Dye Sensitization for Ultraviolet Upconversion Enhancement

Upconversion nanocrystals that converted near-infrared radiation into emission in the ultraviolet spectral region offer many exciting opportunities for drug release, photocatalysis, photodynamic therapy, and solid-state lasing. However, a key challenge is the development of lanthanide-doped nanocrystals with efficient ultraviolet emission, due to low conversion efficiency. Here, we develop a dye-sensitized, heterogeneous core–multishelled lanthanide nanoparticle for ultraviolet upconversion enhancement. We systematically study the main influencing factors on ultraviolet upconversion emission, including dye concentration, excitation wavelength, and dye-sensitizer distance. Interestingly, our experimental results demonstrate a largely promoted multiphoton upconversion. The underlying mechanism and detailed energy transfer pathway are illustrated. These findings offer insights into future developments of highly ultraviolet-emissive nanohybrids and provide more opportunities for applications in photo-catalysis, biomedicine, and environmental science.

Volume and surface effects on two-photonic and three-photonic processes in dry co-doped upconversion nanocrystals

Despite considerable advances in synthesizing high-quality core/shell upconversion (UC) nanocrystals (NC; UCNC) and UCNC photophysics, the application of near-infrared (NIR)-excitable lanthanide-doped UCNC in the life and material sciences is still hampered by the relatively low upconversion luminescence (UCL) of UCNC of small size or thin protecting shell. To obtain deeper insights into energy transfer and surface quenching processes involving Yb3+ and Er3+ ions, we examined energy loss processes in differently sized solid core NaYF4 nanocrystals doped with either Yb3+ (YbNC; 20% Yb3+) or Er3+ (ErNC; 2% Er3+) and co-doped with Yb3+ and Er3+ (YbErNC; 20% Yb3+ and 2% Er3+) without a surface protection shell and coated with a thin and a thick NaYF4 shell in comparison to single and co-doped bulk materials. Luminescence studies at 375 nm excitation demonstrate back-energy transfer (BET) from the 4G11/2 state of Er3+ to the 2F5/2 state of Yb3+, through which the red Er3+4F9/2 state is efficiently populated. Excitation power density (P)-dependent steady state and time-resolved photoluminescence measurements at different excitation and emission wavelengths enable to separate surface-related and volume-related effects for two-photonic and three-photonic processes involved in UCL and indicate a different influence of surface passivation on the green and red Er3+ emission. The intensity and lifetime of the latter respond particularly to an increase in volume of the active UCNC core. We provide a three-dimensional random walk model to describe these effects that can be used in the future to predict the UCL behavior of UCNC.

Dual-wavelength switchable single-mode lasing from a lanthanide-doped resonator

Multi-wavelength lasing, with dynamic switching functionality, high spectral purity and contrast, plays an essential role in photonic devices. Lanthanide(Ln3+)-doped upconversion nanocrystals(UCNCs) featured with plentiful energy levels act as ideal candidates for the gain medium. However, it remains a daunting challenge to develop a tunable Ln3+-based single-mode laser across a wide wavelength range due to the absence of an appropriate mode-selection mechanism. Here, we have demonstrated the first active control of unidirectional single-mode lasing with switchable wavelength spanning beyond a record range (~ 300 nm). This is accomplished through the integration of two size-mismatched coupled microdisks cavity and inversely designed dual-mode UCNCs incorporated with selective sensitizer-pair and activator ions. By asymmetric pumping, a reversible and crosstalk-free ultraviolet-to-red single-mode operation were formed respectively from such UCNCs-based system through changing the pumping wavelengths from 980 nm to 808 nm. The results enlighten the rational design of luminescent materials and microcavity. With remarkable doping flexibility, our approach would pave an avenue to a class of UCNCs-based photonic devices for on-chip optical filter, switch, sensor and other devices.