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We report Ge23Sb7S70 chalcogenide ring resonators with up to 8 × 104 quality factors operating around 3.6 µm wavelength fabricated through e-beam lithography. Their rib waveguide geometry can be engineered to support close-to-zero dispersion modes needed for mid-infrared microcomb generation. 

A new nanofabrication approach is reported for the scalable production of plasmonically modulated upconverting nanoparticles, with the potential for force sensing. 

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. 

Abstract A novel force sensor exploiting the interaction between plasmonic nanostructures and upconversion nanoparticles (UCNPs) is reported. The nanosensor is composed of a gold nanodisk and UCNPs separated by a flexible polymer layer. The gold nanodisk is designed to exhibit a plasmon resonance that selectively enhances one of the emission bands of the UCNPs while leaving the other ones largely unaffected. As the nanosensor is compressed or stretched by an external force, the polymer layer thickness changes, modulating the plasmon‐UCNP coupling. The resulting changes in the luminescence intensity provide the basis for sensing. Furthermore, the nanosensor employs ratiometric sensing, which makes it highly robust against any environmental variations. The nanosensors exhibit two orders of magnitude higher responsivity than previously reported UCNP‐based force sensors. They can be prepared as an on‐chip sensor array or in a colloidal solution, making them suitable for a variety of applications in biology and robotics. 

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. 

Abstract Förster resonance energy transfer (FRET)‐based devices have been extensively researched as potential biosensors due to their highly localized responsivity. In particular, dye‐conjugated upconverting nanoparticles (UCNPs) are among the most promising FRET‐based sensor candidates. UCNPs have a multi‐modal emission profile that allows for ratiometric sensing, and by conjugating a biosensitive dye to their surface, this profile can be used to measure localized variations in biological parameters. However, the complex nature of the UCNP energy profile as well as reabsorption of emitted photons must be taken into account in order to properly sense the target parameters. To the authors’ knowledge, no proposed UCNP‐based sensor has accurately taken care of these intricacies. In this article, the authors account for these complexities by creating a FRET‐based sensor that measures pH. This sensor utilizes Thulium (Tm³⁺)‐doped UCNPs and the fluorescent dye fluorescein isothiocyanate (FITC). It is first demonstrated that photon reabsorption is a serious issue for the 475 nm Tm³⁺emission, thereby limiting its use in FRET‐based sensing. It is then shown that by taking the ratio of the 646 and 800 nm emissions rather than the more popular 475 nm one, it is possible to measure pH exclusively through FRET.