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Preprints
201. R. Rosati, I. Paradisanos, E. Malic, and B. Urbaszek "Two dimensional semiconductors: optical and electronic properties", arXiv: 2405.04222
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 moiré 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 moire 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.
arXiv: 2405.04222200. R. Perea-Causin, S. Brem, F. Buchner, K. Watanabe, T. Taniguchi, J. M. Lupton, K.-Q. Lin, E. Malic, "Electrically tunable layer-hybridized trions in doped WSe2 bilayers", arXiv: 2404.18716
Doped van der Waals heterostructures host layer-hybridized trions, i.e. charged excitons with layer-delocalized constituents holding promise for highly controllable optoelectronics. Combining a microscopic theory with photoluminescence (PL) experiments, we demonstrate the electrical tunability of the trion energy landscape in naturally stacked WSe2 bilayers. We show that an out-of-plane electric field modifies the energetic ordering of the lowest lying trion states, which consist of layer-hybridized Λ-point electrons and layer-localized K-point holes. At small fields, intralayer-like trions yield distinct PL signatures in opposite doping regimes characterized by weak Stark shifts in both cases. Above a doping-asymmetric critical field, interlayer-like species are energetically favored and produce PL peaks with a pronounced Stark red-shift and a counter-intuitively large intensity arising from efficient phonon-assisted recombination. Our work presents an important step forward in the microscopic understanding of layer-hybridized trions in van der Waals heterostructures and paves the way towards optoelectronic applications based on electrically controllable atomically-thin semiconductors.
arXiv: 2404.18716199. A. de la Torre, D. M. Kennes, E. Malic, S. Kar "Review: Spatial inhomogeneities, moiré potential and moiré excitons", arXiv: 2402.19236
In this short review, we provide an overview of recent progress in deploying advanced characterization techniques to understand the effects of local inhomogeneities in moiré heterostructures over multiple length scales. Particular emphasis is placed on correlating the impact of twist angle misalignment, nano-scale disorder, and atomic relaxation on the moiré potential and its collective excitations, particularly moiré excitons. Finally, we discuss future technological applications leveraging based on moié excitons.
arXiv: 2402.19236198. J. M. Fitzgerald, R. Rosati, B. Ferreira, H. Shan, C. Schneider, E. Malic, "Circumventing the polariton bottleneck via dark excitons in 2D semiconductors", arXiv: 2401.03825v1
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.
arXiv: 2401.03825v1197. A. Kumar, D. Yagodkin, R. Rosati, D. J Bock, C. Schattauer, S. Tobisch, J. Hagel, B. Höfer, J. N Kirchhof, P. Hernández López, K. Burfeindt, S. Heeg, C. Gahl, F. Libisch, E. Malic, K. I Bolotin, "Strain fingerprinting of exciton valley character", arXiv: 2312.07332
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 (WSe2, WS2) 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.
arXiv: 2312.07332196. D. Schmitt, J. P. Bange, W. Bennecke, G. Meneghini, A. AlMutairi, M.Merboldt, J. Pöhls, K. Watanabe, T. Taniguchi, S. Steil, D. Steil, R. T. Weitz, S. Hofmann, S. Brem, G. S. Matthijs Jansen, E. Malic, S. Mathias and M. Reutzel "Ultrafast nano-imaging of dark excitons", arXiv: 2305.18908
The role and impact of spatial heterogeneity in two-dimensional quantum materials represents one of the major research quests regarding the future application of these materials in optoelectronics and quantum information science. In the case of transition-metal dichalcogenide heterostructures, in particular, direct access to heterogeneities in the dark-exciton landscape with nanometer spatial and ultrafast time resolution is highly desired, but remains largely elusive. Here, we introduce ultrafast dark field momentum microscopy to spatio-temporally resolve dark exciton formation dynamics in a twisted WSe2/MoS2 heterostructure with 55 femtosecond time- and 500~nm spatial resolution. This allows us to directly map spatial heterogeneity in the electronic and excitonic structure, and to correlate these with the dark exciton formation and relaxation dynamics. The benefits of simultaneous ultrafast nanoscale dark-field momentum microscopy and spectroscopy is groundbreaking for the present study, and opens the door to new types of experiments with unprecedented spectroscopic and spatiotemporal capabilities.
arXiv: 2305.18908195. Q. Lin, H. Fang, Y. Liu, Y. Zhang, M. Fischer, J. Li, J. Hagel, S. Brem, E. Malic, N. Stenger, Z. Sun, M. Wubs, S. Xiao, "A room-temperature moiré interlayer exciton laser" arXiv: 2302.01266
Moiré superlattices in van der Waals heterostructures offer highly tunable quantum systems with emergent electronic and excitonic properties such as superconductivity, topological edge states, and moiré-trapped excitons. Theoretical calculations predicted the existence of the moiré potential at elevated temperatures; however, its impact on the optical properties of interlayer excitons (IXs) at room temperature is lacking, and the benefits of the moiré effects for lasing applications remain unexplored. We report that the moiré potential in a molybdenum disulfide/tungsten diselenide (MoS2/WSe2) heterobilayer system can significantly enhance light emission, elongate the IX lifetime, and modulate the IX emission energy at room temperature. By integrating a moiré superlattice with a silicon topological nanocavity, we achieve ultra-low-threshold lasing at the technologically important telecommunication O-band thanks to the significant moiré modulation. Moreover, the high-quality topological nanocavities facilitate the highest spectral coherence of < 0.1 nm linewidth among all reported two-dimensional material-based laser systems. Our findings not only open a new avenue for studying correlated states at elevated temperatures, but also enable novel architectures for integrated on-chip photonics and optoelectronics.
arXiv: 2302.01266