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1. Spatial coherence of room-temperature monolayer WSe2 exciton-polaritons in a trap

   https://doi.org/10.1038/s41467-021-26715-9  

   Shan, Hangyong; Lackner, Lukas; Han, Bo; Sedov, Evgeny; Rupprecht, Christoph; Knopf, Heiko; Eilenberger, Falk; Beierlein, Johannes; Kunte, Nils; Esmann, Martin; et al (December 2021, Nature Communications)

   Abstract The emergence of spatial and temporal coherence of light emitted from solid-state systems is a fundamental phenomenon intrinsically aligned with the control of light-matter coupling. It is canonical for laser oscillation, emerges in the superradiance of collective emitters, and has been investigated in bosonic condensates of thermalized light, as well as exciton-polaritons. Our room temperature experiments show the strong light-matter coupling between microcavity photons and excitons in atomically thin WSe 2 . We evidence the density-dependent expansion of spatial and temporal coherence of the emitted light from the spatially confined system ground-state, which is accompanied by a threshold-like response of the emitted light intensity. Additionally, valley-physics is manifested in the presence of an external magnetic field, which allows us to manipulate K and K’ polaritons via the valley-Zeeman-effect. Our findings validate the potential of atomically thin crystals as versatile components of coherent light-sources, and in valleytronic applications at room temperature. 

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2. Tunable exciton-polaritons emerging from WS2 monolayer excitons in a photonic lattice at room temperature

   https://doi.org/10.1038/s41467-021-24925-9  

   Lackner, L.; Dusel, M.; Egorov, O. A.; Han, B.; Knopf, H.; Eilenberger, F.; Schröder, S.; Watanabe, K.; Taniguchi, T.; Tongay, S.; et al (December 2021, Nature Communications)

   Abstract Engineering non-linear hybrid light-matter states in tailored lattices is a central research strategy for the simulation of complex Hamiltonians. Excitons in atomically thin crystals are an ideal active medium for such purposes, since they couple strongly with light and bear the potential to harness giant non-linearities and interactions while presenting a simple sample-processing and room temperature operability. We demonstrate lattice polaritons, based on an open, high-quality optical cavity, with an imprinted photonic lattice strongly coupled to excitons in a WS 2 monolayer. We experimentally observe the emergence of the canonical band-structure of particles in a one-dimensional lattice at room temperature, and demonstrate frequency reconfigurability over a spectral window exceeding 85 meV, as well as the systematic variation of the nearest-neighbour coupling, reflected by a tunability in the bandwidth of the p-band polaritons by 7 meV. The technology presented in this work is a critical demonstration towards reconfigurable photonic emulators operated with non-linear photonic fluids, offering a simple experimental implementation and working at ambient conditions. 

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3. Experimental observation of topological Z2 exciton-polaritons in transition metal dichalcogenide monolayers

   https://doi.org/10.1038/s41467-021-24728-y  

   Li, Mengyao; Sinev, Ivan; Benimetskiy, Fedor; Ivanova, Tatyana; Khestanova, Ekaterina; Kiriushechkina, Svetlana; Vakulenko, Anton; Guddala, Sriram; Skolnick, Maurice; Menon, Vinod M.; et al (December 2021, Nature Communications)

   null (Ed.)

   Abstract The rise of quantum science and technologies motivates photonics research to seek new platforms with strong light-matter interactions to facilitate quantum behaviors at moderate light intensities. Topological polaritons (TPs) offer an ideal platform in this context, with unique properties stemming from resilient topological states of light strongly coupled with matter. Here we explore polaritonic metasurfaces based on 2D transition metal dichalcogenides (TMDs) as a promising platform for topological polaritonics. We show that the strong coupling between topological photonic modes of the metasurface and excitons in TMDs yields a topological polaritonic Z 2 phase. We experimentally confirm the emergence of one-way spin-polarized edge TPs in metasurfaces integrating MoSe 2 and WSe 2 . Combined with the valley polarization in TMD monolayers, the proposed system enables an approach to engage the photonic angular momentum and valley and spin of excitons, offering a promising platform for photonic/solid-state interfaces for valleytronics and spintronics. 

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4. Stable gigahertz- and mmWave-repetition-rate soliton microcombs on X-cut lithium niobate

   https://doi.org/10.1364/OPTICA.562208  

   Song, Yunxiang; Zhu, Xinrui; Zuo, Xiangying; Huang, Guanhao; Lončar, Marko (January 2025, Optica)

   Soliton microcombs are a cornerstone of integrated frequency comb technologies, with applications spanning photonic computing, ranging, microwave synthesis, optical communications, and quantum light generation. In nearly all such applications, electro-optic (EO) components play a critical role in generating, monitoring, stabilizing, and modulating solitons. Toward building photonic integrated circuits for next-generation applications that will simultaneously maximize system performance and minimize size, weight, and power consumption metrics, achieving soliton microcombs and efficient EO modulation on a chip is essential. X-cut thin-film lithium niobate (TFLN) has emerged as a leading photonic platform for the realization of high-performance integrated EO devices and systems. However, despite extensive research, soliton microcombs have remained elusive to X-cut TFLN due to its multiple strong Raman-active modes, in-plane refractive index anisotropy, and photorefractive effects. Here, we address this long-standing challenge and demonstrate versatile soliton microcombs on X-cut TFLN, with repetition rates spanning from the gigahertz (∼26GHz, single soliton) up to the millimeter-wave (∼0.156THz, soliton crystal) regime. The combs feature exceptional long-term stability, maintaining a direct injection-locked state for over 90 min (manually terminated), with repetition-rate phase noise closely tracking that of a high-quality electronic microwave synthesizer. Our finding broadly advances both the fundamental science and practical applications of integrated comb sources by enabling efficient EO modulation and broadband coherent solitons to be monolithically combined on the same chip. 

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5. Chiral flat-band optical cavity with atomically thin mirrors

   https://doi.org/10.1126/sciadv.adr5904  

   Suárez-Forero, Daniel G; Ni, Ruihao; Sarkar, Supratik; Jalali_Mehrabad, Mahmoud; Mechtel, Erik; Simonyan, Valery; Grankin, Andrey; Watanabe, Kenji; Taniguchi, Takashi; Park, Suji; et al (December 2024, Science Advances)

   A fundamental requirement for photonic technologies is the ability to control the confinement and propagation of light. Widely used platforms include two-dimensional (2D) optical microcavities in which electromagnetic waves are confined in either metallic or distributed Bragg reflectors. Recently, transition metal dichalcogenides hosting tightly bound excitons with high optical quality have emerged as promising atomically thin mirrors. In this work, we propose and experimentally demonstrate a subwavelength 2D nanocavity using two atomically thin mirrors with degenerate resonances. Angle-resolved measurements show a flat band, which sets this system apart from conventional photonic cavities. We demonstrate how the excitonic nature of the mirrors enables the formation of chiral and tunable optical modes upon the application of an external magnetic field. Moreover, we show the electrical tunability of the confined mode. Our work demonstrates a mechanism for confining light with high-quality excitonic materials, opening perspectives for spin-photon interfaces, and chiral cavity electrodynamics. 

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