Effect of Network Modifier on Spontaneous Emission Probabilities of Er3+ in Oxide Glasses

Compositional dependence of spontaneous emission probabilities of Er₃₊ was studied for silicate, borate, and phosphate glasses using the Judd–Ofelt theory. Through all of the glass systems, spontaneous emission probabilities of the ⁴ I _13/2→⁴ I _15/29→⁴ I _9/2→⁴ I _J, and ⁴ S _3/2→⁴ I _J ( J = 9/2, 11/2,13/2,15/2) transitions increased with increased ionic packing ratio of the glass host, which varied markedly with the type of network modifier. The effect of Er–O covalency on the transition probabilities was discussed from the spectral profile of the ⁴ I _15/2→⁴ I _3/2transition in terms of nephelauxetic effect.     

Energy Transfer Process for the Blue Up-Conversion in Calcium Aluminate Glasses Doped with Tm3+ and Nd3+

Excitation of Tm³⁺ to ³ H ₄ using the 791 nm pump source showed the frequency up-converted blue emission (∼480 nm) due to the Tm³⁺:¹ G ₄→³ H ₆ transition in Tm³⁺/Nd³⁺ codoped CaO·Al₂O₃ glasses. Intensity and lifetime changes with rare-earth concentrations suggested the efficient energy transfer of Tm³⁺:³ H ₄→ Nd³⁺:⁴ F _5/2 and Nd³⁺:⁴ F _3/2→ Tm³⁺:¹ G ₄. The latter transfer enabled Tm³⁺ to reach its ¹ G ₄ level, and the blue emission became possible through the ¹ G ₄→³ H ₆ transition. Quantitative analysis with rate equations proved that these two transitions were the most efficient among all the possible energy transfer routes between Tm³⁺ and Nd³⁺. Calculated up-conversion efficiency of the Tm³⁺/Nd³⁺ combination in CaO·Al₂O₃ glass was 6.6 × 10⁻³, and it was ∼4 orders of magnitude larger than those reported for other oxide glasses.     

Microstructure and Photoluminescence Properties of Mg-Doped BaTiO3:Pr3+ Phosphor

The photoluminescence of Mg-doped BaTiO₃:Pr³⁺ (Pr³⁺: 0.1 mol%) ceramics was investigated by changing the doping concentration of Mg and the sintering temperature. The results indicated that the intensity of red emission due to the ¹ D ₂→³ H ₄ transition of Pr³⁺ exhibited significant dependence on both the Mg doping content and the sintering temperature; the strongest red emission intensity was observed for 2.0 mol% Mg-doped ceramics sintered at 1050°C. An interpretation of the results obtained was made in terms of the changes in the crystal structure and microstructure of the ceramics.     

Correlation between Radiative Transition Probabilities of Nd3+ and Composition in Silicate, Borate, and Phosphate Glasses

Compositional dependence of spontaneous emission probabilities between initial ⁴ F _3/2 and terminal ⁴ I _J J = 9/2, 11/2, 13/2, 15/2) levels of Nd³⁺ were studied for about 90 samples of silicate, borate, and phosphate glasses using the Judd–Ofelt theory. The effect of the covalency of the Nd–O bond on the magnitude of intensity parameters was estimated from the variation of spectral profiles of the ⁴ I _9/2→⁴ G _5/2, ² G _7/2 and ⁴ F _7/2, ⁴ S _3/2 transitions. Intensity parameters Ω₄ and Ω₆ and the spontaneous emission probabilities were strongly affected by the ionic packing ratio of the glass host. The results were discussed in terms of the site selectivity of Nd³⁺ ions in glasses.     

Upconversion Luminescence in γ-AlON:Yb3+,Tm3+ Ceramic Phosphors

Upconversion emission properties of γ-AlON:Yb³⁺,Tm³⁺ phosphors were investigated under single-wavelength diode laser excitation of 980 nm. Blue (479 nm) and red (653 nm) emission bands were observed which correspond to the transitions of ¹G₄→³H₆ and ¹G₄→³F₄ of Tm³⁺ ions, respectively. The upconversion spectra show a concentration-dependent luminescence intensity, reaching its peak at a concentration of 1.2 mol% Yb and 0.5 mol% Tm. Pump power dependence of the upconversion emission intensity ( P – I ) revealed that a two-photon process was involved in the blue and red emissions.     

Mechanism of the Blue Up-Conversion in Tm3+/Nd3+-doped Calcium Aluminate Glasses

Blue up-conversion fluorescence from the Tm³⁺:¹ G ₄→³ H ₆ (480 nm) transition has been observed from calcium aluminate glass codoped with Tm³⁺/Nd³⁺. The mechanism for the up-conversion process consists of a two-photon process. An excitation beam with a wavelength of 791 nm first excites Tm³⁺ to the ³ H ₄ level, where Tm³⁺ again absorbs the 1060 nm emission from Nd³⁺:⁴ F _3/2→⁴ I _11/2 to attain the Tm³⁺:¹G₄ level. Lifetime and intensity variations with compositions suggest the presence of an efficient energy transfer from Nd³⁺ to Tm³⁺. The highest 480 nm emission intensity has been obtained from the glass with 0.1 mol% of Nd₂O₃ and 0.2 mol% of Tm₂O₃.     

Structural Characterization and Luminescent Properties of a Red Phosphor Series: Y2−xEux(MoO4)3 (x=0.4–2.0)

A series of rare earth molybdates, Y_{2− x} Eu _x (MoO₄)₃ for x =0.4, 0.8, 1.2, 1.6 and 2.0 were prepared by solid-state method and their crystal structures, photo luminescent characteristics were investigated. The powders are mainly studied for their red light emission efficiency under near UV excitation. The crystal structures of the powders were found to depend on annealing temperature and the yttrium concentration. Mixtures of monoclinic ( C 2 /c ) and orthorhombic ( Pba 2, Pbna ) structures were formed in varying proportions depending on the value of x and annealing temperatures (700°–800°C). The luminescence behavior depended on the resultant composition of the crystal phase and the Eu³⁺ concentration. The excitation spectra showed the characteristic and broad O→Mo charge transfer (CT) band of the MoO₄ tetrahedra and the sharp intra-configurational 4 f –4 f transitions of Eu³⁺ in the host lattice. The integrated emission ratio (⁵D₀→⁷F₂/⁵D₀→⁷F₁) of Eu³⁺ depends on the annealing temperature and reveals that the local site symmetry of Eu³⁺ ions decreases with increasing concentration of Eu³⁺. The emission spectra obtained by exciting at 396 nm, gave highest red emission intensity for Y_0.4Eu_1.6(MoO₄)₃ annealed at 700°C/6 h among this series of samples.     

Comparison of the Luminescence Properties of Dy3+ in α-Sialon and Oxynitride Glass

Dy-α-sialon and β-Si₃N₄ materials containing Dy-oxynitride glass were hot pressed at 1800°C for 1 h. The luminescence spectra of Dy³⁺ in these samples were compared when excited at 350 nm. The results showed that two strong emission bands in the region 470–500 nm and 570-600 nm, associated with the ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions of Dy³⁺ ions, were observed in Dy-α-sialon. However, no emission peak was detected from the β-Si₃N₄ sample, despite it containing the same amount of Dy³⁺ cations. This proved that only the Dy³⁺ cations in the α-sialon structure, not those in the oxynitride glass, produce the luminescence spectrum.     

Photoluminescence of Rare-Earth-Doped Ca-α-SiAlON Phosphors: Composition and Concentration Dependence

Rare-earth-doped Ca-α-SiAlON phosphors, with the compositions of (Ca_{1−3/2 x} RE _x ) _{m /2}Si_{12− m − n} Al _m+n O _n N_{16− n} (RE=Ce, Sm, and Dy, 0.5≤ m =2 n ≤3.0), were prepared by sintering at 1700°C for 2 h under 10 atm N₂. The concentration of rare earths varied from 3 to 30 at.% with respect to Ca. The photoluminescence (PL) properties were investigated as functions of the composition of the host matrix (i.e., m ) and the concentration of rare earths (i.e., x ). The results show that the emission properties can be optimized by tailoring m and x . The Ce³⁺ luminescence originating from the 4 f –5 d interconfigurational transitions is greatly affected by the environment surrounding the Ce³⁺ ions, which differs from the Sm³⁺ or Dy³⁺ luminescence arising from the 4 f –4 f intraconfigurational transitions. X-ray diffraction and scanning electron microscopy were used to explain the composition and concentration dependence of PL properties.     

Judd-Ofelt Parameters, Hypersensitivity, and Emission Characteristics of Ln3+ (Nd3+, Ho3+, and Er3+) Ions Doped in PbO-PbF2 Glasses

The Judd–Ofelt parameters, Ω₂ and ΣΩ_λ (λ= 2, 4, 6), for Nd³⁺, Ho³⁺, and Er³⁺ doped in the oxyfluoride glass 30PbO70PbF₂ lie intermediate between fluoride glasses and oxide glasses such as borate and phosphate glasses, providing evidence for the sensitivity of these parameters to the bonding environment. The variation of Ω₂ unlike Ω₄ and Ω₆ for the lanthanide series is qualitatively different for glass matrices compared to crystalline matrices. Plots of oscillator strengths of hypersensitive transitions for these ions against ΣΩ_λ (λ= 2, 4, 6) are found useful in discerning the degree of hypersensitivity of these transitions due to change in the host matrix. The ⁵F₂³K₈⁵G₆←⁵I₈ transition of Ho³⁺ is found to be the most hypersensitive. The radiative parameters for the oxyfluoride glasses are close to fluoride glasses and the branching ratio of the important lasing transition, viz., ⁴F_3/2→⁴I_11/2, of Nd³⁺ is higher for the present case compared to fluoride glass. The results suggest that the oxyfluoride glasses may be used as hosts in the place of fluoride glasses wherever suitable as they are more stable and easy to prepare and have similar radiative properties.     

Structural and Photoluminescence Properties of Ce3+- and Tb3+-Activated Lu-α-Sialon

The crystal structure and photoluminescence properties of undoped and Ce³⁺- or Tb³⁺-doped Lu-α-Sialon are reported. Lu-α-Sialon with the composition of Lu_0.367Si_9.9Al_2.1ON₁₅ crystallizes in a trigonal system with a =7.8013(1) Å, c =5.6827(1) Å, in the space group P 31 c . The Lu³⁺ ion is accommodated at the interstice site at 2 b and directly connected by seven (N, O) atoms with an average bond length of 2.624 Å. The incorporation of large Ce³⁺ and Tb³⁺ ions on small Lu³⁺ site results in the expansion of the unit cell as expected but keeping the average Lu_Ln-(N, O) (Ln=Ce, Tb) distances nearly constant with slight distortion in the lattices. The optical band gap of Lu-α-Sialon is about 5.25 eV determined by the diffused reflection spectrum. Lu-α-Sialon:Ce³⁺ (2 mol%) exhibits efficiently greenish-blue emission at about 486 nm under near-ultraviolet (UV) excitation originating from the 5 d →4 f ¹5 d ⁰ transition of Ce³⁺. Lu-α-Sialon:Ce³⁺ is a potential candidate phosphor for white UV-LED applications due to its high quantum efficiency and excellent thermal stability. Lu-α-Sialon:Tb³⁺ (2 mol%) emits strong mixed blue and green light in equivalent due to the transitions of ⁵D₃→⁷F_J and ⁵D₄→⁷F_J of Tb³⁺.     

Processing and Properties of Eu3+:LiTaO3 TransparentGlass–Ceramic Nanocomposites

We report here the processing and properties of transparent glass and glass–ceramic nanocomposites in the Li₂O–Ta₂O₅–SiO₂–Al₂O₃ system in the presence of Eu₂O₃ as luminescent probe. The formation of the LiTaO₃ crystal phase, the crystallite size, and the morphology with the progression of heat treatment have been examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transformed infrared reflectance spectroscopy measurements. The crystallite sizes obtained from XRD and TEM are found to increase with heat-treatment time and vary in the range of 2–20 nm. The measured photoluminescence spectra exhibit emission transitions of ⁵D_0,1→⁷F _j ( j =0, 1, 2, 3, and 4) of Eu³⁺ ions. From the nature of the emission transitions, the site symmetry in the vicinity of Eu³⁺ ions has been found to be near C_3v in the glass–ceramic nanocomposites. An inverse correlation has been observed between the asymmetric ratio ( I _ED/ I _MD) of Eu³⁺ ions and the dielectric constant (ɛ_r), with an increase in the heat-treatment time of glass, which is caused by the dipole–dipole interaction.     

Compositional Dependence of Infrared-to-Visible Upconversion in Yb3+- and Er3+-Codoped Germanate, Gallate, and Tellurite Glasses

Upconversion fluorescences of the green ⁴S_3/2→⁴I_15/2 and red ⁴F_9/2→⁴I_15/2 transitions of the Er³⁺ ion are studied for Yb³⁺- and Er³⁺-codoped sodium germanate, potassium tantalum gallate, and barium tellurite glasses by InGaAs laser-diode pumping. The phonon energies of the host glasses are determined by infrared-reflection measurements. Compositional effects on the Judd—;Ofelt parameters for the Er³⁺ ion, the spontaneous emission probability (SPE) of the ²F_5/2→²F_7/2 transition for the Yb³⁺ ion, and the phonon energy of the glass network are discussed in terms of glass structure. The factors that affect the upconversion fluorescence intensities of the Er³⁺ ion are discussed, using the phonon energy of the host glass and the SPE for the Yb³⁺ ion in the germanate, gallate, and tellurite glasses.     

Energy Transfer and Population Inversion in Heavy Metal Oxide Glasses Doped with Tm3+ and Tb3+

Emission properties and energy transfer of PbO–Bi₂O₃–Ga₂O₃–GeO₂ glasses codoped with Tm³⁺ and Tb³⁺ ions were investigated. The 1.48-μm emission due to the Tm³⁺:³H₄→³F₄ transition can be used to amplify the S-band (1460–1530-nm) signal light. With Tb³⁺ addition, the lifetime and emission intensity of the Tm³⁺:³F₄ level decreased sharply via the Tm³⁺:³F₄→Tb³⁺:⁷F_0,1,2 energy transfer. Population densities of the ³F₄ and ³H₄ levels in Tm³⁺ calculated from rate equations clearly verified that population inversion in Tm³⁺ ions became possible with as little as 0.1 mol% of Tb³⁺ addition.     

Preparation and Optical Properties of Transparent Eu3+:Y3Al5(1−x)Sc5xO12 Ceramics

Using a novel combustion method, Eu-doped Eu:yttrium aluminum garnet (YAG) and Eu:YSAG powders, and transparent Eu:YSAG ceramics were fabricated. The optical properties of these transparent ceramics have been measured, and a reduced peak splitting of Eu³⁺ for ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ was observed when 10 at.% Al³⁺ was substituted by Sc³⁺. The enhanced symmetry of the Eu sites in YAG lattice, which resulted from the expanded YSAG lattice by Sc³⁺ doping, is the main reason for the reduced peak splitting.     

Pr3+/Er3+ Codoped Ge-As-Ga-S Glasses as Dual-Wavelength Fiber-optic Amplifiers for 1.31 and 1.55 μm Windows

Fluorescence emissions at both 1.31 and 1.55 μm communication windows were observed from Pr³⁺/Er³⁺ codoped Ge-As-Ga-S glasses with a single wavelength pumping at 986 nm. The lifetime of the Er³⁺:⁴ I _11/2 level decreased as the Pr³⁺ concentration increased, and that of the Pr³⁺:¹ G ₄ level increased as the Er³⁺ concentration increased. Energy transfer from the Er³⁺:⁴ I _11/2 level to the Pr³⁺:¹ G ₄ level was responsible for emission of the 1.31 μm fluorescence from the Pr³⁺:¹ G ₄ level. Ge-As-Ga-S glasses that have been doped with Pr³⁺ and Er³⁺ cations are promising amplifier materials for both 1.31 and 1.55 μm communication windows.     

Compositional Dependence of Judd-Ofelt Parameters in Silicate, Borate, and Phosphate Glasses

Judd-Ofelt parameters Ω _t with t = 2,4, 6 for the rare-earth ions Pr³⁺, Nd³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, and Tm³⁺ in alkali and/or alkaline-earth silicate, borate, and phosphate glasses have been determined. The variations of Ω _t with the number of 4 f electrons of the rare-earth ions are demonstrated, and factors affecting the Judd-Ofelt parameter Ω₆are discussed. The intensity parameter Ω₆ depends on the ionic packing ratio of the glass host by changing modifier type in silicate and borate glasses, and it is independent of that in a series of borate glasses as a function of modifier content and phosphate glasses. The peak wavenumbers of the transitions whose intensities are determined mainly by the Ω₆<| U ⁽⁶⁾|>² term—where <| U ⁽⁶⁾|> is one of the reduced matrix elements—shift systematically with the values of Ω₆ for all the rare-earth ions.     

Enhanced Phase Stability and Photoluminescence of Eu3+ Modified t-ZrO2 Nanoparticles

Tetragonal ( t ) ZrO₂ nanoparticles have been obtained by a partial Eu³⁺→Zr⁴⁺ substitution, synthesized using a simple oxalate method at a moderate temperature of 650°C in air. The Eu³⁺ additive, 2 mol% used according to the optimal photoluminescence (PL), gives small crystallites of the sample. On raising the temperature further, the average crystallite size D grows slowly from 16 nm to a value as big as 49 nm at 1200°C. The Eu³⁺: t -ZrO₂ nanoparticles have a wide PL spectrum at room temperature in the visible to near-IR regions (550–730 nm) in the ⁵D₀→⁷F_J (Eu³⁺), J =1–4, electronic transitions. The intensity of the ⁵D₀→⁷F₄ group is as large as that of the characteristic ⁵D₀→⁷F₂ group of the spectrum in the forced electric-dipole allowed transitions. The enhanced t -ZrO₂ phase stability and wide PL can be attributed to the combined effects of an amorphous Eu³⁺-rich surface and part of the Eu³⁺ doping of ZrO₂ of small crystallites.     

Photoluminescence of Cerium-Doped α-SiAlON Materials

Cerium-doped α-SiAlON (M _x Si_{12−( m + n )}Al _{m + n} O _n N_{16– n} ) materials have been prepared by gas-pressure sintering and post-hot-isostatic-press (HIP) annealing, using four powder mixtures of α-Si₃N₄, AlN, and either (i) CeO₂, (ii) CeO₂+ Y-α-SiAlON seed, (iii) CeO₂+ Y₂O₃, or (iv) CeO₂+ CaO. Cerium-containing CeAl(Si_{6– z} Al _z )(N_{10– z} O _z ) (JEM) phase, rather than Ce-α-SiAlON phase, forms in the sample with only CeO₂, whereas a single-phase α-SiAlON generates in samples with dual doping (CeO₂+ Y₂O₃ and CeO₂+ CaO). On ultraviolet-light excitation, JEM gives one broad emission band with maximum at 465 nm and a shoulder at 498 nm; α-SiAlON shows an intense and broad emission band that peaks at 500 nm. The unusual long-wavelength emissions in JEM and α-SiAlON are due to increases in the nephelauxetic effect and the ligand-field splitting of the 5 d band, because the coordination of Ce³⁺ in JEM and α-SiAlON is nitrogen enriched.     

Optical Reflectance Spectra of Cr3+ in MgO Solid Solutions

The effect of Cr³⁺ concentration in an MgO crystal lattice on the crystal field splitting energy, 10 Dq , and on other electronic transitions was studied. Optical reflectance spectra showed a linear increase in 10 Dq with increasing Cr³⁺ concentration up to the limit of solid solubility. The Racah parameter, B, decreased and the energy of the ⁴T_1g→⁴A_2g transition increased with increasing Cr³⁺ concentration. The increase in 10 Dq was related to the decrease in lattice parameter.