Laser-diode-pumped Tm:SrF2 single crystal for high efficiency CW laser operation at ∼2 µm.

A Tm:SrF2 single crystal is grown by a modified temperature gradient technique (TGT). A study of the fluorescence characteristics and laser performance is carried out. Under laser-diode (LD) end-pumping, a continuous-wave (CW) laser is demonstrated. A slope efficiency of up to 81.8% is first achieved in the Tm-doped single crystals, which is extremely close to the photon quantum efficiency of 2.00. Such a high photon quantum efficiency is caused by the effective cross-relaxation process between Tm3+ ions. A maximum output power of 4.082 W is also obtained. The laser performance indicates that Tm:SrF2 is a promising candidate for highly efficient ∼2 µm lasers.

Site Occupancies, Electron-Vibration Interaction and Energy Transfer of CaMgSi2 O6 : Eu2+ , Mn2+ Phosphors for Potential Temperature-Sensing and Anti-counterfeiting Applications.

Eu2+ -, Mn2+ - and Eu2+ -Mn2+ -doped CaMgSi2 O6 phosphors have been prepared by a high-temperature solid-state reaction. Systematic investigation of the concentration- and temperature-dependent luminescence of Mn2+ showed that Mn2+ ions occupy two distinct sites in CaMgSi2 O6 . Electron-vibration interaction (EVI) analyses of Mn2+ ions revealed Huang-Rhys factors of 4.73 and 2.82 as well as effective phonon energies of 313 and 383 cm-1 for the two sites. Eu2+ -Mn2+ energy transfer is also discussed, and its efficiency is estimated by lifetime and luminescence spectra. The different thermal quenching behaviours of Eu2+ and Mn2+ , the distinct emission colours of Eu2+ (blue, band peak at ∼451 nm) and Mn2+ (yellow-red range, band peaks at ∼583 and 693 nm) endow the co-doped samples with potential applications in luminescence thermometry and temperature-/excitation wavelength-responsive dual anti-counterfeiting.

Intense 2.1-4.2 µm broadband emission of Co/Er:PbF2 mid-infrared laser crystal.

A novel mid-infrared (MIR) laser crystal Co/Er:PbF2 was successfully grown. The use of Er3+ ion co-doping to sensitize a Co2+ ion and enhance the 2.1-4.2 µm broadband MIR emission of the Co2+ ion in a PbF2 crystal was studied for the first time, to the best of our knowledge. The Er3+ ion was demonstrated to be an effective sensitizer of the Co2+ ion, making the Co/Er:PbF2 crystal propitious to be pumped by commercialized laser diodes. Furthermore, with Er3+ ion co-doping, the local symmetry of Co2+ and Er3+ ions was seriously distorted, thereby enhancing the 2.1-4.2 µm MIR emissions. This study provides a path for designing MIR laser materials with optimal performance.

Tailoring local coordination structure of the Er3+ ions for tuning the up-conversion multicolor luminescence.

The regulation of the local structure around Er3+ ions is an important channel for adjusting the characteristic of up-conversion luminescence. In this paper, the cubic-phased Er3+:CaF2 crystals with different Er3+ doping concentrations were fabricated with temperature gradient technique (TGT) method and the effect of the local coordination structure of the Er3+ ions on its luminescence performance was investigated. The local coordination structure of Er3+ ions was simulated by density functional theory. The computational results show that clusters evolve from low order to high order with the increase of Er3+ ion doping concentration. In this evolution process, the local structure transforms from cubic structure to the co-existence of cubic and lower symmetric square anti-prism structures. Meanwhile, the distance between Er3+ ions in the cluster decreased first and then increased slightly, and in dimers and trimers this distance reached the minimum. Under 980 nm excitation, with the increase of Er3+ ion concentration, the intensity ratios of the red and green emissions of Er3+:CaF2 first increased from 0.61 to 42.03 and then decreased to 12.11. The corresponding up-conversion luminescence gamut was adjusted from monochrome green to red to red-yellow. This work provides a new thread for realizing upconversion multicolor luminescence by regulating the clusters of rare earth ions.

Site Occupancies, VUV-UV-vis Photoluminescence, and X-ray Radioluminescence of Eu2+-Doped RbBaPO4.

RbBaPO4:Eu2+ phosphors have been prepared by a high-temperature solid-state reaction method, and the structure was determined by Rietveld refinement based on powder X-ray diffraction (P-XRD) data. Their VUV-UV-vis photoluminescence properties are systematically investigated with three objectives: (1) based on low-temperature spectra, we clarify the site occupancies of Eu2+, and demonstrate that the doublet emission bands at ∼406 and ∼431 nm originate from Eu2+ in Ba2+ [Eu2+(I)] and Rb+ [Eu2+(II)] sites, respectively; (2) an electron-vibrational interaction (EVI) analysis is conducted to estimate the Huang-Rhys factors, the zero-phonon lines (ZPLs) and the Stokes shifts of Eu2+ in Rb+ and Ba2+ sites; (3) the studies on luminescence decay of Eu2+(I) reveal that dipole-dipole interaction is mainly responsible for the energy transfer from Eu2+(I) to Eu2+(II), and the energy migration between Eu2+(I) is weak. Finally, the X-ray excited luminescence (XEL) spectrum indicates that the light yield of the sample RbBa0.995Eu0.005PO4 is ∼17 700 ph/MeV, showing its potential application in X-ray detecting.

The stability of coordination polyhedrons and distribution of europium ions in Ca6BaP4O17.

In this work, the coordination polyhedron stabilities and distributions of europium ions in Ca6BaP4O17 (CBPO) luminescent materials are investigated. The density functional theory (DFT)-based first principles calculation results show that the PO4 tetrahedrons can tilt in the structure, which leads to the atomic distortion of O13 and O12 in CBPO and the Eu2+/Eu3+-doped systems. The energy scale of about ∼0.1 eV suggests that stabilities of coordination polyhedrons are easily influenced by dynamic factors. The atomic distortion and vacancy of work as charge compensations in CBPO:Eu3+, and three lattice sites of europium are extracted and summarized. The X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) confirm that Eu3+ can occupy the Ca1, Ca2 and Ba sites of CBPO. The combination of first principles calculation and X-ray absorption fine structure (XAFS) provides more information about microstructures of luminescent materials.

Host Differential Sensitization toward Color/Lifetime-Tuned Lanthanide Coordination Polymers for Optical Multiplexing.

Optical multiplexing based on luminescent materials with tunable color/lifetime has potential applications in information storage and security. However, the available tunable luminescent materials reported so far still suffer from several drawbacks of low efficiency or poor stability, thus restraining their further applications. Herein, we demonstrate a strategy to develop efficient and stable lanthanide coordination polymers (LCPs) with tunable luminescence as a new option for optical multiplexing. Their multicolor emission from green to red and naked-eye-sensitive green emission with tunable lifetime (from ca. 300 to ca. 600 µs) can be controlled by host differential sensitization and energy transfer between lanthanide ions. The quantum efficiencies of developed samples range from around 20 % to 46 % and the luminescence intensity/lifetime appear quite stable in polar solvents up to ten weeks. Furthermore, with the aid of inkjet printing and concepts of luminescence lifetime imaging and time-gated imaging, we illustrate their promising applications of information storage and security in spatial and temporal domains.

Broadband, Polarization-Sensitive, and Self-Powered High-Performance Photodetection of Hetero-Integrated MoS2 on Lithium Niobate.

High-performance photodetectors hold promising potential in optical communication and imaging systems. However, conventional counterparts are suffering narrow detection range, high power consumption, and poor polarization sensitivity. Characteristics originating from switchable polarization in ferroelectrics can be used to optimize the photo-to-electric procedure and improve the photodetection performance. In this regard, we constructed a configuration by integrating 2-dimensional molybdenum disulfide (MoS2) with ferroelectric lithium niobate (LiNbO3), resulting in the MoS2/LiNbO3 heterostructured photodetector. Benefiting from the pyroelectric effect of LiNbO3, the limitation of bandgap on the detection range can be broken, thus broadening the response band of the detector to 365 to 1,064 nm, as well as enabling the self-powered characteristic. Meanwhile, high carrier mobility and decent light absorbance of MoS2 introduce robust light-matter interactions with the underlying LiNbO3, leading to ultrafast rise/fall times of ≈150 µs/250 µs and switching ratios of up to ≈190. Moreover, the highest responsivity, specific detectivity, and external quantum efficiency achieved were 17.3 A·W-1, 4.3 × 1011 Jones, and 4,645.78%, respectively. Furthermore, because of the anisotropy of the spontaneous-polarized LiNbO3 substrate, the photocurrent of the device achieved a dichroic ratio of 7.42, comparing favorably to most MoS2-based photodetectors. This work demonstrates the integration potential between ferroelectric LiNbO3 and 2-dimensional materials for high-performance photodetection.

Shining Transparent Displays with Stable Narrow-Band Blue-Emitting Phosphor in Layered Film.

Transparent displays (TDs) rendering "levitating" images on screen have appeared as an emerging technology toward augmented/mixed reality applications. However, the traditional phosphor design and screen construction have severely limited the TD performance owing to the lack of efficient narrow-band blue emitters and stable screen structure. Herein, the novel narrow-band (full width at half maximum: 32 nm) NaLi3 SiO4 :Eu2+ phosphor with a peak at 467 nm as a key blue emitter is explored, and it is sandwiched in layered film as a unique screen design. The devised screen features decent transparency, high emission color purity, and good reliability, and the TD prototype renders "floating" static images and vivid animation with broad viewing angle (15°-165°) and large color gamut (97% of National Television Standards Committee). Spectroscopic and microstructural characterizations reveal the TD superior performance originates from synergistic contributions of moderate crystal field effect (εc  ≈ 1.13 eV; εcfs  ≈ 1.60 eV), weak vibronic coupling (S ≈ 3; hω ≈ 285 cm-1 ), and limited thermal ionization of 5d electrons (Ea  ≈ 0.43 eV) for NaLi3 SiO4 :Eu2+ emission and layered architecture for screen film. These findings establish fundamental guidelines for narrow-band emitting materials design and shine light on superior TD innovative development.