Nanosystèmes graphitiques (cavités optiques ajustables et détection spectrale des contraintes dans un nanorésonateur mécanique)

Le graphène et les nanotubes de carbone, assimilés à des nano-systèmes graphitiques, partagent des propriétés mécaniques, optiques, électroniques et vibrationnelles uniques. Associant faible masse, grande rigidité et comportement semi-transparent, des membranes de 10 à 100 couches de graphène ont été suspendues au dessus d'un substrat réfléchissant, formant ainsi un résonateur mécanique couplé à une cavité optique. Le projet de cette thèse repose sur les diffusions élastiques et inélastiques de la lumière pour sonder les contraintes mécaniques et les effets thermiques dans ces nano-systèmes graphitiques. Ce type de mesure repose sur la spectroscopie Raman, sensible aux phonons optiques du matériau sondé. Un premier aspect du présent projet de thèse porte sur l'utilisation de cavités optiques à base de graphène comme élément de base pour constituer un système hybride. Après avoir déposé une couche de molécules à la surface de ces membranes, nous avons montré que le signal Raman des molécules est exalté par un effet d'interférences optiques constructives. Nous avons mis en évidence la possibilité de moduler ce signal en se déplaçant le long de l'échantillon, ou en variant la position de la membrane à l'aide d'une actuation électrostatique. De plus, on peut observer des effets thermiques importants associés aux phénomènes d'interférences optiques dans ces membranes à base de graphène. Le second axe de cette thèse est la détection du mouvement et des contraintes mécaniques dans un résonateur graphitique (membranes de graphène multicouche, nanotubes, etc.). Au travers d'expériences menées sur des membranes suspendues de graphène multicouche, nous avons détecté la résonance mécanique de deux façons : en analysant la modulation de la lumière réfléchie et en mesurant les variations de la réponse Raman du résonateur. Cette détection, reposant sur l'augmentation des contraintes mécaniques à résonance, associe le mouvement mécanique du résonateur à un décalage en énergie des photons Raman et représente un schéma original de couplage optomécanique.Graphitic nano systems, such as graphene or carbon nanotubes, share unique mechanical, optical, electrical and vibrational properties. Combining low mass, high rigidity and semi-transparent behavior, membranes made of 10 to 100 graphene layers have been suspended over a reflecting substrate. This results in a nanomechanical resonator coupled to an optical cavity. This Phd project is based on elastic and inelastic scattering of light in order to probe mechanical stress and thermal effects within graphitic nano systems. This type of measurement is made by Raman spectroscopy which is sensitive to optical phonons. A first part of this Phd project is about using graphene based optical cavities as a constitutive blocks to make a hybrid system. We have shown interferential enhancement of Raman signal of molecules grafted on the membrane surface. We have also demonstrated the possibility to tune this molecular Raman signal by moving along the suspended membrane, or by changing the membrane position using electrostatic actuation. Moreover, we have observed important thermal effects associated to optical interferences within these graphene based cantilevers. A second part of this Phd project is the detection of motion and mechanical stress within a graphitic nano resonator. Through experiments on suspended multilayer graphene membranes, we have detected the mechanical resonance by two different means : by analyzing the reflected light modulation, and by measuring the variations of the Raman signal of the resonator. This spectral detection, based on the increase of the mechanical stress at resonance, couples the mechanical motion of the resonator to a shift in energy of the Raman scattered photons. This represents an original scheme for optomechanical coupling.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Epitaxial graphene prepared by chemical vapor deposition on single crystal thin iridium films on sapphire

Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single crystallographic orientation and a very low density of defects, as shown by diffraction, scanning tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of graphene produced on Ir bulk single crystals, i.e. much higher than on metal thin films used so far.Comment: To appear in Appl. Phys. Let

Optomechanical measurement of thermal transport in two-dimensional MoSe2 lattices

Nanomechanical resonators have emerged as sensors with exceptional sensitivities. These sensing capabilities open new possibilities in the studies of the thermodynamic properties in condensed matter. Here, we use mechanical sensing as a novel approach to measure the thermal properties of low-dimensional materials. We measure the temperature dependence of both the thermal conductivity and the specific heat capacity of a transition metal dichalcogenide (TMD) monolayer down to cryogenic temperature, something that has not been achieved thus far with a single nanoscale object. These measurements show how heat is transported by phonons in two-dimensional systems. Both the thermal conductivity and the specific heat capacity measurements are consistent with predictions based on first-principles

Fast electrical modulation of strong near-field interactions between erbium emitters and graphene

Combining the quantum optical properties of single-photon emitters with the strong near-field interactions available in nanophotonic and plasmonic systems is a powerful way of creating quantum manipulation and metrological functionalities. The ability to actively and dynamically modulate emitter-environment interactions is of particular interest in this regard. While thermal, mechanical and optical modulation have been demonstrated, electrical modulation has remained an outstanding challenge. Here we realize fast, all-electrical modulation of the near-field interactions between a nanolayer of erbium emitters and graphene, by in-situ tuning the Fermi energy of graphene. We demonstrate strong interactions with a >1000-fold increased decay rate for ~25% of the emitters, and electrically modulate these interactions with frequencies up to 300 kHz - orders of magnitude faster than the emitter's radiative decay (~100 Hz). This constitutes an enabling platform for integrated quantum technologies, opening routes to quantum entanglement generation by collective plasmon emission or photon emission with controlled waveform

Strain superlattices and macroscale suspension of Graphene induced by corrugated substrates

We investigate the organized formation of strain, ripples and suspended features in macroscopic CVD-prepared graphene sheets transferred onto a corrugated substrate made of an ordered arrays of silica pillars of variable geometries. Depending on the aspect ratio and sharpness of the corrugated array, graphene can conformally coat the surface, partially collapse, or lay, fakir-like, fully suspended between pillars over tens of micrometers. Upon increase of pillar density, ripples in collapsed films display a transition from random oriented pleats emerging from pillars to ripples linking nearest neighboring pillars organized in domains of given orientation. Spatially-resolved Raman spectroscopy, atomic force microscopy and electronic microscopy reveal uniaxial strain domains in the transferred graphene, which are induced and controlled by the geometry. We propose a simple theoretical model to explain the transition between suspended and collapsed graphene. For the arrays with high aspect ratio pillars, graphene membranes stays suspended over macroscopic distances with minimal interaction with pillars tip apex. It offers a platform to tailor stress in graphene layers and open perspectives for electron transport and nanomechanical applications

Quantum nanophotonics in two-dimensional materials

The field of two-dimensional (2D) materials-based nanophotonics has been growing at a rapid pace, triggered by the ability to design nanophotonic systems with in situ control, unprecedented number of degrees of freedom, and to build material heterostructures from the bottom up with atomic precision. A wide palette of polaritonic classes have been identified, comprising ultraconfined optical fields, even approaching characteristic length-scales of a single atom. These advances have been a real boost for the emerging field of quantum nanophotonics, where the quantum mechanical nature of the electrons and polaritons and their interactions become relevant. Examples include quantum nonlocal effects, ultrastrong light–matter interactions, Cherenkov radiation, access to forbidden transitions, hydrodynamic effects, single-plasmon nonlinearities, polaritonic quantization, topological effects, and so on. In addition to these intrinsic quantum nanophotonic phenomena, 2D material systems can also be used as sensitive probes for the quantum properties of the material that carries the nanophotonics modes or quantum materials in its vicinity. Here, polaritons act as a probe for otherwise invisible excitations, for example, in superconductors, or as a new tool to monitor the existence of Berry curvature in topological materials and superlattice effects in twisted 2D materials. In this Perspective, we present an overview of the emergent field of 2D-material quantum nanophotonics and provide a future perspective on the prospects of both fundamental emergent phenomena and emergent quantum technologies, such as quantum sensing, single-photon sources, and quantum emitters manipulation. We address four main implications: (i) quantum sensing, featuring polaritons to probe superconductivity and explore new electronic transport hydrodynamic behaviors, (ii) quantum technologies harnessing single-photon generation, manipulation, and detection using 2D materials, (iii) polariton engineering with quantum materials enabled by twist angle and stacking order control in van der Waals heterostructures, and (iv) extreme light−matter interactions enabled by the strong confinement of light at atomic level by 2D materials, which provide new tools to manipulate light fields at the nanoscale (e.g., quantum chemistry, nonlocal effects, high Purcell enhancement).H.L.K. acknowledges support from the Government of Spain (FIS2017-91599-EXP; Severo Ochoa CEX2019-000910-S), Fundacio ' Cellex, Fundacio ' Mir-Puig, and Generalitat de Catalunya (CERCA, AGAUR, SGR 1656). Furthermore, the research leading to these results has received funding from the European Union's Horizon 2020 under Grant Agreements 785219 (Graphene flagship Core2), 881603 (Graphene flagship Core3), and 820378 (Quantum flagship). This work was also supported by the ERC TOPONANOP under Grant Agreement No. 726001. I.T. acknowledges funding from the Spanish Ministry of Science, Innovation and Universities (MCIU) and State Research Agency (AEI) via the Juan de la Cierva Fellowship No. FJC2018-037098-I. F.H.L. K. and A.R.-P. acknowledge BIST Ignite Programme Grant from the Barcelona Institute of Science and Technology (QEE2DUP). N.M.R.P. acknowledges support from the European Commission through the project "Graphene-Driven Revolutions in ICT and Beyond" (ref. No. 881603, CORE 3), COMPETE 2020, PORTUGAL 2020, FEDER, and the Portuguese Foundation for Science and Technology (FCT) through Project POCI-01-0145-FEDER028114, and the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Financing UID/FIS/04650/2019. N.A.M. is a VILLUM Investigator supported by VILLUM FONDEN (Grant No. 16498) and Independent Research Fund Denmark (Grant No. 702600117B). The Center for Nano Optics is financially supported by the University of Southern Denmark (SDU 2020 funding). The Center for Nanostructured Graphene (CNG) is sponsored by the Danish National Research Foundation (Project No. DNRF103). J.C.W.S. acknowledges support from the National Research Foundation (NRF) Singapore under its NRF fellowship programme Award No. NRF-NRFF2016-05 and the Ministry of Education (MOE) Singapore under its MOE AcRF Tier 3 Award MOE2018-T3-1-002

Giant ultra-broadband photoconductivity in twisted graphene heterostructures

The requirements for broadband photodetection are becoming exceedingly demanding in hyperspectral imaging. Whilst intrinsic photoconductor arrays based on mercury cadmium telluride represent the most sensitive and suitable technology, their optical spectrum imposes a narrow spectral range with a sharp absorption edge that cuts their operation to < 25 um. Here, we demonstrate a giant ultra-broadband photoconductivity in twisted double bilayer graphene heterostructures spanning a spectral range of 2 - 100 um with internal quantum efficiencies ~ 40 % at speeds of 100 kHz. The giant response originates from unique properties of twist-decoupled heterostructures including pristine, crystal field induced terahertz band gaps, parallel photoactive channels, and strong photoconductivity enhancements caused by interlayer screening of electronic interactions by respective layers acting as sub-atomic spaced proximity screening gates. Our work demonstrates a rare instance of an intrinsic infrared-terahertz photoconductor that is complementary metal-oxide-semiconductor compatible and array integratable, and introduces twist-decoupled graphene heterostructures as a viable route for engineering gapped graphene photodetectors with 3D scalability

Valley-hybridized gate-tunable 1D exciton confinement in MoSe2

Controlling excitons at the nanoscale in semiconductor materials represents a formidable challenge in the fields of quantum photonics and optoelectronics. Achieving this control holds great potential for unlocking strong exciton-exciton interaction regimes, enabling exciton-based logic operations, exploring exotic quantum phases of matter, facilitating deterministic positioning and tuning of quantum emitters, and designing advanced optoelectronic devices. Monolayers of transition metal dichalcogenides (TMDs) offer inherent two-dimensional confinement and possess significant binding energies, making them particularly promising candidates for achieving electric-field-based confinement of excitons without dissociation. While previous exciton engineering strategies have predominantly focused on local strain gradients, the recent emergence of electrically confined states in TMDs has paved the way for novel approaches. Exploiting the valley degree of freedom associated with these confined states further broadens the prospects for exciton engineering. Here, we show electric control of light polarization emitted from one-dimensional (1D) quantum confined states in MoSe2. By employing non-uniform in-plane electric fields, we demonstrate the in-situ tuning of the trapping potential and reveal how gate-tunable valley-hybridization gives rise to linearly polarized emission from these localized states. Remarkably, the polarization of the localized states can be entirely engineered through either the spatial geometry of the 1D confinement potential or the application of an out-of-plane magnetic field