Ultraviolet and visible upconversion luminescence of Tm(0.1)Yb(5):FOV oxyfluoride nanophase vitroceramics

The ultraviolet upconversion luminescence of Tm³⁺ ions sensitized by Yb³⁺ ions in oxyfluoride nanophase vitroceramics when excited by a 975 nm diode laser was studied. An ultraviolet upconversion luminescence line positioned at 363.6 nm was found. It was attributed to the fluorescence transition of ¹D₂→³H₆ of Tm³⁺ ion. Several visible upconversion luminescence lines at 450.7 nm, (477.0 nm, 462.5 nm), 648.5 nm, (680.5 nm, 699.5 nm) and (777.2 nm, 800.7 nm) were also found, which result respectively from the fluorescence transitions of ¹D₂→³F₄, ¹G₄→³H₆, ¹G₄→³F₄, ³F₃→³H₆ and ³H₄→³H₆ of Tm³⁺ ion. The careful measurement and analysis of the variation of upconversion luminescence intensity F as a function of the 975 nm pumping laser power P prove that the upconversion luminescence of ¹D₂ state is partly a five-photon upconversion luminescence, and the upconversion luminescence of ¹G₄ state and ³H₄ state are respectively the three-photon and two-photon upconversion luminescence. The theoretical analysis suggested that the upconversion mechanism of the 363.6 nm ¹D₂→³H₆ upconversion luminescence is partly the cross energy transfer of {³H₄(Tm³⁺), ³F₄(Tm³⁺), ¹G₄(Tm³⁺)→¹D₂(Tm³⁺)} and {¹G₄(Tm³⁺)→³F₄(Tm³⁺), ³H₄(Tm³⁺)→¹D₂(Tm³⁺)} between Tm³⁺ ions. In addition, the upconversion luminescence of ¹G₄ and ³H₄ state results respectively from the sequential energy transfer {²F_5/2(Yb³⁺)→²F_7/2(Yb³⁺), ³H₄(Tm³⁺)→¹G₄(Tm³⁺)} and {²F_5/2(Yb³⁺) →²F_7/2(Yb³⁺), ³F₄(Tm³⁺)→³F₂(Tm³⁺)} from Yb³⁺ ions to Tm³⁺ ions. Supported by the National Natural Science Foundation of China (Grant No. 10674019)     

Optical gain by upconversion in Tm–Yb oxyfluoride glass ceramic

Evidence of positive optical gain is observed in Tm³⁺–Yb³⁺-codoped oxyfluoride glass ceramic in an upconversion pump and probe experiment. The ¹G₄ level of the Tm³⁺ ions is populated by an upconversion mechanism under excitation of the Yb³⁺ ions at 975 nm with a high-power pulsed laser and give rise to an intense emission from the ¹G₄ to the ³F₄ levels. The ¹G₄→³F₄ electronic transition is stimulated with a low signal at 650 nm as a probe.     

650-nm laser emission under upconversion pumping at 1.064 μm in Tm<Superscript>+3</Superscript>: ZBLAN fiber laser

We report for the first time continuous wave (CW) red laser emission in Tm⁺³-doped ZBLAN fiber laser, operated at 650 nm (¹ G ₄ → ³ F ₄ transition of Tm⁺³). The excitation uses a three step upconversion scheme. The pump source is a Nd:YAG laser operated at 1.064 μm. A laser output power of CW 80 mW was obtained for 1.42 W of launched pump power. The slope efficiency with respect to launched pump power was measured to be 7.7%. The temporal behavior of the emitted laser is also addressed.     

Upconversion excitations in Pr<Superscript>3+</Superscript>-doped BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystal

We report the orange-to-blue and infrared-(IR)-to-blue wavelengths upconversion luminescence in Pr³⁺:BaY₂F₈ crystals. Mechanism of the orange light upconversion into blue ³P₀ state emission was confirmed to be energy transfer between two Pr³⁺ ions in the ¹D₂ state. IR-to-blue upconversion has only been observed under two different color IR pumping. The first resonant step was the ³H₄→¹G₄ ground state absorption transition, and the second resonant transition was the excited state absorption from the ¹G₄ to ¹I₆ and ³P_J levels. A comparison of the efficiency of the IR-to-blue upconversion in several praseodymium activated host is presented and discussed. A model of the IR pumped upconversion praseodymium blue laser is presented and the population inversion conditions are calculated.     

Upconversion in Ho<Superscript>3+</Superscript>-doped YbF<Subscript>3</Subscript> particle prepared by coprecipitation method

YbF₃ particles doped with Ho³⁺ were synthesized by coprecipitation method, from which the ultraviolet and visible emission bands of the Ho³⁺ and the 480 nm cooperative upconversion emission of Yb³⁺–Yb³⁺ are observed under 980 nm excitation. Under the same excitation power, the emission intensity of Ho³⁺ in coprecipitation method is enhanced by about two times comparing to that in solid-state reaction method. The novel ultraviolet and violet emissions of the Ho³⁺ are firstly obtained which are centered at 360 (⁵G₂→⁵I₈),391 (³K₇→⁵I₈),412 (⁵G₄→⁵I₈), and 446 nm (⁵G₅→⁵I₈). The luminescence decay profiles of 545 and 652 nm visible emissions were obtained with a 980 nm pulsed laser. The excitation power dependence of the emission intensity was also measured and intensity saturation was observed. Based on the level structures of Ho³⁺, two- and three-photon processes are suggested to perform populations of ⁵S₂ and ⁵G₃ (Ho³⁺) levels, respectively. The dominant upconversion mechanism may be attributed to a cooperative sensitization process of two excited states of Yb³⁺ and energy transfers from Yb³⁺ to Ho³⁺.     

Emission analysis of RE3+ (Eu3+, Tb3+ & Ho3+): B2O3-BaO-LiF/AIF3 glasses

We report here the luminescence spectra of certain rare earth ions (Eu³⁺, Tb³⁺ & Ho³⁺) doped B₂O₃-BaO-LiF/AiF₃ based on the measurements of emission and decay curves of prominent emission transitions. For both the reference host glasses, FTIR, XRD, DTA-TG profiles have been recorded to understand their structural and thermal properties. Eu³⁺ doped glasses have shown five emission transitions of ⁵D₀ → ⁷F_{01,2,3 & 4} located at 580nm, 593nm, 615nm, 655nm and 704nm respectively with an excitation at λ_exci = 392 nm (⁷F₀ → ⁵L₆). Also under an UV source, these europium glasses have displayed a bright red emission from their surfaces. Tb³⁺ glasses have exhibited four emission bands of ⁵D₄ → ⁷F_6,5,4,3 at 491nm, 547nm, 588nm and 625nm respectively with an excitation at λ_exci = 376 nm (⁷F₆ → ⁵G₆). Intense green emission from the glass surfaces has been noticed upon exposure to the UV source. Prominently bluish-green emission has been noticed from the surfaces of the holmium glasses under an UV source and same emission transition (⁵F₄ → ⁵I₈) at 519 nm with an excitation at λ_exci = 389 nm (⁵I₈ − ⁵G₄) has also been obtained from their measured emission spectra. For all the prominent emissions of the rare earth glasses, decay curves have been measured to compute their lifetimes.     

Enhancement of blue emission in β-NaYbF4:Tm3+/Nd3+ nanophosphors synthesized by nonclosed hydrothermal synthesis method

An improved nonclosed hydrothermal synthetic processing is used to synthesize Tm³⁺ and Nd³⁺ doped β-NaYbF₄ nanophosphors at 85°C in the air without any high-temperature and high-pressure treatments as a final step. The particles have average crystallite size around 40 nm as obtained by TEM and calculation in terms of the XRD data. Intense 475 nm blue upconversion emission originated from the ¹G₄→³H₆ transition of Tm³⁺ is observed under 808 nm excitation, and its intensity can be enhanced onefold by introducing Nd³⁺ ion. The dominant populating mechanisms for the β-NaYbF₄:Tm³⁺ and β-NaYbF₄:Tm³⁺/Nd³⁺ are thought as Tm³⁺→Yb³⁺→Tm³⁺ and Nd³⁺→Yb³⁺→Tm³⁺ energy transfer processes, respectively. The concentration quenching processes for blue and red emissions are discussed.     

Multiphoton ultraviolet and visible upconversion luminescence of TmYb co-doped oxyfluoride glass

The ultraviolet upconversion luminescence of Tm³⁺ ions sensitized by Yb³⁺ ions in oxyfluoride glass when excited by a 975 nm diode laser was studied in this paper. One typical ultraviolet upconversion luminescence lines positioned at 362.3 nm was found. It can be attributed to the five-photon upconversion luminescence transition of ¹D₂ → ³H₆. Several visible upconversion luminescence lines at 451.1 nm, (477.9 nm, 462.5 nm), 648.7 nm, (680.5 nm, 699.5 nm) and (777.5 nm, 800.7 nm) were found also, which results from the fluorescence transitions of five-photon ¹D₂ → ³F₄, three-photon ¹G₄ → ³H₆, three-photon ¹G₄ → ³F₄, two-photon ³F₃ → ³H₆ and two-photon ³H₄ → ³H₆ of Tm³⁺ ion, respectively. The theoretical analysis suggests that the upconversion mechanism of the 362.3 nm ¹D₂ → ³H₆ upconversion luminescence is the cross energy transfer of {³H₄(Tm³⁺) → ³F₄(Tm³⁺), ¹G₄(Tm³⁺) → ¹D₂(Tm³⁺)} and {¹G₄(Tm³⁺) → ³F₄(Tm³⁺), ³H₄(Tm³⁺) → ¹D₂(Tm³⁺)} between Tm³⁺ ions. In addition, the upconversion luminescence of ¹G₄ and ³H₄ state results from the sequential energy transfer {²F_5/2(Yb³⁺) → ²F_7/2(Yb³⁺), ³H₄(Tm³⁺) → ¹G₄(Tm³⁺)} and {²F_5/2(Yb³⁺) → ²F_7/2(Yb³⁺), ³F₄(Tm³⁺) → ³F₂(Tm³⁺)} from Yb³⁺ ions to Tm³⁺ions, respectively.     

Double Fluorescence Conversion in Ultraviolet and Visible Region for Some Praseodymium Complexes of Aromatic Carboxyates

Four praseodymium complexes of aromatic carboxylates (benzoate, 4-tert-butylbenzoate, 2-benzoylbe-noate, and benzimidazole-5-carboxylate) have been synthesized and characterized, whose photophysical properties have been studied with ultraviolet spectra, phosphorescence spectra, and fluorescence spectra. The fluorescent emission spectra of all praseodymium complexes show two emission peaks under the excitation band of 245 nm at about 395 and 595 nm respectively, while one peak under 415 nm at about 595 nm, which attributed to be ¹S₀ → ¹I₆ (395 nm) transition and the characteristic emission ¹D₂ → ³H₄ (595 nm) transition of Pr³⁺ ion. The ¹S₀ → ¹I₆ transition can be ascribed as the transition of charge transfer state, and the ¹D₂ → ³H₄ can be further proved that there exists an antenna effect in the fluorescence of praseodymium with aromatic carboxylic acids. In conclusion, the praseodymium complexes systems can realize the double fluorescent conversion in both ultraviolet and visible region and can be further studied the application of this conversion.     

Energy transfer and frequency upconversion in Yb3+–Er3+-doped PbO-GeO2 glass containing silver nanoparticles

We report the infrared-to-visible frequency upconversion in Er³⁺–Yb³⁺-codoped PbO-GeO₂ glass containing silver nanoparticles (NPs). The optical excitation is made with a laser at 980 nm in resonance with the ²F_5/2→²F_7/2 transition of Yb³⁺ ions. Intense emission bands centered at 525, 550, and 662 nm were observed corresponding to Er³⁺ transitions. The simultaneous influence of the Yb³⁺→Er³⁺ energy transfer and the contribution of the intensified local field effect due to the silver NPs give origin to the enhancement of the whole frequency upconversion spectra.     

Dy3+ emission in M5(PO4)3F (M = Ca, Ba) phosphor

Ultrafine M₅(PO₄)₃F:Dy³⁺ (M = Ca, Ba) phosphors were prepared via combustion process using metal nitrates as precursors. The formation of crystalline phosphate was confirmed by X-ray diffraction pattern. The PL excitation spectra show the excitation peaks observed at 250 to 400 nm due to f → f transition of Dy³⁺ ion, which are useful for solid-state lighting purpose (mercury free excitation). The PL emission of Dy³⁺ ion by 348 nm excitation gave an emission at 489 nm (blue), 582 nm (yellow) and 675 nm (red). All the characteristics of BYR emissions like BGR indicate that Dy doped Ca₅(PO₄)₃F and Ba₅(PO₄)₃F phosphors are good candidates that can be applied in solid-state lighting phosphor (mercury free excited lamp phosphor) and white light LED.      

Luminescence of praseodymium (III) in solutions of mixed-ligand complexes with β-diketones

We have found that it is possible to enhance the luminescence of Pr(III) in solutions of complexes with β-diketones: acetylacetone derivatives containing fluoroalkyl substituents of different lengths and structures. We have established that in the presence of organic solvents, second (additional) ligands, and surfactants shielding the central ion from the quenching effect of the water molecules (OH oscillators), the intensity of luminescence for Pr(III) increases by a factor of 65, 38, and 45 respectively. In this case, cationic surfactants form ionic associates with Pr(III) β-diketonates, with incorporation of one more β-diketone molecule (ratio Pr:β-diketonate:surfactant = 1:4:1). As a result of suppression of intramolecular energy losses in solutions of Pr(III) β-diketonates, it is possible to observe its rather intense luminescence at 605 nm (the transition ¹D₂ → ³H₄), 612 nm (³P₀ → ³H₆), and 646 nm (³P₀ → 3F₂). __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 746–750, November–December, 2006.     

Ultraviolet upconversion in Pr3+:Y2SiO5 crystal by Ar+ laser (488 nm) excitation

Ultraviolet upconversion fluorescence band (260–350 nm) has been observed from Pr³⁺:Y₂SiO₅ pumped by Ar⁺ ion laser (488 nm). Power dependence of the fluorescence emitted from 4f5d, ³P₀ and ¹D₂ were measured. The upconversion mechanism was analyzed using the rate equations with a simplified three level model. It appears that excited state absorption (ESA) is the dominant upconversion process for lower Pr³⁺ concentration and energy transfer upconversion (ETU) is dominant for higher Pr³⁺ concentration.     

Conversion of infrared radiation into visible emission in NaGd(WO4)2:Yb3+, Ho3+ crystals

NaGd(WO₄)₂:Yb³⁺, Ho³⁺ single crystals have been grown by the Czochralski technique along the (0 0 1) orientation. Conversion of the infrared (IR) radiation at 980 nm into the visible emission in NaGd(WO₄)₂ crystals containing several different concentrations of Yb³⁺ and Ho³⁺ has been investigated. The NaGd(WO₄)₂: 8 at. % Yb³⁺, 4 at. % Ho³⁺ system exhibits intense red upconverted emission originating from the ⁵F₅ level. The upconversion mechanism in a Ho³⁺-Yb³⁺ system under near infrared excitation is discussed. It is concluded that the green emission is excited by energy transfers from Yb³⁺ to Ho³⁺, whereas excited state absorption is involved in the excitation of red emission. The emission cross-section of the ⁵F₅→⁵I₈ transition at about 660 nm was estimated by using the Füchtbauer–Ladengurg formula. PACS 78.55.Hx; 78.20.-e     

Cooperative energy transfer and frequency upconversion in Yb3+-Tb 3+ and Nd 3+-Yb 3+-Tb 3+ codoped GdAl3(BO3)4 phosphors

Polycrystalline GdAl₃(BO₃)₄ phosphors codoped with Yb³⁺/Tb³⁺ and/or Nd³⁺/Yb³⁺/Tb³⁺ have been synthesized by combustion method. Upon excitation with a 980 nm laser diode, an intense green upconversion luminescence has been observed in GdAl₃(BO₃)₄:Yb,Tb phosphor. The quadratic dependence of the luminescence on the pump-laser power indicating a cooperative energy transfer process. Meanwhile, it is noticed that upon excitation with 808 nm laser diode, intense luminescence has clearly been detected in GdAl₃(BO₃)₄:Nd,Yb,Tb phosphor. The luminescence intensity exhibits also a quadratic dependence on incident pump-laser power. However, no green-emission has been observed in GdAl₃(BO₃)₄ phosphors codoped with Yb³⁺/Tb³⁺ or Nd³⁺/Tb³⁺ respectively upon excited at 808 nm laser diode. A proposed upconversion mechanism involving energy transfer from Nd³⁺ to Yb³⁺, and then a cooperative energy transfer process from two excited Yb³⁺ to Tb³⁺ has been presented.     

Ultraviolet and visible upconversion emission in Tb3+/Yb3+ co-doped fluorophosphate glasses

Ultraviolet and visible upconversion emissions in Tb³⁺/Yb³⁺ co-doped YF₃–BaF₂–Ba(PO₃)₂ glasses were observed under 980-nm laser diode excitation. The dependence of the emission intensities of Tb³⁺ on the pump power reveals that two-photon processes account for blue cooperative emission of Yb³⁺ at 476 nm and green upconversion emission of Tb³⁺ at 543 nm, and three-photon processes for ultraviolet emission of Tb³⁺ in the wavelength range of 379–435 nm. The effects of Tb³⁺ concentration on the emission intensity and the lifetime of Tb³⁺ and Yb³⁺ are investigated in detail. It is found that the cooperative energy transfer from a pair of excited Yb³⁺ ions to a ground Tb³⁺ ion is responsible for the appearance of blue and green upconversion emissions due to the ⁵D₄→⁷F _J (J=6,5,4,3) transitions of Tb³⁺, and the resonance energy transfer from Yb³⁺ to Tb³⁺ accounts for the population on the ⁵D₃,⁵G₆ level and ultraviolet upconversion emission.     

Strong cooperative upconversion luminescence of ytterbium doped oxyfluoride nanophase vitroceramics

The strong 479.1 nm blue cooperative upconversion luminescence of ytterbium Yb³⁺ ion doped oxyfluoride nanophase vitroceramics (Yb:FOV) is studied in this article. It is found that the 479.1 nm blue cooperative upconversion luminescence strength of Yb(5):FOV is 230 times greater than that of fluoride glass Yb(3):ZBLAN. The large enhancement on cooperative upconversion blue luminescence of this article results from the comprehensive improvement on the aspects of better coupled chance of the Yb³⁺-Yb³⁺ cluster, less cross-relaxation, better concentration contribution of Yb³⁺ activator, non-saturation, and better upconversion luminescence efficiency.     

Optical properties and upconversions in Er doped Li:TeO2 glass

A NIR excitation of Er³⁺ doped Lithium modified tellurite (Li:TeO₂) glass results in antistokes fluorescent emission near 380, 530, 551 and 654 nm (ultraviolet, green and red regions) in addition to NIR Stokes emission. The antistokes emissions are ascribed to transition from the excited ⁴G_11/2, ⁴S_3/2(²H_11/2) and ⁴F_9/2 levels in Er³⁺. The excitation involves three and two incident photons. On excitation with the green laser line at 532 nm also leads to similar emissions. The mechanisms involved in these processes are discussed on the basis of the known energy level diagram and the upconversion efficiency has been calculated. Lifetime of the ⁴S_3/2 level has been measured. The temperature dependence of the upconversion process has also been investigated.     

Effects of Zn impurities on the electronic properties of Pr doped CaTiO3

Microcosmic investigations of weak red-emitting materials are crucial for their further development and application. In this work, we have focused on the band structures and electronic properties of Pr mono- and (Zn, Pr) co-doped CaTiO₃ using density functional theory. Zn substitution for Ca or Ti tends to form clusters energetically with Pr substituting for Ca in CaTiO₃. In Pr mono-doped CaTiO₃, the O_2p→Ti_3d transition in CaTiO₃ host corresponds to the centered 330 nm excitation spectra. The gap states above the valence band of ∼1.30 eV and ∼2.06 eV are hybridized by Pr_4f, O_2p and Ti_3d orbitals. They are mainly due to Pr_4f orbitals in CaTiO₃:Pr. The former gap level is related to red emission at 614 nm due to ¹D₂→³H₄ transition of Pr³⁺ activator. The latter is related to the excitation spectra centered at 380 nm due to the low-lying Pr-to-mental intervalence charge transfer transitions (Pr³⁺-O²⁻-Ti⁴⁺?Pr⁴⁺-O²⁻-Ti³⁺). The band structures of (Zn, Pr) co-doped CaTiO₃ keep the similar gap levels to those in Pr mono-doped CaTiO₃. The incorporation of Zn brings out the two stronger localized gap states, which are hybridized by Pr_4f, O_2p and Ti_3d orbitals, in comparison with those in Pr mono-doped CaTiO₃. Therefore, when Zn impurities are added into Pr doped CaTiO₃, the present calculations visualize the two enhanced levels and the distorted structures around Pr.     

Multifunctional Er3+–Yb3+ codoped Gd2O3 nanocrystalline phosphor synthesized through optimized combustion route

This paper reports the synthesis of high upconversion luminescent Gd₂O₃: Er³⁺, Yb³⁺ nanophosphor through optimized combustion route using urea as a reducing agent. The paper also reports the first observation of upconversion emission bands extending upto the UV region (335, 366 and 380 nm) in Er³⁺–Yb³⁺ co-doped phosphor materials. The fuel to oxidizer ratio has been varied to obtain the maximum upconversion luminescence. Three high intensity bands are found at 408, 523–548 and 667 nm due to the ⁴G_11/2 → ⁴I_15/2, ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions, respectively, along with the other bands. Input excitation power dependence has been studied for different transitions, and the saturation effect and decrease in the slope of different transitions at higher input pump power has been explained. Heat treatments of the samples show change in crystallite phase/size and relative upconversion luminescence intensities of blue, green and red bands. The color of the phosphor emission has shown to be tunable with change in the crystal structure as well as on excitation laser power and Er³⁺–Yb³⁺ concentration. The property of color tunability of the phosphor material has been used to record the fingerprint in different colors. Also, the future prospect of the nanocrystalline phosphor material as a sensor for temperature, using FIR method, has been explored.