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  • 194. X. Sun, E. Malic, Y. Lu, “Dipolar many-body complexes and their interactions in stacked 2D heterobilayers” arXiv: 2402.08394

    Highly customizable interfaces created by van der Waals stacked 2D materials provide an extremely flexible opportunity for engineering and effectively controlling material properties. The atomic-thin nature and strong scalability of transition metal dichalcogenides (TMDs), the star family of two-dimensional semiconducting materials, allow for the modulation of their inherent optical and electrical characteristics by utilizing various environmental stimuli. In such a material system, the stacking mechanism with spatial separation in the structure enables recent observations of dipolar many-body complexes with the interplay of multi-particles, leading to some exotic and novel excitonic phenomena and enabling the closer study of high-correlated quantum physics. The presence of powerful dipole-dipole interactions among long-lived interlayer excitons can cause the system to enter unique classical and quantum phases with multiparticle correlations, such as dipolar liquids, dipolar crystals and superfluids. The strong binding energy of interlayer excitons in TMD-based hetero-bilayers especially enhances the critical temperature of these exotic phenomena. Here, we provide a concise summary of the recent frontier research progress on dipolar complexes and many-body effects in TMD double layers, encompassing fundamental theory and properties modulation. We reveal the significance and current challenges of this research field and present the potential developing directions of the hetero-bilayers in quantum physics and quantum devices by adding new levels of external control or integration.

    arXiv: 2402.08394

  • 193. G. Meneghini, S. Brem, E. Malic "Excitonic thermalization bottleneck in twisted TMD heterostructures", arXiv: 2402.03007

    Twisted van der Waals heterostructures show an intriguing interface exciton physics including hybridization effects and emergence of moiré potentials. Recent experiments have revealed that moiré-trapped excitons exhibit a remarkable dynamics, where excited states show lifetimes that are several orders of magnitude longer than those in monolayers. The origin of this behaviour is still under debate. Based on a microscopic many-particle approach, we investigate the phonon-driven relaxation cascade of non-equilibrium moiré excitons in the exemplary MoSe2-WSe2 heterostructure. We track the exciton relaxation pathway across different moiré mini-bands and identify the phonon-scattering channels assisting the spatial redistribution of excitons into low-energy pockets of the moiré potential. We unravel a phonon bottleneck in the flat band structure at low twist angles preventing excitons to fully thermalize into the lowest state explaining the measured enhanced emission intensity of excited moiré excitons. Overall, our work provides important insights into exciton relaxation dynamics in flatband exciton materials.

    arXiv: 2402.03007

  • 192. B. Ferreira, H. Shan, R. Rosati, J. M. Fitzgerald, L. Lackner, B. Han, M. Esmann, P. Hays, G. Liebling, K. Watanabe, T. Taniguchi, F. Eilenberger, S. Tongay, C. Schneider, E. Malic, "Revealing dark exciton signatures in polariton spectra of 2D materials", arXiv: 2401.04588

    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.

    arXiv: 2401.04588

  • 191. 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: 2401.03825v1

  • 190. 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., 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: 2312.07332

  • 189. C. Chandrakant Palekar, J. Hagel, B. Rosa, S. Brem, C.-W. Shih, I. Limame, M. von Helversen, S. Tongay, E. Malic, S. Reitzenstein, "Twist Angle Dependence of Exciton Resonances in WSe2/MoSe2 Moiré Heterostructures", arXiv: 2309.16822

    Van der Waals heterostructures based on TMDC semiconducting materials have emerged as promising materials due to their spin-valley properties efficiently contrived by the stacking-twist angle. The twist angle drastically alters the interlayer excitonic response by determining the spatial modulation, confining moiré potential, and atomic reconstruction in those systems. Nonetheless, the impact of the interlayer twist angle on the band alignment of the monolayers composing the heterostructure has received scant attention in the current research. Here, we systematically investigate the twist-angle dependence of intra- and interlayer excitons in twisted WSe2/MoSe2 heterobilayers. By performing photoluminescence excitation spectroscopy, we identify the twist-angle dependence of interlayer emission response, where an energy redshift of about 100 meV was observed for increasing twist angles. The applied microscopic theory predicts, on the contrary, a blueshift, which suggests that additional features, such as atomic reconstruction, may also surpass the moiré potential confinement. Those findings also prompt the effects of dielectric screening by addressing the redshift response to the stacking layer order. Furthermore, our findings support the evidence of a band offset dependence on the twist angle for the adjacent monolayers composing the heterobilayer system. Our fundamental study of exciton resonances deepens the current understanding of the physics of twisted TMDC heterostructures and paves the way for future experiments and theoretical works.

    arXiv: 2309.16822

  • 188. 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: 2305.18908

  • 187. 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