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Developing fluorescent materials that can achieve efficient upconversion (UC) emission under low power density excitation has always been a great challenge, especially for Tm3+ ions. Here, Tm3+‐doped Cs2NaYbCl6 double perovskite single‐crystals (DPSCs) are prepared successfully. The DPSCs exhibit efficient UC emission under 980‐nm laser excitation, and the optimum UC efficiency reaches 9.48%. It is a recorded value. More importantly, its efficient UC emission can be achieved under low‐power density of 0.117 W cm−2 excitation. Moreover, the luminescence color of the DPSCs can be regulated from blue to red by increasing the doping concentration of Tm3+ ions. This is because the severe cross‐relaxation occurs between Tm3+ ions at high doping concentrations. The Cs2NaYb0.98Tm0.02Cl6 DPSCs with efficient UC emission are coupled to the CsPbI3 perovskite photodetector, achieving good visible and near‐infrared (NIR) detection without an external bias. It is worth emphasizing that the detectivity for 980 nm light can reach 1.26 × 1011 Jones, which is the highest value for self‐powered NIR photodetectors. Finally, given the dual functional characteristics of the device's response to broadband visible and narrowband NIR light, the dual encryption function is implemented during information transmission. This work will open new chances for applications in advanced integrated photonics devices. [ABSTRACT FROM AUTHOR]

A strategy is demonstrated for simultaneous size and color tuning of KZnF:Yb/Er nanocrystals through transition metal Mn doping. The experimental results indicate that the introduction of Mn into the KZnF:Yb/Er reaction system facilitates the decrease of the crystal size. Moreover, in the case of a high concentration of Mn doping, the downshifting (DS) and upconversion (UC) luminescence all show a single red band emission. In this work, we mainly investigate the DS and UC energy transfer process among Mn/Yb/Er in KZnF nanocrystals by the detection and analysis of the absorption, excitation, emission, and transient fluorescence spectra. The results show that both Mn and Yb ions can decrease the green emission while improving the red luminescence of Er. This DS and UC single red color tuning is significant for the application of nanoparticles in light conversion, biological labeling, or plant photosynthesis. [ABSTRACT FROM AUTHOR]

Fe-doped BaTiO 3 films were prepared on quartz substrates by sol-gel route. Linearly optical constants of the films were obtained by fitting transmittance spectra with Adachi’s model. With increasing Fe composition, refractive index n at 532 nm increases, single oscillator energy E c and dispersion energy E d increase, but optical band gap E g decreases from 3.78 to 3.63 eV. In addition, nonlinear optical properties of the Fe-doped BaTiO 3 films were analyzed by Z-scan method. Nonlinear optical responses of the BaTiO 3 films were enhanced by Fe doping. For BaTi 0.96 Fe 0.04 O 3 film, the third-order nonlinear refractive index coefficient γ , the nonlinear refractive index coefficient n 2 , the nonlinear absorption coefficient β and the third-order nonlinear susceptibility χ (3) are 1.98 × 10 −13 m 2 /W, 1.03 × 10 −6 esu, 1.33 × 10 −7 m/W, 2.38 × 10 −7 esu respectively. The nonlinearity is predominated by the nonlinear refractive behavior. These results suggest that Fe doping may tune the linear and nonlinear optical properties of BaTiO 3 films, which indicates that Fe-doped BaTiO 3 films are promising materials for applications in optical devices. [ABSTRACT FROM AUTHOR]

A series of oxyfluoride glass ceramics containing CaF 2 nano-crystals tri-doped with Ce 3+ /Er 3+ /Yb 3+ ions were prepared by high temperature melting method and subsequent heat treatment. The structural properties were examined by X-ray diffraction measurements. The absorption, excitation, and emission spectra of the glass ceramics were investigated. Difference in erbium emission spectra between glass and glass ceramics had been studied. The emission bands originating from the 4 F 9/2 state of Er 3+ were enhanced when the CaF 2 nano-crystal created. By down-converting the ultraviolet wavelength region (280∼400 nm) light and up-converting the near-infrared wavelength region (900∼1100 nm) light, the glass ceramics can also emit strong reddish orange emission. The emission spectra consisting of bluish violet (400∼500 nm) and reddish orange (640∼680 nm) bands match well with the action spectrum of photosynthesis and absorption spectra of chlorophylls. Our materials will be favored to promote the development of glass greenhouses for green plant. [ABSTRACT FROM AUTHOR]

In this work, monodisperse Mg 2+ doped KMnF 3 :Yb 3+ /Er 3+ nanoparticles (NPs) are prepared via a fast solvothermal method. With the excitation of 980 nm laser diode, a brightly single red upconversion (UC) luminescence can be observed from KMnF 3 :Yb 3+ /Er 3+ NPs, which is further improved by introducing Mg 2+ ions. Moreover, the single red UC fluorescence intensity of the KMnF 3 :Yb 3+ /Er 3+ NPs co-doped with 30 mol% Mg 2+ ions is enhanced by 20.8 times compared to that of Mg 2+ ions-absent sample. Besides, the TEM measurement of the 30 mol% Mg 2+ doped KMnF 3 :Yb 3+ /Er 3+ NPs shows a biocompatible ultra-small crystal size (about 10 nm). All results indicate that Mg 2+ -doped is an effective method to significantly improve the UC single-red light of KMnF 3 :Yb 3+ /Er 3+ , which shows great potential application in vivo bioimaging. [ABSTRACT FROM AUTHOR]

The Bi 3+ /Yb 3+ co-doped transparent oxyfluoride glass–ceramics had been synthesized successfully. The formation of nanocrystals was studied using X-ray diffraction and scanning electron microscopy. The structure of fluoride nanocrystals indicates that the main phase in the oxyfluoride glass–ceramics is CaF 2 nanocrystal sized at 25 nm by thermal treatment at 600 °C for 8 h. The metal ion Bi 3+ has been shown to be an efficient broadband sensitizer for Yb 3+ ion in oxyfluoride glass–ceramics. Under the ultraviolet excitation, an intense near-infrared emission of Yb 3+ : 2 F 5/2 → 2 F 7/2 around 1 µm was observed, due to the cooperative energy transfer from one Bi 3+ to two Yb 3+ ions. Yb 3+ concentration-dependent quantum efficiency was evaluated and the maximum efficiency approached 170.1%. The oxyfluoride glass–ceramics are promising for practical application to enhance the energy conversion efficiency of crystalline Si solar cells via spectrum modification. [ABSTRACT FROM AUTHOR]

The Bi 3+ /Yb 3+ ion co-doped 55SiO 2 –20Al 2 O 3 –5Na 2 CO 3 –20CaF 2 glasses are synthesized successfully by a conventional melting-quenching method. High efficient quantum cutting involving the emission of two near-infrared photons for one ultraviolet photon absorbed is realized in the oxyfluoride glasses co-doped with Bi 3+ and Yb 3+ . An intense characteristic near-infrared emission around 977 nm of Yb 3+ : 2 F 5/2 → 2 F 7/2 transition is obtained when the 303 nm is as excitation wavelength to induce the 1 S 0 → 3 P 1 transition of Bi 3+ . The maximum quantum efficiency of our glasses is estimated to be 164.3%. The energy transfer mechanism is proposed to be a cooperative energy transfer via second-order down-conversion process. The glasses could be a potential quantum cutting converter to improve the photovoltaic energy conversion efficiency of crystalline Si solar cells via spectrum modification. [ABSTRACT FROM AUTHOR]

Design and enhancing the luminescence of rare earth materials under pressure have been important topics in the past few years. Here we report a new strategy for realizing pressure induced emission enhancement of rare earth element thulium (Tm) by confining the dopant luminescentions in a two-dimensional hexagonal NaYF 4 planes, NaYF 4 @β-NaYF 4 :Yb 0.2 ,Tm 0.005 @NaYF 4 delta-doped structure. Upon compression, NaYF 4 @β-NaYF 4 :Yb 0.2 ,Tm 0.005 @NaYF 4 exhibits a 2 fold increase in emissions, obviously higher than that of corresponding homogeneous doped samples. When Tm3+ was overdoped, this pressure induced emissions enhancement would absent in the whole pressure region due to concentration quenching. Moreover, both NaYF 4 @β-NaYF 4 :Yb 0.2 ,Tm 0.005 @NaYF 4 and corresponding homogeneous doped samples undergo structure transition under pressure. Raman study and calculations show that the delta-doping induced phonon peak red shift and reduced defects density increasing rate could play major roles for the pressure-induced emission enhancement of NaYF 4 @β-NaYF 4 :Yb 0.2 ,Tm 0.005 @NaYF 4 , and the deformation of the Y-F 9 polyhedrons at vertices leading to the structure transitions of both delta and homogeneous doped samples. These findings suggest that the pressure induced emission enhancement of rare earth materials can be realized by passivating inner and surface defects using a delta-doped structure. Image 1 • Realizing pressure induced emission enhancement of Tm by using δ-doped structure. • Reduced defect density increasing rate contribute emission enhancement under pressure. • The doping content of Tm plays a key role in their emissions under pressure. • Homogeneous and δ-doped β-NaYF 4 :Yb,Tm undergo structure transitions under pressure. [ABSTRACT FROM AUTHOR]