Microcavity lasing has many excellent properties, such as a small mode volume, high quality factor (Q), high sensitivity, effortless manipulation, and good monochromaticity, which can be applied in sensing,^{1 – 3} nonlinear optics,^{4 – 6} and cavity optomechanics.^7,8 The superior features of lanthanide-doped upconversion nanoparticles (UCNPs), such as their sharp emission bands, long lifetime, nonblinking and nonbleaching characteristics, and high stability,^9,10 make them an attractive gain medium for the development of upconversion microlasers. Moreover, they have real intermediate energy states as efficient media for the development of multiphoton-excited states. Lanthanide-based microcavity lasing has attracted considerable attention owing to its large anti-Stokes shifts, multiple emission bands, and narrow linewidths.^{11 – 14} Many advances have been made, such as the effective regulation of laser modes, the realization of low-threshold microlasers, and the achievement of upconverted lasing.^{15 – 19} With the miniaturization of optoelectronic devices, various applications of lanthanide-based microlasers, such as optical communication, optical storage, and polarization imaging, require selecting the appropriate laser polarization mode and controlling the laser properties remotely on-chip. However, controlling the polarization mode of microlasing, a fundamental property of microlasers, both in situ and remotely, remains challenging.

Localized surface plasmon has been extensively studied and used for the photocatalysis of various chemical reactions. However, the different contributions between plasmon resonance and interband transition in photocatalysis has not been well understood. Here, we study the photothermal and hot electrons effects for crystal transformation by combining controlled experiments with numerical simulations. By photo-excitation of NaYF₄ : Eu³⁺ @Au composite structure, it is found that the plasmonic catalysis is much superior to that of interband transition in the experiments, owing to the hot electrons generated by plasmon decay more energetic to facilitate the reaction. We emphasize that the energy level of hot electrons plays an essential role for improving the photocatalytic activity. The results provide guidelines for improving the efficiency of plasmonic catalysis in future experimental design.

Over 10 000 times fluorescence enhancement is achieved and small splitting is observed in both photoluminescence and scattering spectra of the NRs in the plasmonic nanocavity.

Surface-enhanced Raman scattering (SERS) technology has important applications in many fields, such as biomedicine, environmental monitoring, and food safety. Plasmonic nanocavities have the ability to superdiffract localized light and enhance light-matter interactions. As a key SERS active substrate, research on plasmonic nanocavities has made significant progress regarding the enhancement mechanism, the utilization of hotspots for the detection of specific molecular groups, and practical applications. However, challenges related to improving the enhancement factor of nanocavity SERS, enhancing the stability and reproducibility of hotspots, and enabling the detection of single-molecule layers remain. In this study, we adopt a bottom-up approach to construct a silver microplate–molecule–multi-sized silver nanosphere nanoparticle-on-mirror (NPoM) nanocavity and achieve the efficient stable enhancement of Raman scattering from 4-mercaptobenzoic acid and biphenyl-4,4′-dithiol molecules via the electromagnetic mechanism. By characterizing the fabricated nanocavity using dark-field scattering and micro-confocal Raman scattering, we observed that the Raman scattering intensity in the NPoM nanocavity was enhanced by a factor of 10³ compared to that of individual silver nanospheres. Furthermore, we achieved the efficient stabilization of SERS by precisely tuning the size of the silver nanospheres to match their resonance frequency with the Raman shift of the target molecules. This approach offers a valuable reference for the detection of various single-molecule layers and demonstrates significant potential for applications in biosensing and chemical analysis.