Sylvain Massabeau

Two-dimensional transition metal dichalcogenide (TMD) interfaces offer a versatile platform for studying quantum phenomena and developing device functionalities. When distinct TMD monolayers are stacked vertically or laterally stitched, their interfaces can exhibit unique electronic band alignments, giving rise to long-lived interlayer excitons, charge transfer effects, and moiré superlattices with correlated states. Here, we demonstrate that the interface between a large variety of two different epitaxially grown TMD monolayers controls the intensity and sign of the Rashba spin splitting, which is probed using THz spintronic emission. Optimized TMD heterobilayers, such as HfSe2/PtSe2, show enhanced THz emission that surpasses the spin-to-charge conversion efficiency of bulk TMDs, confirming the presence of Rashba states with large spin splitting at the interface. By combining spin- and angle-resolved photoemission spectroscopy with density functional theory, we reveal that the electronic hybridization between the two different TMD monolayers gives rise to extended in-gap states with strong Rashba spin–orbit coupling. The choice of TMD layers enables to engineer the sign and strength of spin-to-charge conversion in van der Waals heterobilayers, enabling to build efficient and tunable THz spintronic emitters.

Propriétés optiques et électroniques des boîtes quantiques de graphène dans la gamme spectrale Térahertz: L’objectif de cette thèse est d'explorer les propriétés électroniques et optiques des boîtes quantiques de graphène (BQG) dans le domaine spectral THz. En utilisant le formalisme des liaisons fortes, nous calculons d'abord les niveaux d'énergie de BQG (diamètres de 6 à 50 nm) et analysons leurs caractéristiques. Nous calculons ensuite leur couplage à des photons de faible énergie et déterminons les spectres de probabilité d'absorption des BQG dans la gamme spectrale THz, en fonction de leur taille, de la température et du dopage. Ensuite, nous mesurons les propriétés optiques des BQG aux fréquences THz, en utilisant un système de spectroscopie THz dans le domaine temporel. Des échantillons de graphène épitaxial multicouches (GEM) sont sondés puis nanostructurés en réseaux contenant 107 BQG. Nous montrons que la réponse THz de BQG de quelques dizaines de nm de diamètre est principalement caractérisée par une large absorption autour de 6 THz, à basse et à température ambiante. Ces résultats originaux sont étayés par l'analyse théorique et sont fortement différents de ce qui est observé dans le GEM.Enfin, nous étudions les propriétés de transport électronique d’une BQG unique. La BQG est formée à partir de graphène exfolié encapsulé dans des couches de hBN, et est insérée dans un transistor à électron unique couplé à une antenne THz. On observe le régime de blocage de Coulomb et les états excités de la BQG. Enfin, nous étudions la photoréponse à des photons THz incohérents d’une BQG dans le régime de blocage de Coulomb. Ces résultats ouvrent des perspectives très intéressantes pour le développement de dispositifs THz à base de BQG, comme les lasers THz.

In this Letter, we demonstrate an actively mode-locked laser at telecom wavelength made of graphene electro-absorption modulator on a photonic integrated circuit, inserted in a dispersion-engineered cavity with III–V semiconductor optical amplifiers as gain medium. Stable low-jitter pulse trains at 10 GHz with 2.9 ps deconvoluted pulse widths are achieved, with performances comparable to a commercial lithium niobate high-speed modulator. These results highlight the potential of integrated graphene-based devices for the realization of compact and stable optical clocks operating at high repetition rate.

The diversity of 2D materials and their van der Waals (vdW) stacking presents fertile ground for engineering novel multifunctional materials and quantum states of matter. This permits unique opportunities to tailor the electronic properties of vdW heterostructures by the insertion of only a single 2D material layer. However, such vdW materials engineering at the atomic scale has yet to be investigated for spin-charge interconversion phenomena. Here, we report on the control of these effects at the monolayer level, where a drastic increase in intensity and change in sign of THz spintronic emission are demonstrated by inserting a single layer of MoSe_{2} between PtSe_{2} and graphene in a fully epitaxial, large area stacked structure. By using a combination of spin and angle resolved photoemission and density functional theory to reveal the electronic and spin structures, we illustrate two different mechanisms relying on charge transfer and electronic hybridization for the formation of Rashba states, which are responsible for spin-charge conversion and hence the THz spintronic emission. These findings open new pathways to design, at the atomic scale, efficient THz spintronic emitters made of 2D materials and other spintronic devices based on spin-charge interconversion phenomena.

Spintronic terahertz emitters, based on optically triggered spin-to-charge conversion processes, have recently emerged as a novel route toward compact and efficient THz sources. The next challenge for technologically relevant devices, however, remains to modulate the emission with low-energy consumption during operation. In order to achieve this, ferroelectric materials coupled to active spin–orbit layers such as two-dimensional transition metal dichalcogenides are potential candidates. In this work, we present the realization of a large area heterostructure of CoFeB/PtSe2/MoSe2 on a macroscopically bidomain LiNbO3 substrate. Using THz time-domain spectroscopy, we show that the ferroelectric polarization direction induces a sizable modulation of the THz emission. We rationalize these experimental results by using band structure and spin accumulation calculations that are consistent with an interfacial spin-to-charge conversion mediated by the inverse Rashba–Edelstein effect at the MoSe2/PtSe2 interface and being tuned by ferroelectricity in the adjacent LiNbO3 surface. This work demonstrates the relevance and technological potential of field effect spin–orbit architectures for novel THz technologies.