arXiv subjects

Roberto Rosati

Publications and source records attributed to Roberto Rosati.

At least 19 recordsLinked to original sources

arXiv2026

Strain engineering of transient exciton diffusion in WSe2 monolayers at cryogenic temperatures

Tungsten-based transition metal dichalcogenides exhibit dark excitons as the energetically lowest states. These are crucial for exciton thermalization and propagation and they dominate low-temperature photoluminescence via the emergence of pronounced phonon sidebands. After a resonant excitation, highly mobile hot dark excitons are formed, which quickly thermalize into an equilibrium distribution. The application of strain modifies the exciton energy landscape and, in particular, the relative energy separation between bright and dark exciton states. The impact of strain on the transient photoluminescence and diffusion of non-equilibrium excitons has remained largely unexplored so far. In this work, we investigate the spatiotemporal exciton dynamics in strained hBN-encapsulated WSe2 monolayers at cryogenic temperatures. We demonstrate that tensile strain abruptly increases the excess energy of hot excitons, thereby accelerating their transient diffusion. We trace this back to suppressed phonon-mediated scattering from bright to dark excitons. Furthermore, we predict a periodic modulation of the transient exciton diffusion in the presence of a compressive strain resulting from strain-driven emission of M phonons. The gained microscopic insights illustrate how strain can be used to engineer transient photoluminescence and exciton diffusion in technologically promising 2D semiconductors.

cond-mat.mes-hall↗

arXiv2026

Coulomb-mediated interactions of charge-transfer excitons in TMD lateral heterostructures

Lateral heterostructures of transition-metal dichalcogenides (TMDs) host spatially separated charge-transfer (CT) excitons. While analogous to interlayer excitons in vertical TMD heterostructures, these interfacial excitons possess much larger in-plane dipoles of several nanometers and an additional center-of-mass quantization. Here, we study the mutual interactions between these highly dipolar CT excitons on a microscopic footing. Accounting for the dipolar and quantum exchange interactions, we evaluate the experimentally accessible density-dependent energy renormalization and predict a net energy blueshift of a few meV for bound CT excitons. Interestingly, for small dipole moments, the energy renormalization displays a quadratic dependence with respect to the dipole moment, in contrast to the linear dependence found in vertical TMD heterostructures. We show that spatial energy offset and temperature are the key tuning knobs for controlling the density-dependent excitonic response. Overall, our results contribute to a better microscopic understanding of CT excitons and their interactions in lateral TMD heterostructures.

cond-mat.mes-hall↗

arXiv2026

Nonequilibrium transport in epitaxial CsPbBr3 single crystals

Transport of optically excited carriers in semiconductors is typically described within a quasi-equilibrium picture, where energy is carried by a single thermalized quasiparticle population characterized by well-defined transport coefficients. Here, we demonstrate that in epitaxial CsPbBr3 perovskite single crystals, this picture holds near room temperature - but breaks down dramatically at low temperature. Using transient microscopy, we show that optically measured carrier mobilities match device-scale Hall-effect and field-effect transistor measurements across a broad temperature range, resolving reported discrepancies and validating the equilibrium framework in the free-carrier regime. Below ~60 K, however, when excitonic effects become significant, equilibrium models begin to fail. We observe two coupled populations: a transient (<100 ps) hot-exciton gas with a diffusivity ~25-30 cm 2 /s - greatly exceeding the diffusivity expected for thermalized excitons - and a quasi-localized state that is fed by the cooling of the hot-exciton gas. These results reveal that in CsPbBr3, transport and thermalization are not separable processes: carriers move while still redistributing among internal degrees of freedom, breaking the timescale separation that underpins equilibrium transport theory in conventional semiconductors. By resolving transport at the population level, we can directly access the competing kinetics of exciton formation, interconversion, and cooling, offering a new space for controlling energy flow in perovskite materials and their photonic applications.

cond-mat.mes-hall↗

arXiv2026

Valley-controlled many-body exciton interactions in monolayer WSe$_2$ phototransistors

Many-body exciton interactions shape the optoelectronic response of atomically-thin transition metal dichalcogenides, yet optical control of these interactions remains largely unexplored. To date, modulation of exciton-exciton interactions has primarily relied on electrical gating or van der Waals engineering. Here, we demonstrate all-optical control of many-body exciton interactions in monolayer WSe$_2$ via valley-selective excitation using polarization-resolved pulsed-laser photocurrent spectroscopy. Circular excitation selectively populates excitons in a single valley, whereas linear excitation populates both valleys, inducing a valley-dependent nonlinear photoresponse. We observe helicity-dependent exciton renormalization, alongside a two-fold enhancement of sublinear photocurrent scaling under circular excitation, reflecting single-valley population of interacting excitons. A microscopic model incorporating intervalley-exchange and exciton-exciton annihilation mediated by dark and bright exciton populations reproduces the nonlinear valley-selective response. These results establish the valley degree of freedom as an all-optical control parameter for tuning many-body excitonic effects and, exploring correlated exciton states and valleytronic applications in two-dimensional semiconductors.

cond-mat.mes-hall↗

arXiv2025

Polariton transport in 2D semiconductors: Phonon-mediated transitions between ballistic, superdiffusive and exciton-limited regimes

Exciton transport in 2D semiconductors holds promise for room-temperature, ultra-compact optoelectronic devices, but it is limited by short propagation distances. Hybridization of excitons with cavity photons to form exciton-polaritons can enhance the propagation by orders of magnitude, enabling a coherent, ballistic transport. However, a microscopic understanding of the role of phonons is still lacking, particularly regarding their influence on the crossover from the ballistic to the diffusive polariton transport regime. Here, we investigate the spatiotemporal polariton dynamics in \ce{MoSe2} monolayers at moderate to high temperatures, explicitly including the phonon-mediated coupling to the intervalley exciton reservoir. We identify three distinct transport regimes: (i) an initial sub-ps ballistic-like regime characterized by a phonon-induced velocity renormalization, (ii) a transient, few-ps superdiffusive regime characterized by strongly enhanced diffusion, and (iii) a slower, exciton-limited diffusion following thermalization. The gained microscopic insights will trigger and guide future experimental studies on the phonon-mediated polariton transport in atomically thin semiconductors.

cond-mat.mes-hall↗

arXiv2025

Tuning relaxation and nonlinear upconversion of valley-exciton-polaritons in a monolayer semiconductor

Controlling exciton relaxation and energy conversion pathways via their coupling to photonic modes is a central task in cavity-mediated quantum materials research. In this context, the light-matter hybridization in optical cavities can lead to intriguing effects, such as modified carrier transport, enhancement of optical quantum yield, and control of chemical reaction pathways. Here, we investigate the impact of the strong light-matter coupling regime on energy conversion, both in relaxation and upconversion schemes, by utilizing a strongly charged MoSe2 monolayer embedded in a spectrally tunable open-access cavity. We find that the charge carrier gas yields a significantly modified photoluminescence response of cavity exciton-polaritons, dominated by an intra-cavity like pump scheme. In addition, upconversion luminescence emerges from a population transfer from fermionic trions to bosonic exciton-polaritons. Due to the availability of multiple optical modes in the tunable open cavity, it seamlessly meets the cavity-enhanced double resonance condition required for an efficient upconversion. The latter can be actively tuned via the cavity length in-situ, displaying nonlinear scaling in intensity and fingerprints of the valley polarization. This suggests mechanisms that include both trion-trion Auger scattering and phonon absorption as its underlying microscopic origin.

cond-mat.mes-hall↗

arXiv2025

Impact of charge transfer excitons on unidirectional exciton transport in lateral TMD heterostructures

Lateral heterostructures built of monolayers of transition metal dichalcogenides (TMDs) are characterized by a thin 1D interface exhibiting a large energy offset. Recently, the formation of spatially separated charge-transfer (CT) excitons at the interface has been demonstrated. The technologically important exciton propagation across the interface and the impact of CT excitons has remained in the dark so far. In this work, we microscopically investigate the spatiotemporal exciton dynamics in the exemplary hBN-encapsulated WSe$_2$-MoSe$_2$ lateral heterostructure and reveal a highly interesting interplay of energy offset-driven unidirectional exciton drift across the interface and efficient capture into energetically lower CT excitons at the interface. This interplay triggers a counterintuitive thermal control of exciton transport with a less efficient propagation at lower temperatures - opposite to the behavior in conventional semiconductors. We predict clear signatures of this intriguing exciton propagation both in far- and near-field photoluminescence experiments. Our results present an important step toward a microscopic understanding of the technologically relevant unidirectional exciton transport in lateral heterostructures.

cond-mat.mes-hall↗

arXiv2025

Strain engineering of valley-polarized hybrid excitons in a 2D semiconductor

Encoding and manipulating digital information in quantum degrees of freedom is one of the major challenges of today's science and technology. The valley indices of excitons in transition metal dichalcogenides (TMDs) are well-suited to address this challenge. Here, we demonstrate a new class of strain-tunable, valley-polarized hybrid excitons in monolayer TMDs, comprising a pair of energy-resonant intra- and intervalley excitons. These states combine the advantages of bright intravalley excitons, where the valley index directly couples to light polarization, and dark intervalley excitons, characterized by low depolarization rates. We demonstrate that the hybridized state of dark KK' intervalley and defect-localized excitons exhibits a degree of circular polarization of emitted photons that is three times higher than that of the constituent species. Moreover, a bright KK intravalley and a dark KQ exciton form a coherently coupled hybrid state under energetic resonance, with their valley depolarization dynamics slowed down a hundredfold. Overall, these valley-polarized hybrid excitons with strain-tunable valley character emerge as prime candidates for valleytronic applications in future quantum and information technology.

cond-mat.mes-hall↗

arXiv2024

Trion Engineered Multimodal Transistors in Two dimensional Bilayer Semiconductor Lateral Heterostructures

Multimodal device operations are essential to advancing the integration of 2D semiconductors in electronics, photonics, information and quantum technology. Precise control over carrier dynamics, particularly exciton generation and transport, is crucial for finetuning the functionality of optoelectronic devices based on 2D semiconductor heterostructure. However, the traditional exciton engineering methods in 2D semiconductors are mainly restricted to the artificially assembled vertical pn heterostructures with electrical or strain induced confinements. In this study, we utilized bilayer 2D lateral npn multijunction heterostructures with intrinsically spatially separated energy landscapes to achieve preferential exciton generation and manipulation without external confinement. In lateral npn FET geometry, we uncover unique and nontrivial properties, including dynamic tuning of channel photoresponsivity from positive to negative. The multimodal operation of these 2D FETs is achieved by carefully adjusting electrical bias and the impinging photon energy, enabling precise control over the trions generation and transport. Cryogenic photoluminescence measurement revealed the presence of trions in bilayer MoSe2 and intrinsic trap states in WSe2. Measurements in different FET device geometries show the multifunctionality of 2D lateral heterostructure phototransistors for efficient tuning and electrical manipulation of excitonic characteristics. Our findings pave the way for developing practical exciton-based transistors, sensors, multimodal optoelectronic and quantum technologies

physics.app-ph↗

arXiv2024

Infrared magneto-polaritons in MoTe$_2$ mono- and bilayers

MoTe$_2$ monolayers and bilayers are unique within the family of van-der-Waals materials since they pave the way towards atomically thin infrared light-matter quantum interfaces, potentially reaching the important telecommunication windows. Here, we report emergent exciton-polaritons based on MoTe$_2$ monolayer and bilayer in a low-temperature open micro-cavity in a joint experiment-theory study. Our experiments clearly evidence both the enhanced oscillator strength and enhanced luminescence of MoTe$_2$ bilayers, signified by a 38 \% increase of the Rabi-splitting and a strongly enhanced relaxation of polaritons to low-energy states. The latter is distinct from polaritons in MoTe$_2$ monolayers, which feature a bottleneck-like relaxation inhibition. Both the polaritonic spin-valley locking in monolayers and the spin-layer locking in bilayers are revealed via the Zeeman effect, which we map and control via the light-matter composition of our polaritonic resonances.

cond-mat.mes-hall↗

arXiv2024

Two dimensional semiconductors: optical and electronic properties

In the last decade atomically thin 2D materials have emerged as a perfect platform for studying and tuning light-matter interaction and electronic properties in nanostructures. The optoelectronic properties in layered materials such as transition-metal-dichalcogenides (TMDs) are governed by excitons, Coulomb bound electron-hole pairs, even at room temperature. The energy, wave function extension, spin and valley properties of optically excited conduction electrons and valence holes are controllable via multiple experimentally accessible knobs, such as lattice strain, varying atomic registries, dielectric engineering as well as electric and magnetic fields. This results in a multitude of fascinating physical phenomena in optics and transport linked to excitons with very specific properties, such as bright and dark excitons, interlayer and charge transfer excitons as well as hybrid and moir\'e excitons. In this book chapter we introduce general optoelectronic properties of 2D materials and energy landscapes in TMD monolayers as well as their vertical and lateral heterostructures, including twisted TMD hetero- and homobilayer bilayers with moir\'e excitons and lattice recombination effects. We review the recently gained insights and open questions on exciton diffusion, strain- and field-induced exciton drift. We discuss intriguing non-linear many-particle effects, such as exciton halo formation, negative and anomalous diffusion, the surprising anti-funneling of dark excitons.

cond-mat.mes-hall↗

arXiv2024

Revealing dark exciton signatures in polariton spectra of 2D materials

Dark excitons in transition metal dichalcogenides (TMD) have been so far neglected in the context of polariton physics due to their lack of oscillator strength. However, in tungsten-based TMDs, dark excitons are known to be the energetically lowest states and could thus provide important scattering partners for polaritons. In this joint theory-experiment work, we investigate the impact of the full exciton energy landscape on polariton absorption and reflectance. By changing the cavity detuning, we vary the polariton energy relative to the unaffected dark excitons in such a way that we open or close specific phonon-driven scattering channels. We demonstrate both in theory and experiment that this controlled switching of scattering channels manifests in characteristic sharp changes in optical spectra of polaritons. These spectral features can be exploited to extract the position of dark excitons. Our work suggests new possibilities for exploiting polaritons for fingerprinting nanomaterials via their unique exciton landscape.

cond-mat.mes-hall↗

arXiv2024

Circumventing the polariton bottleneck via dark excitons in 2D semiconductors

Efficient scattering into the exciton polariton ground state is a key prerequisite for generating Bose-Einstein condensates and low-threshold polariton lasing. However, this can be challenging to achieve at low densities due to the polariton bottleneck effect that impedes phonon-driven scattering into low-momentum polariton states. The rich exciton landscape of transition metal dichalcogenides (TMDs) provides potential intervalley scattering pathways via dark excitons to rapidly populate these polaritons. Here, we present a microscopic study exploring the time- and momentum-resolved relaxation of exciton polaritons supported by a \ce{MoSe2} monolayer integrated within a Fabry-Perot cavity. By exploiting phonon-assisted transitions between momentum-dark excitons and the lower polariton branch, we demonstrate that it is possible to circumvent the bottleneck region and efficiently populate the polariton ground state. Furthermore, this intervalley pathway is predicted to give rise to, yet unobserved, angle-resolved phonon sidebands in low-temperature photoluminescence spectra that are associated with momentum-dark excitons. This represents a distinctive experimental signature for efficient phonon-mediated polariton-dark-exciton interactions.

cond-mat.mes-hall↗

arXiv2023

Strain fingerprinting of exciton valley character

Momentum-indirect excitons composed of electrons and holes in different valleys define optoelectronic properties of many semiconductors, but are challenging to detect due to their weak coupling to light. The identification of an excitons' valley character is further limited by complexities associated with momentum-selective probes. Here, we study the photoluminescence of indirect excitons in controllably strained prototypical 2D semiconductors (WSe$_2$, WS$_2$) at cryogenic temperatures. We find that these excitons i) exhibit valley-specific energy shifts, enabling their valley fingerprinting, and ii) hybridize with bright excitons, becoming directly accessible to optical spectroscopy methods. This approach allows us to identify multiple previously inaccessible excitons with wavefunctions residing in K, $Γ$, or Q valleys in the momentum space as well as various types of defect-related excitons. Overall, our approach is well-suited to unravel and tune intervalley excitons in various semiconductors.

cond-mat.mes-hall↗

arXiv2023

Interface engineering of charge-transfer excitons in 2D lateral heterostructures

The existence of bound charge transfer (CT) excitons at the interface of monolayer lateral heterojunctions has been debated in literature, but contrary to the case of interlayer excitons in vertical heterostructure their observation still has to be confirmed. Here, we present a microscopic study investigating signatures of bound CT excitons in photoluminescence spectra at the interface of hBN-encapsulated lateral MoSe$_2$-WSe$_2$ heterostructures. Based on a fully microscopic and material-specific theory, we reveal the many-particle processes behind the formation of CT excitons and how they can be tuned via interface- and dielectric engineering. For junction widths smaller than the Coulomb-induced Bohr radius we predict the appearance of a low-energy CT exciton. The theoretical prediction is compared with experimental low-temperature photoluminescence measurements showing emission in the bound CT excitons energy range. Our joint theory-experiment study presents a significant step towards a microscopic understanding of optical properties of technologically promising 2D lateral heterostructures.

cond-mat.mes-hall↗

arXiv2022

Exciton optics, dynamics and transport in atomically thin semiconductors

Atomically thin semiconductors such as transition metal dichalcogenide (TMD) monolayers exhibit a very strong Coulomb interaction, giving rise to a rich exciton landscape. This makes these materials highly attractive for efficient and tunable optoelectronic devices. In this article, we review the recent progress in the understanding of exciton optics, dynamics and transport, which crucially govern the operation of TMD-based devices. We highlight the impact of hBN-encapsulation, which reveals a plethora of many-particle states in optical spectra, and we outline the most novel breakthroughs in the field of exciton-polaritonics. Moreover, we underline the direct observation of exciton formation and thermalization in TMD monolayers and heterostructures in recent time-resolved ARPES studies. We also show the impact of exciton density, strain and dielectric environment on exciton diffusion and funneling. Finally, we put forward relevant research directions in the field of atomically thin semiconductors for the near future.

cond-mat.mes-hall↗

arXiv2022

Signatures of dark excitons in exciton-polariton optics of transition metal dichalcogenides

Integrating 2D materials into high-quality optical microcavities opens the door to fascinating many-particle phenomena including the formation of exciton-polaritons. These are hybrid quasi-particles inheriting properties of both the constituent photons and excitons. In this work, we investigate the so-far overlooked impact of dark excitons on the momentum-resolved absorption spectra of hBN-encapsulated WSe$_2$ and MoSe$_2$ monolayers in the strong-coupling regime. In particular, thanks to the efficient phonon-mediated scattering of polaritons into energetically lower dark exciton states, the absorption of the lower polariton branch in WSe$_2$ is much higher than in MoSe$_2$. It shows unique step-like increases in the momentum-resolved profile indicating opening of specific scattering channels. We study how different externally accessible quantities, such as temperature or mirror reflectance, change the optical response of polaritons. Our study contributes to an improved microscopic understanding of exciton-polaritons and their interaction with phonons, potentially suggesting experiments that could determine the energy of dark exciton states via momentum-resolved polariton absorption.

cond-mat.mes-hall↗

arXiv2022

Microscopic modelling of exciton-polariton diffusion coefficients in atomically thin semiconductors

In the strong light-matter coupling regime realized e.g. by integrating semiconductors into optical microcavities, polaritons as new hybrid light-matter quasi-particles are formed. The corresponding change in the dispersion relation has a large impact on optics, dynamics and transport behaviour of semiconductors. In this work, we investigate the strong-coupling regime in hBN-encapsulated MoSe$_2$ monolayers focusing on exciton-polariton diffusion. Applying a microscopic approach based on the exciton density matrix formalism combined with the Hopfield approach, we predict a drastic increase of the diffusion coefficients by two to three orders of magnitude in the strong coupling regime. We explain this behaviour by the much larger polariton group velocity and suppressed polariton-phonon scattering channels with respect to the case of bare excitons. Our study contributes to a better microscopic understanding of polariton diffusion in atomically thin semiconductors.

cond-mat.mtrl-sci↗