Dirac fermion reflector by ballistic graphene sawtooth-shaped npn junctions

We have realized a Dirac fermion reflector in graphene by controlling the ballistic carrier trajectory in a sawtooth-shaped npn junction. When the carrier density in the inner p-region is much larger than that in the outer n-regions, the first straight np interface works as a collimator and the collimated ballistic carriers can be totally reflected at the second zigzag pn interface. We observed clear resistance enhancement around the np+n regime, which is in good agreement with the numerical simulation. The tunable reflectance of ballistic carriers could be an elementary and important step for realizing ultrahigh-mobility graphene field effect transistors utilizing Dirac fermion optics in the near future

Klein-tunneling transistor with ballistic graphene

Today the availability of high mobility graphene up to room temperature makes ballistic transport in nanodevices achievable. In particular, p-n-p transistor in the ballistic regime gives access to the Klein tunneling physics and allows the realization of devices exploiting the optics-like behavior of Dirac Fermions (DF) as in the Vesalego lens or the Fabry P\'erot cavity. Here we propose a Klein tunneling transistor based on geometrical optics of DF. We consider the case of a prismatic active region delimited by a triangular gate, where total internal reflection may occur, which leads to the tunable suppression of the transistor transmission. We calculate the transmission and the current by means of scattering theory and the finite bias properties using Non Equilibrium Green's Function(NEGF) simulation.Comment: 4 pages, 5 figure

Low-loss, compact, spot-size-converter based vertical couplers for photonic integrated circuits

Funding: (i) European Union Horizon H2020 Programme (H2020-ICT27-2015, COSMICC No. 688516). (ii) European Union Research Council (ERC) starting grant 337508.In recent years, the monolithic integration of new materials such as SiN, Ge and LiNbO3 on silicon (Si) has become important to the Si photonics community due to the possibility of combining the advantages of both material systems. However, efficient coupling between the two different layers is challenging. In this work, we present a spot size converter based on a two-tier taper structure to couple the optical mode adiabatically between Si and SiN. The fabricated devices show a coupling loss as low as 0.058 dB  ±  0.01 dB per transition at 1525 nm. The low coupling loss between the Si to SiN, and vice versa, reveals that this interlayer transition occurs adiabatically for short taper lengths (<200 µm). The high refractive index contrast between the Si and SiN is overcome by matching the optical impedance. The proposed two-tier taper structure provides a new platform for optoelectronic integration and a route towards 3D photonic integrated circuits.PostprintPeer reviewe

A versatile silicon-silicon nitride photonics platform for enhanced functionalities and applications

Silicon photonics is one of the most prominent technology platforms for integrated photonics and can support a wide variety of applications. As we move towards a mature industrial core technology, we present the integration of silicon nitride (SiN) material to extend the capabilities of our silicon photonics platform. Depending on the application being targeted, we have developed several integration strategies for the incorporation of SiN. We present these processes, as well as key components for dedicated applications. In particular, we present the use of SiN for athermal multiplexing in optical transceivers for datacom applications, the nonlinear generation of frequency combs in SiN micro-resonators for ultra-high data rate transmission, spectroscopy or metrology applications and the use of SiN to realize optical phased arrays in the 800&ndash;1000 nm wavelength range for Light Detection And Ranging (LIDAR) applications. These functionalities are demonstrated using a 200 mm complementary metal-oxide-semiconductor (CMOS)-compatible pilot line, showing the versatility and scalability of the Si-SiN platform

Ingénierie du profil de dopage dans le graphène : de l’optique des fermions de Dirac à l'électronique haute fréquence.

This thesis discusses the nanostructuration of local gates for electronic transport in graphene. The nanostructured gates enable a full control of the graphene doping profile at the Fermi wavelength scale which is the primary condition for Dirac Fermion optics experiments. This control of the doping profile proves to be necessary for the realization of high frequency transistors as well.In this work, I first present a new technology based on local bottom gates and high mobility graphene on thin boron nitride. This allows the realization of sharp and tunable p-n junctions which are the building blocks for Dirac Fermion optics. I will discuss a direct application of this technology, the Klein tunneling transistor, which takes advantage of the refractive properties of Dirac Fermions to open a transmission gap in graphene. Then this technology is implemented in a device with a gate located underneath the contact area in order to tune in situ the work function difference between the metal and the contacted graphene. The contact doping is monitored by measuring the resistance of the contact junction. In particular, the contact resistance is tuned and the polarity reversal of the contacted graphene is demonstrated.The last two chapters are devoted to the study of our devices when they are driven at high bias, which is the relevant regime for a high frequency transistor. In this regime, a current saturation is observed due to the electron-phonon inelastic scattering. From the current saturation measurement we extract the relevant phonon energy scale, pointing out a mechanism dominated by the surface phonons of the boron nitride substrate. In addition, we observe and model the non-uniform doping profile that arises in local gated devices at high bias which contributes also to the current saturation. Finally, the devices are measured in the gigahertz range to show how those current saturation mechanisms can improve the power gain of a graphene microwave transistor.Cette thèse traite du contrôle du profil de dopage dans le graphène au moyen de grilles locales nano-structurées, pour l’électronique des fermions de Dirac. Cette nano-structuration à l’échelle de la longueur d’onde de Fermi s’avère essentielle pour réaliser des expériences d’optique de fermion de Dirac ainsi que, dans un registre plus appliqué, pour l’électronique haute-fréquence. Dans ce travail, je commence par présenter notre technologie, qui repose sur des grilles arrières locales et du graphène haute-mobilité sur nitrure de bore hexagonal. Cela nous permet de réaliser des jonctions p-n abruptes, accordables et balistiques, qui sont l’élement de base pour l’électronique des fermions de Dirac. Je traiterai une application possible de cette technologie, le transistor à effet tunnel de Klein, qui utilise la réfraction des fermions de Dirac pour controler l’ouverture et la fermeture du canal d’un transistor graphène. Ensuite, cette technologie est mise en application pour équiper un transistor d’une grille placée sous le métal de contact. Cette grille de contact donne un contrôle complet du dopage du graphène contacté et permet de moduler la resistance de la jonction de contact jusque dans le gigahertz.Les deux derniers chapitres sont dévolus au régime de fort biais qui est pertinent pour les applications hautes fréquences ; dans ce régime le profile de dopage dépend aussi de la tension drain-source appliquée. Nous observons et modélisons la saturation de courant comme la conséquence de deux effets : la diffusion par les phonons de surface du substrat hBN et l’inhomogénéité de dopage dans les dispositifs à grilles locales. Enfin, nous évaluons les performances de nos dispositifs comme transistors radio-fréquences dans ce régime de saturation, notamment en terme de fréquence de coupure du gain de puissance

L’électronique pour la conversion - Projet ciblé OFCOC: Optical Frequency Combs On a Chip

Partenaires FOTONINLC2NIESInternational audienc

L’électronique pour la conversion - Projet ciblé OFCOC: Optical Frequency Combs On a Chip

Partenaires FOTONINLC2NIESInternational audienc

Silicon nitride-on-silicon bi-layer grating couplers designed by a global optimization method

International audienceSilicon nitride-on-silicon bi-layer grating couplers were designed for the O-band using an optimization-based procedure that accounted for design rules and fabricated on a 200 mm wafer. The designs were sufficiently robust to fabrication variations to function well across the wafer. A peak fiber-to-chip coupling efficiency to standard single mode fiber of -2.2 dB and a 1-dB bandwidth of 72.9 nm was achieved in the representative device. Over several chips across the wafer, we measured a median peak coupling efficiency of -2.1 dB and median 1-dB bandwidth of 70.8 nm. The measurements had good correspondence with simulation