Inducing and probing localized excitons in atomically thin semiconductors via tip-enhanced cavity-spectroscopy

Reviews and Highlights	Quantum Science	Molecular and Soft-matter	Ultrafast Nano-optics and Nanophotonics	Mineralogy and Geochemistry

Hyeongwoo Lee, Inki Kim, Chulho Park, Mingu Kang, Jinseong Choi, Kwang-Yong Jeong, Jungho Mun, Yeseul Kim, Jeonghoon Park, Markus B. Raschke, Hong-Gyu Park, Mun Seok Jeong, Junsuk Rho, and Kyoung-Duck Park
Adv. Funct. Mater. 31, 2102893 (2021).
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In atomically thin semiconductors, localized exciton (X_L) coupled to light provides a new class of optical sources for potential applications in quantum communication. However, in most studies, X_L photoluminescence (PL) from crystal defects has mainly been observed in cryogenic conditions because of their sub-wavelength emission region and low quantum yield at room temperature. Hybrid-modality of cavity-spectroscopy to induce and probe the X_L emissions at the nanoscale in atomically thin semiconductors is presented. By placing a WSe₂ monolayer on the two extremely sharp Au tips in a bowtie antenna with a radius of curvature of <1 nm, tensile strain of ≈0.3% is effectively induced in a <30 nm region to create robust X_L states. The Au tip then approaches the strained crystal region to enhance the X_L emissions and probe them with tip-enhanced photoluminescence (TEPL) spectroscopy at room temperature. Through this triple-sharp-tips cavity-spectroscopy with <15 nm spatial resolution, TEPL enhancement as high as ≈4.0 × 10⁴ by the Purcell effect is achieved, and peak energy shifts of X_L up to ≈40 meV are observed. This approach combining nano-cavity and -spectroscopy provides a systematic way to induce and probe the radiative emission of localized excitons in 2D semiconductors offering new strategies for dynamic quantum nano-optical devices.