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Squeezing light into deep sub-wavelength volumes or even into few-nanometer gaps has led to the investigation of interesting phenomena, including strong coupling, quantum plasmonics, nonlinearity enhancement, nonlocality, and molecular junctions. Bowtie nanoantennas, as a common configuration for plasmonic nanocavity, have been extensively studied owing to their great enhancement of the localized field. The enhancement rapidly increases as the tips become sharper and the gap becomes narrower. However, every effort paid to increase the extreme sharpness and nanometer precision of less than 10 nm gaps result in the downfall of fabrication throughput, since it relies on the probability of achieving one satisfactory piece out of the many fabricated. Here, an intuitive “fall-to-rise” schemes are proposed and experimentally validated using cascade domino lithography (CDL) and capillary-force-induced collapse lithography (CCL). In this report, we successfully establish a controllable lithography method of making extremely sharp bowtie-shaped plasmonic nanocavity with sub-1nm radius curvature reaching the size of a gold nanocluster as well as a single-digit-nanometer gap between such sharp tips. By controlling falling mechanisms of photoresist mask structures, a facile route to fabricate sub-10 nm plasmonic nanocavity with high yield is provided. In addition, a proof-of-concept application in surface enhanced Raman spectroscopy (SERS) is demonstrated. The numerically calculated intensity enhancement is over 2.2×10^4 with the confined mode volume below 7.14×10^-6 λ^3, the measured average Raman enhancement factor is of the order of 10^6 with the calculated local Raman enhancement factor over 10^8 promising for single-molecular level sensing applications. Furthermore, such sharp plasmonic nanocavity is used to probe and control localized excitons at room temperature, which offers new strategies for active quantum nano-optical devices. Atomically thin semiconductors, WSe2, are coated on top of plasmonic nanocavity and Au tip for tip-enhanced photoluminescence (TEPL) spectroscopy is added to induce tensile strain in the nanoscale region to create robust localized exciton at the hotspot region. Such an approach provides a systematic way to control localized quantum light. This controlling mechanism of nanostructures falling opens up an unexplored gateway towards conquering the limitations of exploring the realm of plasmonics down to the sub-nanometer regime.
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