Achieving high thermal sensitivity from ratiometric CaGdAlO4:Mn4+,Tb3+ thermometers.

The pursuit of optical temperature sensing with high thermal sensitivity to discriminate small temperature changes without contact with the subject possesses a crucial technological and scientific significance. Ratiometric temperature detection based on transition metals and lanthanides emerges as a promising strategy to achieve the purpose due to the dopants' distinct thermal quenching rates. In this work, a new CaGdAlO4:Mn4+,Tb3+ luminescent thermometer was developed. The combination of the highly-thermal-sensitive red emission from Mn4+ ions with the thermally-robust green emission from Tb3+ ions renders the thermometer with a maximum relative thermal sensitivity of 2.3% K-1 at 398 K. The well-separated red and green channels in digital images enable further evaluation of thermal sensitivity. The estimated thermal sensitivity is 2.23% K-1 at 398 K from the pixel intensity ratio of red and green channels.

Ba3YB3O9 based phosphor ceramic plates with excellent thermal stability for wLED applications.

A series of Eu3+ and Tb3+ singly doped Ba3YB3O9 phosphors were synthesized by a solution combustion approach. The most luminescent phosphors were selected as the starting materials to fabricate phosphor ceramic plates (PCPs). More intense red and green emission is observed from the PCPs than the corresponding phosphors. The emission colour can be effectively tuned by varying the mass ratio of the red and green phosphors and the excitation wavelength. White light emission was obtained by incorporating the blue-emitting Eu2+ doped BaMgAl10O17 phosphor into the PCP component. The synthesized phosphors and PCPs show robust thermal stability, maintaining more than 80% of emission intensity at 423 K of that at 298 K after a long time operation. The results indicate that the prepared red and green PCPs are promising candidates for high-power wLED applications.

Enhanced deep-red emission from Mn4+/Mg2+ co-doped CaGdAlO4 phosphors for plant cultivation.

Mn4+-Doped oxide phosphors are under intensive investigation owing to their low manufacture cost and attractive luminescent features for indoor plant cultivation applications. However, it is still a challenge to develop Mn4+-doped oxides with high luminescence efficiency and thermal stability. Herein, Mn4+-Mg2+ pairs are incorporated into a CaGdAlO4 host to reduce non-radiative channels formed by Mn4+-Mn4+-O2- clusters. The photoluminescence and quantum efficiency are significantly enhanced after the introduction of Mg2+ ions to the host. A prolonged Mn4+ decay time is also obtained from the Mn4+/Mg2+ co-doped samples. Intense red emission with a narrow peak at 712 nm due to the 2Eg → 4A2g transition of Mn4+ ions is observed under 335 nm excitation. LEDs fabricated by coating the synthesized phosphor on a 365 nm near-UV chip exhibit an intense deep-red emission with CIE chromaticity coordinates of (0.712, 0.285). The results indicate Mn4+/Mg2+ co-doped CaGdAlO4 phosphors may be applicable to plant cultivation fields.

High-efficiency watt-level continuous-wave 2.9 µm Ho,Pr:YLF laser.

The bottleneck in Ho3+:I65→I752.9 µm laser emission, where the upper-level lifetime (I65) is significantly shorter than that of the lower-level (I75) lifetime, is overcome by co-doping Ho3+ and Pr3+ ions. A novel kind of Ho3+,Pr3+:YLIF4 (Ho,Pr:YLF) crystal is fabricated by optimizing the doping concentration ratios. By using as-grown Ho,Pr: YLF crystals with doping concentrations of 0.498 at. % Ho3+, 0.115 at. % Pr3+ and 0.489 at. % Ho3+, 0.097 at. % Pr3+ as gain media, both high-efficiency and continuous-wave 2.9 µm laser operations are realized under 1150 nm fiber laser pumping, respectively. With the 0.498 at. % Ho3+, 0.115 at. % Pr3+: YLF crystal, a maximum output power of 1.27 W with a corresponding slope efficiency of 28.3% is yielded under a 4.48 W absorbed pump power, which is the highest output power among the Ho3+-doped 2.9 µm lasers ever reported, to the best of our knowledge. The results well reveal the great potential of Ho,Pr:YLF crystals in developing high-power, high-efficiency 2.9 µm mid-infrared lasers.

NIR Downconversion and Energy Transfer Mechanisms in Tb3+/Yb3+ Codoped Na5Lu9F32 Single Crystals.

Tb3+/Yb3+ codoped Na5Lu9F32 single crystals with near-infrared (NIR) emission are achieved by an improved Bridgman approach. Energy transfer from Tb3+ to Yb3+ ions is affirmed by the photoluminescence (PL) emission spectra and decay curves characterization. On the basis of the analysis of energy transfer rate dependence on the Yb3+ concentration, the interaction between Tb3+ and Yb3+ ions in Na5Lu9F32 single crystals is confirmed through the one-to-one energy transfer process. Results demonstrate that the prepared Na5Lu9F32 single crystals might be promising candidates to convert sunlight to improve the performance of the silicon solar cells.

Continuous Tuning of Organic Phosphorescence by Diluting Triplet Diffusion at the Molecular Level.

Organic long-persistent phosphorescent materials are advantageous due to the cost-effectiveness and easy processability. The organic phosphorescence is achieved by the long-lived triplet excitons, and the challenges are recognized regarding the various nonradiative pathways to quench the emission lifetime. Taming long-lived phosphorescence is generally engaged with the charge-transfer or exciton diffusion in molecular stacking to stabilize triplet excitons or form a photoinduced ionized state. Herein, we elucidate that the triplet-diffusion can cause a significant quenching that is not thermally activated by using a system of perfluorinated organic complexes. Hence, we suggest a coevaporation technique to dilute a single phosphorescence-emitting molecule with another optically inactive molecule to suppress the diffusion-induced quenching, tuning the phosphorescence lifetime and spectral features continuously. The work successfully suggests a general semitheoretical method of quantifying the population equilibrium to elucidate the loss mechanisms for organic phosphorescence.

High-repetition-rate kHz electro-optically Q-switched Ho, Pr: YLF 2.9 µm bulk laser.

In this study, we operated a novel bulk laser: a dual-end-pumped electro-optically (EO) Q-switched Ho, Pr:YLF laser at 2945.9 nm with a repetition rate of kHz level. The shortest pulse duration of 25.2 ns was obtained at the repetition rate of 500 Hz, corresponding to a single pulse energy of 0.4 mJ and a peak power of 15.9 kW. A maximum output power of 268 mW was delivered at the repetition rate of 1.5 kHz. The beam quality factors of the EO Q-switched Ho, Pr:YLF laser at the maximum output power were Mx 2 = 1.50 and My 2 = 1.54 in x- and y- directions, respectively. The long-term output power instability was measured to be ± 1.5% (RMS) within five hours. The achieved results indicated the promising potential of Ho, Pr: YLF crystals for high repetition rate Q-switched mid-infrared laser pulse generation very well.

Sensitization, energy transfer and infra-red emission decay modulation in Yb3+-doped NaYF4 nanoparticles with visible light through a perfluoroanthraquinone chromophore.

Infra-red emission (980 nm) of sub 10 nm Yb3+-doped NaYF4 nanoparticles has been sensitized through the excitation of 2-hydroxyperfluoroanthraquinone chromophore (1,2,3,4,5,6,7-heptafluro-8-hydroxyanthracene-9,10-dione) functionalizing the nanoparticle surface. The sensitization is achieved with a broad range of visible light excitation (400-600 nm). The overall near infra-red (NIR) emission intensity of Yb3+ ions is increased by a factor 300 as a result of the broad and strong absorption of the chromophore compared with ytterbium's intrinsic absorption. Besides the Yb3+ NIR emission, the hybrid composite shows organic chromophore-based visible emission in the orange-red region of the spectrum. We observe the energy migration process from the sensitized Yb3+ ions at the surface to those in the core of the particle using time-resolved optical spectroscopy. This highlights that the local environments for emitting Yb3+ ions at the surface and center of the nanoparticle are not identical, which causes important differences in the NIR emission dynamics. Based on the understanding of these processes, we suggest a simple strategy to control and modulate the decay time of the functionalized Yb3+-doped nanoparticles over a relatively large range by changing physical or chemical parameters in this model system.