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1. Enhanced nonlinear interaction of polaritons via excitonic Rydberg states in monolayer WSe2

   https://doi.org/10.1038/s41467-021-22537-x  

   Gu, Jie; Walther, Valentin; Waldecker, Lutz; Rhodes, Daniel; Raja, Archana; Hone, James C.; Heinz, Tony F.; Kéna-Cohen, Stéphane; Pohl, Thomas; Menon, Vinod M. (December 2021, Nature Communications)

   Abstract Strong optical nonlinearities play a central role in realizing quantum photonic technologies. Exciton-polaritons, which result from the hybridization of material excitations and cavity photons, are an attractive candidate to realize such nonlinearities. While the interaction between ground state excitons generates a notable optical nonlinearity, the strength of such interactions is generally not sufficient to reach the regime of quantum nonlinear optics. Excited states, however, feature enhanced interactions and therefore hold promise for accessing the quantum domain of single-photon nonlinearities. Here we demonstrate the formation of exciton-polaritons using excited excitonic states in monolayer tungsten diselenide (WSe 2 ) embedded in a microcavity. The realized excited-state polaritons exhibit an enhanced nonlinear response ∼ $${g}_{{pol}-{pol}}^{2s} \sim 46.4\pm 13.9\,\mu {eV}\mu {m}^{2}$$ g p o l − p o l 2 s ~ 46.4 ± 13.9 μ e V μ m 2 which is ∼4.6 times that for the ground-state exciton. The demonstration of enhanced nonlinear response from excited exciton-polaritons presents the potential of generating strong exciton-polariton interactions, a necessary building block for solid-state quantum photonic technologies. 

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2. Observation of Rydberg exciton polaritons and their condensate in a perovskite cavity

   https://doi.org/10.1073/pnas.1909948116  

   Bao, Wei; Liu, Xiaoze; Xue, Fei; Zheng, Fan; Tao, Renjie; Wang, Siqi; Xia, Yang; Zhao, Mervin; Kim, Jeongmin; Yang, Sui; et al (October 2019, Proceedings of the National Academy of Sciences)

   The condensation of half-light half-matter exciton polaritons in semiconductor optical cavities is a striking example of macroscopic quantum coherence in a solid-state platform. Quantum coherence is possible only when there are strong interactions between the exciton polaritons provided by their excitonic constituents. Rydberg excitons with high principal value exhibit strong dipole–dipole interactions in cold atoms. However, polaritons with the excitonic constituent that is an excited state, namely Rydberg exciton polaritons (REPs), have not yet been experimentally observed. Here, we observe the formation of REPs in a single crystal CsPbBr 3 perovskite cavity without any external fields. These polaritons exhibit strong nonlinear behavior that leads to a coherent polariton condensate with a prominent blue shift. Furthermore, the REPs in CsPbBr 3 are highly anisotropic and have a large extinction ratio, arising from the perovskite’s orthorhombic crystal structure. Our observation not only sheds light on the importance of many-body physics in coherent polariton systems involving higher-order excited states, but also paves the way for exploring these coherent interactions for solid-state quantum optical information processing. 

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3. Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

   https://doi.org/10.1038/s41467-023-40288-9  

   Lian, Zhen; Chen, Dongxue; Ma, Lei; Meng, Yuze; Su, Ying; Yan, Li; Huang, Xiong; Wu, Qiran; Chen, Xinyue; Blei, Mark; et al (August 2023, Nature Communications)

   Abstract Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe₂-WS₂-WSe₂trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe₂valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe₂layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe₂under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions. 

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4. Concerning the stability of biexcitons in hybrid HJ aggregates of π -conjugated polymers

   https://doi.org/10.1063/5.0090515  

   Bittner, Eric R.; Silva, Carlos (May 2022, The Journal of Chemical Physics)

   Frenkel excitons are the primary photoexcitations in organic semiconductors and are ultimately responsible for the optical properties of such materials. They are also predicted to form bound exciton pairs, termed biexcitons, which are consequential intermediates in a wide range of photophysical processes. Generally, we think of bound states as arising from an attractive interaction. However, here, we report on our recent theoretical analysis, predicting the formation of stable biexciton states in a conjugated polymer material arising from both attractive and repulsive interactions. We show that in J-aggregate systems, 2J-biexcitons can arise from repulsive dipolar interactions with energies E 2 J > 2 E J , while in H-aggregates, 2H-biexciton states with energies E 2 H < 2 E H can arise corresponding to attractive dipole exciton/exciton interactions. These predictions are corroborated by using ultrafast double-quantum coherence spectroscopy on a [poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene)] material that exhibits both J- and H-like excitonic behavior. 

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5. The ultrafast onset of exciton formation in 2D semiconductors

   https://doi.org/10.1038/s41467-020-18835-5  

   Trovatello, Chiara; Katsch, Florian; Borys, Nicholas J.; Selig, Malte; Yao, Kaiyuan; Borrego-Varillas, Rocio; Scotognella, Francesco; Kriegel, Ilka; Yan, Aiming; Zettl, Alex; et al (December 2020, Nature Communications)

   null (Ed.)

   Abstract The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS 2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process. 

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