Sensitized red luminescence from Bi co-doped Eu: ZnO-B2O3 glasses

Photoluminescence properties of Bi³⁺ co-doped Eu³⁺ containing zinc borate glasses have been investigated and the results are reported here. Bright red emission due to a dominant electric dipole transition ⁵D₀→⁷F₂ of the Eu³⁺ ions has been observed from these glasses. The nature of Stark components from the measured fluorescence transitions of Eu³⁺ ions reveal that the rare earth ions could take the lattice sites of C_s or lower point symmetry in the zinc borate glass hosts. The significant enhancement of Eu³⁺ emission intensity by 346 nm excitation (¹S₀→³P₁ of Bi³⁺ ions) elucidates the sensitization effect of co-dopant. The energy transfer mechanism between sensitizer (Bi³⁺) and activator (Eu³⁺) ions has been explained.     

Luminescence of Ln3+ and WO 2 2+ ions in wolframylphosphate glasses

We investigate the luminescent properties of potassium wolframylphosphate glasses doped with Eu³⁺, Tb³⁺, and Dy³⁺ ions whose luminescence is excited by donor-acceptor interaction between the active WO ₂ ²⁺ and Ln³⁺ ions, as well as the migration of energy in the subsystems of each type of the active ions. Comparison of the obtained data with the results of investigation of the spectroscopic properties of Ln³⁺ in uranylphosphate materials shows that a sufficiently high degree of the ionicity of bonds of Ln³⁺ with the atoms of its first coordination sphere is preserved in wolframylphosphate matrices. We show that three stages of the decomposition of electron excitations are typical of the WO ₂ ²⁺ ions in wolframylphosphate glasses doped with Ln³⁺ and two stages in nonactivated glasses. The electron excitation energy transfer in the WO ₂ ²⁺ −Ln³⁺ system occurs due to induction-resonance interaction. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 620–625, September–October, 1997.     

Optical spectroscopy of Cr3+ and Cr3+-Tm3+ in alkaline silicate glasses

Conclusions Alkaline silicate glasses seem to be good candidates as host materials for codoping with Cr³⁺ and Tm³⁺. Cr³⁺ ions occupy mainly low-field sites in them, and their broadband emission overlaps Tm³⁺ excitation. As a consequence very high efficiency for the energy transfer Cr³⁺→Tm³⁺ is achieved with moderate concentrations of these ions. Moreover, the average lifetime of the Cr³⁺ ions in codoped glasses is long enough to allow significant energy storage by flashlamp pumping. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 5, pp. 115–120, September–October, 1995.     

Optical properties of Eu-doped zinc borate glasses

Photoluminescence, absorption and optical excitation spectra of undoped and Eu³⁺-doped binary zinc borate glasses of varied composition prepared by the melt quenching technique in air atmosphere were investigated. From photoluminescence measurements it was found that growing Eu³⁺ concentration leads to gradual disappearance of the broadband glass emission in the near-UV spectral range, while the red Eu³⁺ emission at 613 nm shows a considerable increase. This result together with excitation spectra suggests that the Eu³⁺ ions are excited via energy transfer from the initially excited glassy host. The glass composition strongly affects the position of the excitation maximum in the UV energy range.     

Photoluminescence characteristics and energy transfer between Bi3+ and Eu3+ in Gd2O3: Eu3+, Bi3+ nanophosphors

Bi³⁺ and Eu³⁺ codoped cubic Gd₂O₃ nanocrystals were prepared by the Pechini sol-gel method. Their photoluminescent properties were investigated under ultraviolet light excitation. The introduction of Bi³⁺ ions broadened the excitation band of Eu³⁺ emission, of which a new strong band occurred ranging from 320 to 380 nm due to the 6s²→6s6p transition of Bi³⁺ ions, implying a very efficient energy transfer from Bi³⁺ ions to Eu³⁺ ions. Upon 325 and 355 nm light excitation, the luminescent intensity of Eu³⁺ ions was remarkably improved by the incorporation of Bi³⁺ ions. But a significant quenching of Eu³⁺ emission was observed under 266 nm light excitation when Bi³⁺ was codoped. The possible energy transfer processes between Bi³⁺ and Eu³⁺ were discussed. The decay curves of Eu³⁺ emission under the excitation of 266 nm pulsed laser were measured and gave further evidence for our discussion.     

Synthesis and luminescent properties of Eu<Superscript>3+</Superscript>- and Eu<Superscript>2+</Superscript>-activated sodium pyrophosphate Na<Subscript>4</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

The luminescent properties of Eu³⁺ and Eu²⁺ ions in sodium pyrophosphate, Na₄P₂O₇, have been studied. The excitation spectrum of the Eu³⁺ emission in Na₄P₂O₇ consists of several sets of bands in the range 280–535 nm due to 4f–4f transitions of Eu³⁺ ions and a broad band with a maximum at about 240 nm interpreted to be due to a charge transfer (CT) transition from oxygen 2p states to empty states of the Eu³⁺ 4f⁶-configuration. Although the CT band energy is large enough, the quantum efficiency (η) of the Eu³⁺ emission in Na₄P₂O₇ under CT excitation was estimated to be very low (η ≤ 0.01). In terms of a configurational coordinate model, this fact is interpreted as a result of the high efficiency of a radiationless relaxation from the CT state to the ⁷F₀ ground state of Eu³⁺ ions occupying sodium sites in Na₄P₂O₇. A strong reducing agent is required in order to stabilize Eu²⁺ ions in Na₄P₂O₇ during the synthesis. Several nonequivalent Eu²⁺ luminescence centers in Na₄P₂O₇ were found.     

The dependence of luminescence on reduction of Sm2+ ions doped in lithium barium borate glasses

Sm³⁺-doped Li₂O–BaO–B₂O₃ glass was prepared by the conventional melt quenching method in air atmosphere. Sm²⁺ ions were obtained by two methods, i.e. heating the as-made glass in a reducing atmosphere and irradiating the sample under X-rays. The two obtained samples were investigated by luminescence spectra and lifetime measurements. It was found that the conversion of Sm³⁺→Sm²⁺ after X-ray irradiation is efficient in this borate glass. Photo-stability of Sm²⁺ ions was evaluated by the photo-bleaching method. Furthermore, thermo-luminescence was also measured. The different defects and the reduction mechanism of Sm²⁺ ions in this borate glass were discussed. This would be helpful to understand the reduction mechanism of Sm²⁺ ions in borate glasses.     

Study of frequency upconversion in Yb<Superscript>3+</Superscript>/Eu<Superscript>3+</Superscript> by cooperative energy transfer in oxyfluoroborate glass matrix

Oxyfluoroborate glass co-doped with Eu and Yb ions has been prepared and characterized for its optical properties through photoluminescence, absorption and lifetime measurements. An intense red upconversion is observed from the ⁵D₀ level of Eu³⁺ ions through energy transfer from Yb³⁺ to Eu³⁺ ion when excited with 980 nm. The Judd–Ofelt parameters have been evaluated to estimate the local site symmetry around the Eu³⁺ ions. These parameters have been used to derive radiative properties such as transition probabilities, branching ratios, radiative lifetimes and stimulated emission cross-sections for the ⁵D₀–⁷F_J transitions. Decay of excitation of the ⁵D₀ level has been measured by monitoring the ⁵D₀→⁷F₂ transition (613 nm) at room temperature. Quantum efficiency for this transition is found to be 73%. We also monitored the effect of temperature on the fluorescence emission of Eu³⁺. PACS 42.70.Ce; 42.70.Hj     

Luminescence Properties of Dual Valence Eu Doped Nano-crystalline BaF2 Embedded Glass-ceramics and Observation of Eu2+ → Eu3+ Energy Transfer

Europium doped glass-ceramics containing BaF₂ nano-crystals have been prepared by using the controlled crystallization of melt-quenched glasses. X-ray diffraction and transmission electron microscopy have confirmed the presence of cubic BaF₂ nano-crystalline phase in glass matrix in the ceramized samples. Incorporation of rare earth ions into the formed crystalline phase having low phonon energy of 346 cm⁻¹ has been demonstrated from the emission spectra of Eu³⁺ ions showing the transitions from upper excitation states ⁵D_J (J = 1, 2, and 3) to ground states for the glass-ceramics samples. The presence of divalent europium ions in glass and glass-ceramics samples is confirmed from the dominant blue emission corresponding to its 5d-4f transition under an excitation of 300 nm. Increase in the reduction of trivalent europium (Eu³⁺) ions to divalent (Eu²⁺) with the extent of ceramization is explained by charge compensation model based on substitution defect mechanisms. Further, the phenomenon of energy transfer from Eu²⁺ to Eu³⁺ ion by radiative trapping or re-absorption is evidenced which increases with the degree of ceramization. For the first time, the reduction of Eu³⁺ to Eu²⁺ under normal air atmospheric condition has been observed in a BaF₂ containing oxyfluoride glass-ceramics system.     

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.     

Luminescence of europium and ytterbium ions in strontium hexaborate

We have studied the luminescent properties of Eu^2+/3+ and Yb²⁺ ions in strontium hexaborate SrB₆O₁₀ for excitation in the 120–400 nm region. The luminescence spectra of Ln²⁺ ions in SrB₆O₁₀ consist of overlapping bands in the 370–520 nm region, due to 5d → 4f transitions at several nonequivalent centers. In the excitation spectra, besides the bands associated with 4f → 5d transitions in the Ln²⁺ ions, we also observe a band in the 135–160 nm region due to the transitions O(2p) → B(2s,2p) within the borate anions. The luminescence of the Eu³⁺ ions is excited most efficiently in the region of the Eu³⁺ charge transfer band (λ_max = 226 nm). The results obtained are compared with data for Ln in other strontium borates. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 770–774, November–December, 2006.     

Energy transfer from the host to Er<Superscript>3+</Superscript> dopants in semiconductor SnO<Subscript>2</Subscript> nanocrystals segregated in sol–gel silica glasses

Undoped and Er³⁺-doped glass–ceramics of composition (100−x)SiO₂–xSnO₂, with x = 5 or 10 and with 0.4 or 0.8 mol% of Er³⁺ ions, were synthesised by thermal treatment of precursor sol–gel glasses. Structural studies were developed by X-Ray Diffraction. Wide band gap SnO₂ semiconductor quantum-dots embedded in the insulator SiO₂ glass are obtained. The mean radius of the SnO₂ nanocrystals, ranging from 2 to 3.2 nm, is comparable to the exciton Bohr radius. The luminescence properties have been analysed as a function of sample composition and thermal treatment. The results show that Er³⁺ ions are partially partitioned into the nanocrystalline phase. An efficient UV excitation of the Er³⁺ ions by energy transfer from the SnO₂ nanocrystal host is observed. The Er³⁺ ions located in the SnO₂ nanocrystals are selectively excited by this energy transfer mechanism. On the other hand, emission from the Er³⁺ ions remaining in the silica glassy phase is obtained by direct excitation of these ions.     

Energy transfer between Ce3+ and Eu2+ in doped KSrPO4

Powder samples of KSrPO₄ doped with Eu²⁺ and Ce³⁺ were prepared by combustion-assisted synthesis. Their structures and photoluminescence spectra were systemically studied. Energy transfer from Ce³⁺ to Eu²⁺ was observed by investigating the optical properties from photoluminescence spectra in Eu²⁺ single doped and Ce³⁺–Eu²⁺ co-doped KSrPO₄. The enhancement of UV excitation is attributed to energy transfer from Ce³⁺ to Eu²⁺, and Ce³⁺ plays a role as a sensitizer. Ce³⁺–Eu²⁺ co-doped KrSrPO₄ powders can possibly be applied as blue phosphors in the fields of lighting and display.     

Blue light emission from Eu ions in sol-gel-derived Al2O3-SiO2 glasses

Blue light-emitting glasses were successfully prepared by doping Eu²⁺ ions in the system Al₂O₃-SiO₂. The Al₂O₃-SiO₂ glasses doped with Eu³⁺ ions were synthesized using a sol-gel method, followed by heating in hydrogen gas atmosphere to reduce into the Eu²⁺ ions. The obtained glasses exhibited emission spectra with peak at ∼450 nm due to 4f⁶5d→4f⁷ (⁸S_7/2) transition, the intensities of which strongly changed depending on their glass composition and heating conditions. The emission quantum efficiency of 48% was achieved by heating the glass with the ratio of Al³⁺ to Eu³⁺ at about 6 at 1000 °C in hydrogen gas atmosphere. It was found that the Al₂O₃-SiO₂ glasses were appropriate not only for homogeneously doping the Eu³⁺ ions in glass structure but also reducing to Eu²⁺ ions, resulting in enhanced blue light-emission properties.     

Europium doped strontium borate glasses and their optical properties

Optical absorption and luminescence spectra of europium doped strontium borate glasses prepared in different conditions are studied. It is found that the percentage of Eu³⁺ ions varies from 100 to 30% being controlled by the conditions of preparation. The mechanism, favoring reduction of europium to Eu²⁺ state in polycrystalline strontium tetraborate, is much weaker in glasses of the same composition. In samples containing mixed valence europium at densities of 8×10²⁰ cm⁻³, the efficient transfer of optical excitation from Eu³⁺ to Eu²⁺, suppressing the Eu³⁺ luminescence, has been found. The most reliable way of monitoring the percentage of europium ions in different valences for strontium borate glasses is the measuring of absorption at f-f transition ⁷F₀→⁵D₂ of Eu³⁺.     

Relaxation of excited states of Tm3+ and Tm3+-Eu3+ energy transfer in YVO4 crystal

Tm³⁺-Eu³⁺ energy transfer processes and relaxation dynamics of the ³ H ₄ and ³ F ₄ excited states of Tm³⁺ ions in 1 at. % Tm³⁺, 5 at. % Eu³⁺:YVO₄ single crystal were studied. Contribution of Tm³⁺-Eu³⁺ energy transfer reduces effectively the lifetime of terminal level in a potential ³ H ₄–³ F ₄ laser transition at around 1.48 μm. Adverse quenching of the ³ H ₄ emission by Eu³⁺ ions is found to be less efficient than that reported for Tm³⁺ + Tb³⁺ system in YVO₄. The classical Inokuti–Hirayama model accounts well for an experimental decay curve of the ³ H ₄ emission recorded for co-doped crystal. Stimulated emission cross section for ³ H ₄–³ F ₄ transition of Tm³⁺ at around 1.48 μm was analyzed taking into account the anisotropy of YVO₄ crystal. PACS 42.55.Xi; 42.62.Fi     

luminescence of BaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals activated by Eu<Superscript>2+</Superscript> and Ce<Superscript>3+</Superscript> ions

We have studied the effect of doping with Eu²⁺ and Ce³⁺ ions on the photoluminescence (PL) of BaGa₂Se₄ crystals in the temperature range 77–300 K. We have established that the broad bands with maxima at wavelengths 456 nm and 506 nm observed in the photoluminescence spectra of BaGa₂Se₄:Ce³⁺ crystals are due to intracenter transitions 5d → ²F_7/2 and 5d →²F_5/2 of the Ce³⁺ ions, while the broad photoluminescence band with maximum at 521 nm in the spectrum of BaGa₂Se₄:Eu²⁺ is associated with 4f⁶ 5d → 4f⁷ (⁸S_7/2) transitions of the Eu²⁺ ion. We show that in BaGa₂Se₄:Eu²⁺,Ce³⁺ crystals, excitation energy is transferred from the Ce³⁺ ions to the Eu²⁺ ions.     

Time Resolved Fluorescence and Energy Transfer Analysis of Nd<Superscript>3+</Superscript>–Yb<Superscript>3+</Superscript>–Er<Superscript>3+</Superscript> Triply-Doped Ba–Al-Metaphosphate Glasses for an Eye Safe Emission (1.54 μm)

This paper reports on the preparation and systematic analysis of energy transfer mechanisms in Nd³⁺–Yb³⁺–Er³⁺ co-doped new series of barium-alumino-metaphosphate glasses. The time resolved fluorescence of Nd³⁺ in triply doped Ba–Al-metaphosphate glasses have revealed that, Yb³⁺ ions could function as quite efficient bridge for an energy transfer between Nd³⁺ and Er³⁺ ions. As a result, a fourfold emission enhancement at 1.54 μm of Er³⁺ ions has been achieved through an excitation of ⁴F_5/2 level of Nd³⁺ at 806 nm for the glass having 3 mol% Yb³⁺ with an energy transfer efficiency reaching up to 94%. Decay of donor (Nd³⁺) ion fluorescence has been analyzed based on theoretical models such as direct energy transfer model (Inokuti–Hirayama) and migration assisted energy transfer models (Burshtein’s hopping and Yokota–Tanimoto’s diffusion). The corresponding energy transfer parameters have been evaluated and discussed. Primarily, electrostatic dipole–dipole (s ~ 6) interactions are found to be responsible for the occurrence of energy transfer process in theses glasses.     

Analysis of the optical properties of Er3+-doped strontium barium niobate nanocrystals using time-resolved laser spectroscopy

This paper reports the results obtained in strontium barium niobate (SBN) nanocrystals in glasses doped with 1, 2.5 and 5 mol% of Er³⁺ ions. The melt-quenching method was applied to fabricate the glasses with composition SrO–BaO–Nb₂O₅–B₂O₃ and further thermal treatment was used to obtain glass ceramic samples from the glass precursor. X-ray diffraction patterns confirmed the formation of SBN nanocrystals with an average size of about 50 nm in diameter. Time-resolved fluorescence spectra for the emission of Er³⁺ ions at 1550 nm have been analyzed in order to confirm the incorporation of the Er³⁺ ions into the nanocrystals. Green frequency upconversion emission under excitation at 975 nm coming from the ions in the nanocrystals has been obtained. This intense upconversion is about a factor of 500 higher than that obtained from the ions which reside in the glassy phase. Moreover, temporal evolution studies have been carried out with the purpose of determining the involved upconversion mechanism and the importance of these processes as a source of losses for the optical amplification at 1550 nm.     

Site-selective spectroscopy of Er3+:Ti:LiNbO3 waveguides

Starting from previous investigations in LiNbO₃ bulk crystals, we studied the optical properties of Er³⁺ ions in Ti:LiNbO₃ channel waveguides and investigated the waveguide-specific lattice environment of the Er³⁺ ions (“sites”) caused by the doping method used and the presence of a large number of Ti⁴⁺ ions. For that purpose the method of combined excitation–emission spectroscopy was applied for the first time to waveguides at low temperatures. Comparing the spectroscopic results obtained for the green, red, and near-IR luminescence (λ≈550, ≈650 and ≈980 nm) under direct (450 nm), 2-step (980 nm), and 3-step (1.5 μm) laser excitation, we found several distinguishable Er³⁺ sites which in terms of energy levels and relative numbers are similar to those in bulk material, but exhibit significantly different up-conversion efficiencies and strongly inhomogeneously broadened transitions. Moreover, we were able to distinguish isolated and cluster Er³⁺ sites by their characteristic excitation and emission transition energies and studied the respective excitation/relaxation channels. The cluster sites are most efficient in the up-conversion process, especially under 3-step excitation. Using accepted microscopic models for Er³⁺ and Ti⁴⁺ incorporation into the LiNbO₃ crystal lattice, the site distribution and up-conversion mechanisms are elucidated and their consequences for laser applications in different spectral regions are discussed. Received: 16 November 2000 / Published online: 21 March 2001