Out-of-plane (OP) exciton-based emitters in twodimensional semiconductor materials are attractive candidates for novel photonic applications, such as radially polarized sources, integrated photonic chips, and quantum communications. However, their low quantum efficiency resulting from forbidden transitions limits their practicality. In this work, we achieve a giant enhancement of up to 34000 for OP exciton emission in indium selenide (InSe) via a designed Ag nanocube-over-Au film plasmonic nanocavity. The large photoluminescence enhancement factor (PLEF) is attributed to the induced OP local electric field (E z ) within the nanocavity, which facilitates effective OP exciton− plasmon interaction and subsequent tremendous enhancement. Moreover, the nanoantenna effect resulting from the effective interaction improves the directivity of spontaneous radiation. Our results not only reveal an effective photoluminescence enhancement approach for OP excitons but also present an avenue for designing on-chip photonic devices with an OP dipole orientation.

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More molecules mean a stronger signal for surface-enhanced Raman spectroscopy (SERS), as usually expected. However, this is not always true in many cases. The factual molecular concentration dependence of the SERS intensity has always been controversial and confusing. Here, we present a chemical interface damping (CID)-induced SERS signal attenuation mechanism to reveal it based on electromagnetic theory and experimentally demonstrate its validity. It has been revealed that the SERS intensity is the result of competition between the two opposite effects of molecular adsorption-induced Raman scattering centers and CID, but not necessarily increasing with the rising concentration, especially at a relatively high concentration. When the molecular adsorption-induced CID is strong enough, the SERS intensity shows a nonmonotonic concentration dependence, while the weak CID leads to the monotonic concentration dependence under optimal excitation. In addition, the excitation condition also significantly influences the concentration dependence of the SERS intensity. When an excitation wavelength significantly deviates from the local surface plasmon resonance of the SERS substrate, the SERS intensity will increase monotonously with the rising concentration, even in the case of the strong CID, despite the reduced overall signal intensity. This work not only deepens the understanding of the electromagnetic enhancement mechanism in SERS, which is also of significance in the quantitative analyses, but also is suitable for the other plasmon-enhanced molecular spectroscopies.