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Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted widespread interest in bioimaging and sensing due to their photostability, low excitation energy, and good tissue penetration. Plasmonic nanostructures, on the other hand, can enhance the luminescence of UCNPs by concentrating electric fields into a nanoscale volume. While the enhanced luminescence intensity is in principle beneficial to sensing, intensity-based sensing has limitations in absolute measurements. This deficiency can be overcome by employing ratiometric sensing in which intensity ratio, rather than intensity itself, is used to quantitatively determine the presence of analytes. The ratiometric sensing is advantageous because the intensity ratio is much less sensitive to the variations in the environment and the number of probe materials in the sensing volume. Here, we demonstrate a plasmonic nanostructure with upconversion nanoparticles for an enhanced ratiometric sensing platform. The plasmonic nanostructure is composed of UCNPs, an indium tin oxide (ITO) spacer layer and an Au nanodisk. The nanostructure is designed such that the plasmon resonance selectively enhances the red luminescence of NaYGdF 4 :Yb 3+ , Er 3+ UCNPs while leaving the green luminescence unaffected, thereby increasing the dynamic range and achievable sensitivity of the red-to-green (R/G) intensity ratio. We observed a 4-fold enhancement in the R/G ratio and also a drastic reduction in the signal uncertainty. This work advances our knowledge of the optical interaction between UCNPs and plasmonic nanostructures and also provides a foundation for improved ratiometric sensing in biomedical applications. 

We report a ∼3-fold enhancement of third-harmonic generation (THG) conversion efficiency using indium tin oxide (ITO) nanoparticles on the surface of an ultra-high-Qsilica microsphere. This is one of the largest microcavity-based THG enhancements reported. Phase-matching and spatial mode overlap are explored numerically to determine the microsphere radius (∼29 µm) and resonant mode numbers that maximize THG. Furthermore, the ITO nanoparticles are uniformly bonded to the cavity surface by drop-casting, eliminating the need for complex fabrication. The significant improvement in THG conversion efficiency establishes functionalized ITO microcavities as a promising tool for broadband frequency conversion, nonlinear enhancement, and applications in integrated photonics.