally strong optical nonlinearities in graphene–insulator–metal heterostructures, which demonstrate an enhancement by three orders of magnitude in the third-harmonic signal compared with that of bare graphene. Furthermore, by increasing the graphene Fermi energy through an external gate voltage, we find that graphene plasmons mediate the optical nonlinearity and modify the third-harmonic signal. Our findings show that graphene–insulator–metal is a promising heterostructure for optically controlled and electrically tunable nano-optoelectronic components.

The so-called buffer layer (BL) is a carbon rich reconstructed layer formed during SiC (0001) sublimation.The covalent bonds between some carbon atoms in this layer and underlying silicon atoms makes itdifferent from epitaxial graphene. We report a systematical and statistical investigation of the BLsignature and its coupling with epitaxial graphene by Raman spectroscopy. Three different BLs arestudied: bare buffer layer obtained by direct growth (BL0), interfacial buffer layer between graphene andSiC (c-BL1) and the interfacial buffer layer without graphene above (u-BL1). To obtain the latter, wedevelop a mechanical exfoliation of graphene by removing an epoxy-based resin or nickel layer. The BLsare ordered-like on the whole BL growth temperature range. BL0 Raman signature may vary from sampleto sample but forms patches on the same terrace. u-BL1 share similar properties with BL0, albeit withmore variability. These BLs have a strikingly larger overall intensity than BL with graphene on top. Thesignal high frequency side onset upshifts upon graphene coverage, unexplainable by a simple straineffect. Two fine peaks (1235, 1360 cm1), present for epitaxial monolayer and absent for BL and transferred graphene. These findings point to a coupling between graphene and BL.

L’obtention de films de graphène aux caractéristiques contrôlées, comme le nombre de couches ou le dopage, anime toujours une partie de la communauté. La décomposition thermique contrôlée du carbure de silicium (SiC) permet d’envisager la croissance homogène de graphène directement sur des substrats de grande surface. Ce mode de croissance du graphène a initialement été développé à des pressions proches de la pression atmosphérique sous argon et à haute température. Dans l’étude présentée, les croissances par sublimation ont été réalisées à basse pression d’argon (10 mbar). Dans tous les cas, lors de la sublimation du SiC du côté dit « face silicium », il se forme une première couche, dite tampon, qui deviendra, par la suite, la couche de graphène sous laquelle se formera une nouvelle couche tampon. Nous discuterons de la présence et des caractéristiques de cette couche tampon. Nous mettrons notamment en évidence l’inhomogénéité de sa signature Raman, et la nécessité d’une procédure du traitement des données robuste. Puis, nous discuterons, à partir d’une étude complémentaire Raman et AFM, les premiers instants de croissance de la monocouche de graphène, par un mécanisme en bord de marches du SiC. Enfin, des mesures électriques à basse température (1.7 K) et sous champ magnétique (jusqu’à 13 T) complèteront les études menées en confirmant la continuité et la qualité de la monocouche de graphène.

Since 2008, epitaxial graphene growth has been developed in terms of homogeneity and scale by using a 1 ATM argon pressure at high temperature (>1650°C). Until now, it still remains challenging to obtain films with different and controlled characteristics such as the number of graphene layers or the doping by tuning the growth parameters. Here, we optimized the epitaxial growth of monolayer graphene (1LG) on 4H-SiC (0001) under a low argon pressure of 10 mbar. This intermediate pressure allows growing a continued 1LG in a short process time ~1h30. First, we discuss the initial growth stages from buffer layer (BL) to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and AFM analyses show that a BL, fully covering the Si-face of SiC, forms as the first step of growth. Subsequently, 1LG starts to grow at step edges and continue to cover the BL on terraces with a step-flow growth mechanism, as demonstrated in Fig.1. Then, we investigate the structural and electronic properties of the 1LG films. The integrated intensity of G-band in Raman spectra normalized with respect to a HOPG reference, AG/AG-HOPG, of each spectrum in Raman map of our continued graphene film is very close to the experimental value reported for a 1LG, demonstrating the good homogeneity. Regarding transport measurement, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the 1LG film.

T he main obstacle to the use of graphene on the industrial scale is the growth of a large and homogenous monolayer graphene Concerning this issue, it is worthnoting that our group has recently developed a reproducible and controlled growth process of a monolayer graphene on SiC 0001 by sublimation at low Ar pressure i e 10 mbar Still, the control of the electronic properties of the obtained graphene by this process is very challenging E g the mobility on our graphene on 4 H SiC (0001) is around 2000 cm 2 v 1 s 1 at RT which is in the range of the measured mobilities on similar substrates Yet, it is still very low when compared with the mobilities reported for suspended graphene. It is well acceptedthat the electronic properties of graphene on SiC are highly sensitive to the substrate underneath It was reported that the mobility of graphene on SiC 0001 increases with increasing SiC steps width, and its resistance increases with increasing SiC steps height. This means that the electronic properties of graphene on SiC 0001 can be tuned by controlling the height and width of the terraces that results from the surface reconstruction of SiC before the growth i e Step bunching phenomenon

We report on observation of an unconventional structure of the quantum Hall effect (QHE) in a p-type HgTe/CdxHg1−xTe double quantum well (DQW) consisting of two HgTe layers of critical width. The observed QHE is a reentrant function of magnetic field between two i = 2 states (plateaus at ρxy = h/ie 2) separated by an intermediate i = 1 state, which looks like some anomalous peak on the extra-long i = 2 plateau when weakly expressed. The anomalous peak apparently separates two different regimes: a traditional QHE at relatively weak fields for a small density of mobile holes ps and a high-field QH structure with a 2 − 1 plateau-plateau transition corresponding to much larger ps. We show that only a part of holes, residing in an additional light hole subband in the DQW, participate in QHE at weak fields while the rest of holes is excluded into the reservoir formed in the lateral maximum of the valence subband. All the holes come into play at high fields due to a peculiar behavior of the zero-mode levels.

This paper describes the design and performance of the search coil magnetometers (SCM), which are part of the Plasma Wave Investigation (PWI) instrument onboard the BepiColombo/Mio spacecraft (Mercury Magnetospheric Orbiter), which will measure the electric field, plasma waves and radio waves for the first time in Mercury’s plasma environment. The SCM consists of two low-frequency orthogonal search coil sensors (LF-SC) measuring two components of the magnetic field (0.1 Hz – 20 kHz) in the spacecraft spin plane, and a dual-band search coil sensor (DB-SC) picking up the third component along the spin axis at both low-frequencies (LF: 0.1 Hz – 20 kHz) and high-frequencies (HF: 10 kHz – 640 kHz). The DB-SC and the two LF-SC sensors form a tri-axial configuration at the tip of a 4.6-m coilable mast (MAST-SC) extending from the spacecraft body, to minimize artificial magnetic field contamination emitted by the spacecraft electronics. After the successful launch of the spacecraft on 20 October 2018, an initial function check for the SCM was conducted. The nominal function and performance of the sensors and preamplifiers were confirmed, even with the MAST-SC being retracted and stowed in the spacecraft body, resulting in the detection of large interference signals likely from spacecraft electronics. The MAST-SC is scheduled for deployment after the Mercury orbit insertion of Mio in 2025, allowing the SCM to make the first higher frequency measurements of magnetic fluctuations in the Hermean magnetosphere and exosphere, and the local solar wind. These measurements will contribute to the investigation of fundamental problems in the Hermean plasma environment, including turbulence, magnetic reconnection, wave-particle interactions and particle acceleration.