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1. Exciton-polaron Rydberg states in monolayer MoSe2 and WSe2

   https://doi.org/10.1038/s41467-021-26304-w  

   Liu, Erfu; van Baren, Jeremiah; Lu, Zhengguang; Taniguchi, Takashi; Watanabe, Kenji; Smirnov, Dmitry; Chang, Yia-Chung; Lui, Chun Hung (October 2021, Nature Communications)

   Abstract Exciton polaron is a hypothetical many-body quasiparticle that involves an exciton dressed with a polarized electron-hole cloud in the Fermi sea. It has been evoked to explain the excitonic spectra of charged monolayer transition metal dichalcogenides, but the studies were limited to the ground state. Here we measure the reflection and photoluminescence of monolayer MoSe₂and WSe₂gating devices encapsulated by boron nitride. We observe gate-tunable exciton polarons associated with the 1 s–3 s exciton Rydberg states. The ground and excited exciton polarons exhibit comparable energy redshift (15~30 meV) from their respective bare excitons. The robust excited states contradict the trion picture because the trions are expected to dissociate in the excited states. When the Fermi sea expands, we observe increasingly severe suppression and steep energy shift from low to high exciton-polaron Rydberg states. Their gate-dependent energy shifts go beyond the trion description but match our exciton-polaron theory. Our experiment and theory demonstrate the exciton-polaron nature of both the ground and excited excitonic states in charged monolayer MoSe₂and WSe₂. 

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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. Interplay of valley polarized dark trion and dark exciton-polaron in monolayer WSe2

   https://doi.org/10.1038/s41467-023-41475-4  

   Cong, Xin; Mohammadi, Parisa Ali; Zheng, Mingyang; Watanabe, Kenji; Taniguchi, Takashi; Rhodes, Daniel; Zhang, Xiao-Xiao (September 2023, Nature Communications)

   Abstract The interactions between charges and excitons involve complex many-body interactions at high densities. The exciton-polaron model has been adopted to understand the Fermi sea screening of charged excitons in monolayer transition metal dichalcogenides. The results provide good agreement with absorption measurements, which are dominated by dilute bright exciton responses. Here we investigate the Fermi sea dressing of spin-forbidden dark excitons in monolayer WSe₂. With a Zeeman field, the valley-polarized dark excitons show distinct p-doping dependence in photoluminescence when the carriers reach a critical density. This density can be interpreted as the onset of strongly modified Fermi sea interactions and shifts with increasing exciton density. Through valley-selective excitation and dynamics measurements, we also infer an intervalley coupling between the dark trions and exciton-polarons mediated by the many-body interactions. Our results reveal the evolution of Fermi sea screening with increasing exciton density and the impacts of polaron-polaron interactions, which lay the foundation for understanding electronic correlations and many-body interactions in 2D systems. 

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4. Narrow-band high-lying excitons with negative-mass electrons in monolayer WSe2

   https://doi.org/10.1038/s41467-021-25499-2  

   Lin, Kai-Qiang; Ong, Chin Shen; Bange, Sebastian; Faria Junior, Paulo E.; Peng, Bo; Ziegler, Jonas D.; Zipfel, Jonas; Bäuml, Christian; Paradiso, Nicola; Watanabe, Kenji; et al (December 2021, Nature Communications)

   Abstract Monolayer transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. Here, we report the existence of a new excitonic species, the high-lying exciton (HX), in single-layer WSe 2 with an energy of ~3.4 eV, almost twice the band-edge A-exciton energy, with a linewidth as narrow as 5.8 meV. The HX is populated through momentum-selective optical excitation in the K -valleys and is identified in upconverted photoluminescence (UPL) in the UV spectral region. Strong electron-phonon coupling results in a cascaded phonon progression with equidistant peaks in the luminescence spectrum, resolvable to ninth order. Ab initio GW -BSE calculations with full electron-hole correlations explain HX formation and unmask the admixture of upper conduction-band states to this complex many-body excitation. These calculations suggest that the HX is comprised of electrons of negative mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalizes the excitonic quantum-interference phenomenon recently discovered in optical second-harmonic generation (SHG) and explains the efficient Auger-like annihilation of band-edge excitons. 

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5. 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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