Observation of in-plane exciton-polaritons in monolayer WSe driven by plasmonic nanofingers.

The transition metal dichalcogenides (TMDs) have drawn great research attention, motivated by the derived remarkable optoelectronic properties and the potentials for high-efficient excitonic devices. The plasmonic nanocavity, integrating deep-sub wavelength confinement of optical mode with dramatic localized field enhancement, provides a practical platform to manipulate light-matter interaction. In order to obtain strong exciton-plasmon coupling effects, it's crucial to match the vibration direction of exciton to the available strong localized in-plane electric field. Herein, we demonstrate the coupling effect of in-plane exciton in monolayer tungsten diselenide (WSe) to deterministic gap-plasmon field which is produced by nanometrically gapped collapsed nanofingers. The gap-plasmon field which is completely parallel to the in-plane excitons in WSe will drive a strong exciton-plasmon coupling at room temperature. More interestingly, it is experimentally observed that the luminescence of exciton-polariton cannot be influenced by the temperature in the range from 77 K to 300 K due to the presence of nanofingers. According to the theoretical analysis results, we attribute this finding to the dielectric screening effect arising from the extremely strong localized electric field of plasmonic nanofingers. This work proposes a feasible way to harness and manipulate the exciton of low-dimensional semiconductor, which might be potential for quantum optoelectronics.