The strong excitonic properties of transition metal dichalcogenides (TMD) have led to the successful demonstration of exciton‐polaritons (EPs) in various optical cavity structures. Recently, self‐hybridized EPs have been discovered in a bare TMD layer, but experimental investigation is still lacking because of their nonradiative nature. Herein, the direct observation of self‐hybridized EPs in a bare multilayer WS2 via the evanescent field coupling technique is reported. Because of the thickness‐dependent Rabi splitting energy, the dispersion curves of the EPs change sensitively with sample thickness. Moreover, continuous tuning of EP dispersion curves is demonstrated by controlling the excitation laser power. Lastly, it is observed that guided EPs retain valley polarization up to 0.2 at room temperature, representing a valley‐preserved strong coupling regime. It is believed that the high tunability and valley polarization properties of the guided EPs in bare TMD layers can facilitate new nanophotonic and valleytronic applications.
Understanding the chiral light-matter interaction offers a new way to control the direction of light. Here, we present an unprecedently long-range transport of valley information of a 2D-layered semiconductor via the directional emission through a dielectric waveguide. In the evanescent near field region of the dielectric waveguide, robust and homogeneous transverse optical spin exists regardless of the size of the waveguide. The handedness of transverse optical spin, determined by the direction of guided light mode, leads to the chiral coupling of light with valley-polarized excitons. Experimentally, we demonstrated ultra-low propagation loss which enabled a 16 µm long propagation of directional emission from valley-polarized excitons through a ZnO waveguide. The estimated directionality of exciton emission from a valley was about 0.7. We confirmed that a dielectric waveguide leads to a better performance than does a plasmonic waveguide in terms of both the directional selectivity of guided emission and the efficiency of optical power reaching the ends of the waveguide when a propagation length is greater than ∼10 µm. The proposed dielectric waveguide system represents an essential platform for efficient spin/valley–photon interfaces.
An ultra-thin transition metal dichalcogenide (TMDC) layer can support guided exciton-polariton modes due to the strong coupling between excitons and photons. Herein, we report the guided mode resonance in an ultra-thin TMDC grating structure. Owing to the strong exciton resonances in TMDCs, a TMDC grating structure shows guided-mode resonance even at a thickness limit of ∼10 nm and is capable of realizing polaritonic dispersion in a monolithic grating structure. We investigated the polarization and thickness dependence of the optical dispersion relations of the tungsten disulfide (WS2) grating structure. In addition, we confirmed that the monolithic WS2 grating coupler can be used to couple the near-field guided exciton-polariton out into the far field. We believe that ultra-thin TMDC layers can facilitate sub-wavelength nanophotonic applications.
Transition metal dichalcogenides multilayer is indirect band material. We observe that a WS2 disk supports whispering gallery modes in a spectral range of 700 ~ 900 nm and exhibits lasing action.
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