VUV and UV Florescence and Absorption Studies of Tb3+ and Tm3+ Trivalent Ions in LiYF4 Single Crystal Hosts

Abstract

The laser induced fluorescence spectra of LiYF₄:Tb³⁺ (YLF:Tb) and LiYF₄:Tm³⁺ (YLF:Tm) single crystals, pumped by an F₂ pulsed discharge molecular laser at 157 nm, were obtained in the vacuum ultraviolet (VUV) and ultraviolet (UV) regions of the spectrum, at room temperature. A number of new fluorescence peaks were observed for the first time. They were assigned to the dipole allowed transitions 4f⁷5d → 4f⁸ and 4f¹¹5d → 4f¹² of Tb³⁺ and Tm³⁺ ions respectively. The absorption spectra of the same crystal samples in the VUV and UV regions were taken as well. The edge (onset) and the energy of the states with 4f^{N ? 1}5d configuration were determined.     

Multicolor upconversion of Er3+/Tm3+/Yb3+ doped oxyfluoride glass ceramics

Er³⁺/Tm³⁺/Yb³⁺ tridoped oxyfluoride glass ceramics was synthesized in a general way. Under 980 nm LD pumping, intense red, green and blue upconversion was obtained. And with those primary colors, multicolor luminescence was observed in oxyfluoride glass ceramics with various dopant concentrations. The red and green upconversion is consistent with ⁴F_9/2 → ⁴I_15/2 and ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺ respectively. While the blue upconversion originates from ¹G₄ → ³H₆ transition of Tm³⁺. This is similar to that in Er³⁺/Yb³⁺ and/or Tm³⁺/Yb³⁺ codoped glass ceramics. However the upconversion of Tm³⁺ is enhanced by the energy transfer between Er³⁺ and Tm³⁺.     

Synthesis and luminescence properties of Yb3+, Tm3+ and Ho3+ co-doped SrGd2(WO4)2(MoO4)2 nano-crystal

Novel up-conversion luminescent SrGd₂(WO₄)₂(MoO₄)₂: Yb³⁺/Tm³⁺/Ho³⁺ nano-crystals were synthesized by hydrothermal method. The composition ratio of rare earth had been investigated. It indicated that when C_Yb³⁺ = 10 mol% and C_Yb³⁺/C_Tm³⁺/C_Ho³⁺ = 10:1.5:2, the emission intensities were the highest. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and up-conversion luminescence spectra were used to characterize SrGd₂(WO₄)₂(MoO₄)₂: Yb³⁺/Tm³⁺/Ho³⁺ nano-crystals and they showed that the sample had high degree of crystallinity, the sample was tetragonal system, and the grain size of the sample was about 56 nm. Three emission peaks, including blue emission peak, green emission peak and red emission peak were observed at 477, 543 and 651 nm corresponding to ¹G₄ → ³H₆ and ¹G₄ → ³F₄ transitions of Tm³⁺, ⁵F₄ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺ respectively. All the emission peaks were observed by excitation of 980 nm semiconductor laser. The relationship between up-conversion intensity and excitation power revealed that blue emission at 477 nm was a three-photon absorption process, green emission at 543 nm and red emission at 651 nm was a two-photon absorption process. The quantum yields of the sample were near 3.2%.     

Synthesis of β–NaYF4: Yb3+, Tm3+ @ TiO2 and β–NaYF4: Yb3+, Tm3+ @ TiO2 @ Au nanocomposites and effective upconversion–driven photocatalytic properties

The β–NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂ nanocomposite has been prepared by a facile hydrothermal method followed by the hydrolysis of TBOT, and then NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂, HAuCl₄ and sodium citrate were put into an oil bath for reaction to obtain the β–NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂ @ Au core–shell nanocomposite. XRD and HRTEM show that the samples exhibit the hexagonal phase NaYF₄, anatase TiO₂ and cubic Au, indicating that the core–shell phases of NaYF₄−TiO₂ or NaYF₄−TiO₂−Au coexist in these samples. EDS and XPS results show the presence of Na, Y, F, Ti, O and Au elements. When TiO₂ was coated on the surface of upconversion nanomaterials of NaYF₄: Yb³⁺, Tm³⁺, the photocatalytic activity was improved significantly, and the β–NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂ nanocomposite gives the highest photodegradation efficiency for MB and RhB, and decomposes about 73% of MB or 80% of RhB within 4.5 h under simulated solar light irradiation respectively. When the ultraviolet light from simulated sunlight irradiation was removed by the addition of a UV filter, the β–NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂ nanocomposite decomposes about 42% of MB or 48% of RhB within 4.5 h. It means that the upconversion–driven photocatalytic performance (decomposes 42% of MB or 48% of RhB) is more effective than UV light–driven photocatalytic performance (31% of MB or 32% of RhB) in the photodegradation process. In addition, the β–NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂ @ Au core–shell nanocomposite does not exhibit the better photocatalytic activity, and the optimal research will be carried out in the future.     

Luminescent branching ratios and radiative lifetimes of Tm3+ in POCl3:SnCl4 laser liquid

For the first time, the optical spectrum of Tm³⁺ ion in POCl₃:SnCl₄ laser liquid was recorded in the UV-VIS and NIR regions. From the observed energy levels, Racah (E^h), spin-orbit (ξ_4f) and configuration interaction (α, β) parameters we evaluated by the least-squares fit method. Radiative transition probabilities and luminescent branching ratios were calculated for the excited fluorescent levels of Tm³⁺ ion using the Judd-Ofelt theory and the intensity parameters obtained from the measured intensities of the absorption bands.     

Design and achieving mechanism of upconversion white emission based on Yb/Tm/Er tri-doped KY3F10 nanocrystals

KY₃F₁₀:Yb³⁺/Tm³⁺/Er³⁺ upconversion nanocrystals are synthesized via a simple hydrothermal procedure. The nanocrystals emit the near equal energy white light with high brightness and favorable color balance when excited using a 980 nm continuous wave diode laser. The research of upconversion mechanism indicates that in addition to the energy transfer processes from Yb³⁺ to Tm³⁺ and Er³⁺, respectively, there exists a new process ¹G₄ (Tm³⁺) + ⁴I_11/2 (Er³⁺) → ³H₄ (Tm³⁺) + ⁴S_3/2 (Er³⁺).     

Photon-avalanche upconversion of Ho in NaYF4 and enhancement of violet, blue and green emissions induced by sensitization of Tm

Ho³⁺ singly doped and Ho³⁺/Tm³⁺ co-doped hexagonal NaYF₄ powders have been synthesized by a solid-state reaction method. Under excitation of 671 nm diode laser, upconverted blue, green and red emission bands are observed in Ho³⁺ singly doped sample. Temporal evolution and excitation power dependent behavior for the green emission are explored, indicating that a photon-avalanche mechanism is responsible for the upconversion processes in Ho³⁺ singly doped hexagonal NaYF₄ sample. With the introduction of Tm³⁺, the intensities of both blue and green emissions of Ho³⁺ are efficiently enhanced, which are attributed to two energy transfer processes from Tm³⁺ to Ho³⁺, i.e., ³F₄ (Tm³⁺) + ⁵I₈(Ho³⁺) → ³H₆ (Tm³⁺) + ⁵I₇(Ho³⁺) and ¹G₄ (Tm³⁺) + ⁵I₈(Ho³⁺) → ³H₆ (Tm³⁺) + ⁵F₃(Ho³⁺). The result offers a new sensitization approach to enhance the upconversion efficiency of Ho³⁺ under 671 nm excitation.     

Spectroscopic properties and energy transfer in Er–Tm co-doped bismuth silicate glass

In this paper, we investigate the spectroscopic properties of and energy transfer processes in Er–Tm co-doped bismuth silicate glass. The Judd–Ofelt parameters of Er³⁺ and Tm³⁺ are calculated, and the similar values indicate that the local environments of these two kinds of rare earth ions are almost the same. When the samples are pumped at 980 nm, the emission intensity ratio of Tm:³F₄ → ³H₆ to Er:⁴I_13/2 → ⁴I_15/2 increases with increased Er³⁺ and Tm³⁺ contents, indicating energy transfer from Er:⁴I_13/2 to Tm:³F₄. When the samples are pumped at 800 nm, the emission intensity ratio of Er:⁴I_13/2 → ⁴I_15/2 to Tm:³H₄ → ³F₄ increases with increased Tm₂O₃ concentration, indicating energy transfer from Tm:³H₄ to Er:⁴I_13/2. The rate equations are given to explain the variations. The microscopic and macroscopic energy transfer parameters are calculated, and the values of energy transfer from Er:⁴I_13/2 to Tm:³F₄ are found to be higher than those of the other processes. For the Tm singly-doped glass pumped at 800 nm and Er–Tm co-doped glass pumped at 980 nm, the pumping rate needed to realize population reversion is calculated. The result shows that when the Er₂O₃ doping level is high, pumping the co-doped glass by a 980 nm laser is an effective way of obtaining a low-threshold ∼2 μm gain.     

Luminescence properties and energy transfer studies of a color tunable BaY2Si3O10:Tm3+,Dy3+ phosphor

A series of color-tunable and white light emitting phosphors BaY₂Si₃O₁₀:Tm³⁺,Dy³⁺ were synthesized by a high temperature solid-state reaction, and their phase structure, photoluminescence properties, and energy transfer processes between rare-earth ions were investigated in detail. Upon UV excitation, white light emission depending on dopant concentrations could be achieved by integrating a blue emission band located at 458 nm and an orange one located at 576 nm attributed to Tm³⁺ and Dy³⁺ ions, respectively. In addition, the energy transfer process between Tm³⁺ and Dy³⁺ ions was demonstrated to be a resonant type via a dipole–quadrupole mechanism. Preliminary studies showed that the phosphor might be promising as a single-phased white-light-emitting phosphor for UV chip pumped white-light LEDs.     

Investigation of near-infrared-to-ultraviolet upconversion luminescence of Tm doped NaYF4 phosphors by Yb codoping

2 mol% Tm³⁺ doped NaYF₄ phosphors with 0–98 mol% Yb³⁺ codoping were synthesized by sol–gel method. The phase transition from the mixture of hexagonal and cubic phases to single cubic phase of Tm³⁺–Yb³⁺:NaYF₄ phosphors was investigated with increasing of Yb³⁺ concentration. Near-infrared, red, blue, violet and ultraviolet upconversion emissions of Tm³⁺ were observed from the Tm³⁺–Yb³⁺:NaYF₄ phosphors under 976 nm laser diode excitation, with the strongest near-infrared to ultraviolet emissions at 20 mol% Yb³⁺ codoping. The violet and blue emissions for the ¹D₂ → ³F₄ and ¹G₄ → ³H₆ transitions of Tm³⁺ can be tuned by varying Yb³⁺ codoping concentration, which was elucidated using steady-state equations. The intensity ratio of red emissions for the ³F₂ → ³H₆ and ³F₃→³H₆ transitions of Tm³⁺ was strongly related to the Yb³⁺ codoping concentration and temperature, implying a potential application of Tm³⁺–Yb³⁺:NaYF₄ phosphors for optical temperature sensing.     

Tm3+/Er3+/Yb3+-codoped oxyhalide tellurite glasses as materials for three-dimensional display

Blue, green and red emissions through frequency upconversion and energy transfer processes in Tm³⁺/Er³⁺/Yb³⁺-codoped oxyhalide tellurite glass under 980 nm excitation are investigated. The intense blue (476 nm), green (530 and 545 nm) and red (656 nm) emissions are simultaneously observed at room temperature. The blue (476 nm) emission was originated from the ¹G₄→³H₆ transition of Tm³⁺. The green (530 and 545 nm), and red (656 nm) upconversion luminescences were identified from the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺, respectively. The energy transfer processes and possible upconversion mechanisms are evaluated.     

Preparation and luminescence characteristics of LiYF4: Tm3+/Dy3+ single crystals for white-light LEDs

Tm³⁺ /Dy³⁺ co-doped LiYF₄ single crystals were synthesized by using vertical Bridgman method in sealed Pt crucibles. When excited by a proper UV-light, the crystals show blue emission band centered at 485 nm, which overlaps between the transition of Tm³⁺ (¹G₄ → ³H₆) and Dy³⁺ (⁴F_9/2 → ⁶H_15/2) ions, and yellow band of 573 nm ascribed to Dy³⁺ (⁴F_9/2 → ⁶H_13/2) ions. Both chromaticity coordinates and photoluminescence intensity vary with the excitation wavelengths and the concentration of rare earth dopants. A white light can be achieved from Tm³⁺ (0.6 mol%), Dy³⁺ (2.25 mol%) co-doped LiYF₄ crystal with chromaticity coordinates of x ≈ 0.2836, y ≈ 0.3229, and color temperature T _c = 8419 K by the excitation of a 350 nm light. It indicates that this crystal can be a potential candidate for the UV-light excited white-light emitting diodes.     

Realization of modulating broadband emission from Er–Tm codoped calcium boroaluminate glasses by dual-wavelength pumping

Near-infrared photoluminescence (PL) of calcium boroaluminate (CABAL) glasses codoped with Er₂O₃ and Tm₂O₃ has been investigated by dual-wavelength pumping at 795 and 476 nm. Spectrum shape of broadband emission could be modulated by controlling the power ratio of two pumping lines (P₄₇₆/P₇₉₅). The result shows that the full width at half maximum can reach ∼500 nm in the wavelength range from 1.3 to 2.0 μm by controlling P₄₇₆/P₇₉₅ = 12. The PL spectra show four characteristic peaks located at 1.46, 1.53, 1.58 and 1.80 μm, corresponding to Tm³⁺: ³H₄ → ³F₄, Er³⁺: ⁴I_13/2 → ⁴I_15/2, Tm³⁺: ¹G₄ → ³F₂ and Tm³⁺: ³F₄ → ³H₆ emissions, respectively. The energy transfer (ET) (ET1: Er³⁺: ⁴I_13/2, Tm³⁺: ³F₄ → Er³⁺: ⁴I_15/2, Tm³⁺: ³H₄ and ET2: Er³⁺: ⁴I_13/2, Tm³⁺: ³H₆ → Er³⁺: ⁴I_15/2, Tm³⁺: ³F₄) between Er³⁺ and Tm³⁺ ions play important roles in the luminescence mechanisms. In addition, a new ET process (ET: Tm³⁺: ¹G₄, Er³⁺: ⁴F_9/2 → Tm³⁺: ³F₂, Er³⁺: ⁴F_7/2) was identified. The flat broadband emission with the bandwidth of ∼500 nm could be realized by changing P₄₇₆/P₇₉₅ as a result of the radiative transitions, Tm–Tm cross-relaxation (Tm³⁺: ³H₄, ³H₆ → Tm³⁺: ³F₄, ³F₄) and Er–Tm ET processes.     

Blue and white light emission in Tm3+ and Tm3+/Dy3+ doped zinc phosphate glasses upon UV light excitation

A spectroscopic study based on photoluminescence spectra and decay time profiles in Tm³⁺ and Tm³⁺/Dy³⁺ doped Zn(PO₃)₂ glasses is reported. The Tm³⁺ doped Zn(PO₃)₂ glass, upon 357 nm excitation, exhibits blue emission with CIE1931 chromaticity coordinates, x = 0.157 and y = 0.030, and color purity of about 96%. Under excitations at 348, 352 and 363 nm, which match with the emissions of AlGaN and GaN based LEDs, the Tm³⁺/Dy³⁺ co-doped Zn(PO₃)₂ glass displays natural white, bluish white and cool white overall emissions, with correlated color temperature values of 4523, 10700 and 7788 K, respectively, depending strongly on the excitation wavelength. The shortening of the Dy³⁺ emission decay time in presence of Tm³⁺ suggests that Dy³⁺→Tm³⁺ non-radiative energy transfer occurs. By using the Inokuti-Hirayama model, it is inferred that an electric quadrupole-quadrupole interaction might be the dominant mechanism involved in the energy transfer. The efficiency and probability of this energy transfer are 0.12 and 126.70 s⁻¹, respectively.     

Photoluminescence of nanocrystalline YVO4:TmxDy1−x prepared by a modified Pechini method

This paper reports the luminescence effects of Tm³⁺ doped YVO₄:Dy nanocrystalline synthesized by a modified Pechini method. The structure and morphology were characterized by using X-ray diffraction (XRD) and transmission electron microscope (TEM). The relationship between the ratio of Tm³⁺/Dy³⁺ and the chromaticity is studied, i.e. Tm³⁺ ion doping effectively tunes the emission color of YVO₄:Tm_xDy_1−x phosphors. The best white light emission was observed with YVO₄:1%(Tm_0.6Dy_0.4). These results indicate that thulium doped YVO₄:Dy phosphors are promising white-emitting luminescence materials.     

Investigation on broadband near-infrared emission and energy transfer in Er-Tm codoped germanate glasses

Broadband near-infrared emission has been investigated in a new type host composition of Er³⁺-Tm³⁺ codoped germanate glass. A broadband emission extend from 1350 to 1675 nm with the full width at half maximum (FWHM) around 138 nm is obtained in the germanate glass which codoped with 0.2 wt.% Er₂O₃ and 0.8 wt.% Tm₂O₃. The energy transfer between Er³⁺ and Tm³⁺ plays an important role in the emission mechanism, which is evidenced by the visible upconversion and the lifetime of Er³⁺:⁴I_13/2 level effected by the addition of Tm₂O₃. And energy transfer efficiency from Er³⁺ to Tm³⁺ reaches 76% for the highest Tm³⁺ concentration of 0.8 wt.%. These results suggest that this glass would be a promising material for broadband light source and broadband amplifier for the wavelength division multiplexing transmission systems.     

Optical properties of the Tm and energy transfer between Tm→Pr ions in P2O5-CaO-SrO-BaO phosphate glass

A P₂O₅-CaO-SrO-BaO phosphate glass doped with Tm³⁺ and glasses doped with (Tm³⁺, Pr³⁺) were used for this study. The photo-luminescence behaviors of Tm³⁺ and Pr³⁺ in phosphate glass were investigated by absorption, excitation and emission spectroscopy. The energy transfer between Tm³⁺ and Pr³⁺ in phosphate glasses (which exhibit a variety of transfer efficiencies) was studied. The experimental quantum efficiencies of the luminescence of Tm³⁺ η₀ and (Tm³⁺, Pr³⁺) doped phosphate glasses were measured to give η/η₀ = 0.447, 0.305, and 0.179 for (0.4 mol% Pr³⁺, 1.0 mol% Tm³⁺), (0.8%Pr³⁺, 1.0%Tm³⁺) and (1.6 mol% Pr³⁺, 1.0 mol% Tm³⁺), respectively. In order to verify the nature of the ion coupling in our phosphate glass system, we applied the Inokuti-Hirayama model. The non-radiative energy transfer rate from Tm³⁺ to Pr³⁺, transfer efficiencies, and the donor-acceptor distance have been calculated and compared with obtained experimental data. As usual, the efficiency and the probability of energy transfer increase with the concentration of the acceptor.     

Controllable phase/morphology tailoring of REF3 and NaREF4 (RE = La-Lu,Y), and insights into the up-conversion luminescence of GdF3:Yb3+/Tm3+ spheres

Four typical rare earth fluorides of hexagonal/orthorhombic REF₃ and hexagonal/cubic NaREF₄ (RE = La-Lu, Y, excluding radioactive Pm), have been hydrothermally synthesized from mixed solutions of the rare earth nitrate solutions and ammonium fluoride (NH₄F), in the presence of EDTA-2Na. The phase and the morphology are closely related to the ionic radius of RE³⁺. Large RE³⁺ ions, La³⁺, Ce³⁺, Pr³⁺, Nd³⁺ and Sm³⁺, crystallize in hexagonal REF₃ (nanocrystals), and middle RE³⁺ ions (RE = Eu-Dy) crystallize in orthorhombic REF₃ (micron-sized spheres/polyhedral micro-crystallites). For small RE³⁺ ions, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺ and Y³⁺, they tend to being incorporated into the host lattice of cubic NaREF₄ (submicron spheres). However, the smallest Lu³⁺ tends to crystallizing in orthorhombic LuF₃. Varying the F^-/RE³⁺ molar ratio (R) and the reaction temperature can efficiently induce the phase/morphology evolution of rare earth fluorides. Increasing R up to 10–30 yielded hexagonal NaREF₄ (RE = Dy, Tb, Gd, Eu, Sm, micron prisms), although the original products were orthorhombic REF₃ (RE = Dy, Tb, Gd, Eu) and hexagonal SmF₃ at R = 5. However, increasing R is inefficient to the phase transition for large RE elements of La, Ce, Pr, Nd. Lowering the reaction temperature from 180 °C to 80 °C gave rise to the phase transition from orthorhombic LuF₃ to cubic NaLuF₄. GdF₃:Yb³⁺,Tm³⁺ spheres exhibited intense blue emission at ~476 nm (¹G₄ → ³H₆ transition of Tm³⁺), deep-red emission at ~646 nm (¹G₄→³F₄ transition of Tm³⁺), and near-infrared emission at ~695 nm (³F_2,3 → ³H₆ transitions of Tm³⁺) under the 980 nm infrared excitation. The intense near-infrared emission under the infrared excitation suggested that they might hold great potential in bio imaging application.     

Spectroscopic and laser characterization of Yb,Tm:KLu(WO4)2 crystal

We report on a comprehensive spectroscopic and laser characterization of monoclinic Yb,Tm:KLu(WO₄)₂ crystals. Stimulated-emission cross-section spectra corresponding to the ³F₄ → ³H₆ transition of Tm³⁺ ions are determined. The radiative lifetime of the ³F₄ state of Tm³⁺ ions is 0.82 ms. The maximum Yb³⁺ → Tm³⁺ energy transfer efficiency is 83.9% for 5 at.% Yb – 8 at.% Tm doping. The fractional heat loading for Yb,Tm:KLu(WO₄)₂ is 0.45 ± 0.05. Using a hemispherical cavity and 5 at.% Yb – 6 at.% Tm doped crystal, a maximum CW power of 227 mW is achieved at 1.983–2.011 μm with a maximum slope efficiency η = 14%. In the microchip laser set-up, the highest slope efficiency is 20% for a 5 at.% Yb– 8 at.% Tm doped crystal with a maximum output power of 201 mW at 1.99–2.007 μm. Operation of Yb,Tm:KLu(WO₄)₂ as a vibronic laser emitting at 2.081–2.093 μm is also demonstrated.     

Upconversion emissions in Yb<Superscript>3+</Superscript>–Tm<Superscript>3+</Superscript>-doped tellurite glasses excited at 976 nm

Intense Tm³⁺ blue upconversion emission has been observed in Tm³⁺–Yb³⁺ codoped oxyfluoride tellurite glass under excitation with a diode laser at 976 nm. Three emission bands centered at 475, 650 and 796 nm corresponding to the transitions ¹G₄→ ³H₆, ¹G₄→ ³H₄ and ³F₄→ ³H₆, respectively, simultaneously occur. The dependence of upconversion intensities on Tm³⁺ ions concentration and excitation power are investigated. For fixed Yb₂O₃ concentrations of 5.0 mol%, the maximum upconversion intensity was obtained with Tm₂O₃ concentration of about 0.1 mol%. The blue upconversion luminescence lifetimes of the Tm³⁺ transitions ¹G₄→ ³H₆ are measured. The results are evaluated by the possible upconversion mechanisms.