Growth of large-sized Ce:Y3Al5O12 (Ce:YAG) scintillation crystal by the temperature gradient technique (TGT)

In this work, Ce:YAG crystal with the size of ?4 in was successfully grown by the TGT method. The optical and scintillation properties of as-grown Ce:YAG crystals were investigated. Three obvious absorption bands at 223, 340 and 460 nm and two weak color-center absorption bands at 296 and 370 nm are observed in as-grown Ce:YAG crystal. Fluorescence with an emission peak at 398 nm is observed due to the color centers, and absorption bands of the color centers can be eliminated by annealing in O₂ or H₂ atmosphere at 1673 K for 24 h. Yellow-green fluorescence centered at 530 nm is found when the crystal was excited at 460 nm and the 530 nm excitation spectrum shows two peaks at 340 and 460 nm. X-ray fluorescence spectrum of as-grown crystal shows three emission peaks at 300, 360 and 530 nm. An average light output of 1360 phe/MeV and a single exponential decay with the decay time constant of 62.97 ns are found in as-grown Ce:YAG crystal.     

Cs2LiGdCl6 (Ce): New scintillation material

We investigated the scintillation properties of Cs₂LiGdCl₆:Ce³⁺ as a function of the Ce concentration. X-ray excited luminescence spectra of the scintillation material showed broad emission bands between 360 and 460 nm, with two overlapping peaks, due to the d→f transitions on Ce³⁺ ions. The samples provide good scintillation results. The energy resolution was found to be 5.0% (FWHM) at 662 keV for 10% Ce sample. Under γ-ray excitation, Cs₂LiGdCl₆:Ce³⁺ showed three exponential decay time components of about 130–200 ns decay time constant. The light output of the investigated samples was 20,000 photons/MeV for a 10% Ce concentration. The light output deviation from the linear response is within 7% between the energy range of 31 and 1333 keV. Overall, the scintillation properties confirm that Cs₂LiGdCl₆:Ce³⁺ single crystal is a promising candidate for medical imaging and radiation detection.     

Efficient frequency upconversion emission in Er-doped novel strontium lead bismuth glass

Er³⁺-doped strontium lead bismuth glass for developing upconversion lasers has been fabricated and characterized. The Judd-Ofelt intensity parameters Ω_t (t = 2, 4, 6), calculated based on the experimental absorption spectrum and Judd-Ofelt theory, were found to be Ω₂ = 2.95 × 10⁻²⁰, Ω₄ = 0.91 × 10⁻²⁰, and Ω₆ = 0.36 × 10⁻²⁰ cm². Under 975 nm excitation, intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2, respectively, were observed. The upconversion mechanisms are discussed based on the energy matching and quadratic dependence on excitation power, and the dominant mechanisms are excited state absorption and energy transfer upconversion for the green and red emissions. The long-lived ⁴I_11/2 level is supposed to serve as the intermediate state responsible for the upconversion processes.     

Float zone growth of Dy:GdVO4 single crystals for potential use in solid-state yellow lasers

Single crystals of dysprosium-doped gadolinium orthovanadate (Dy:GdVO₄) were successfully grown by the floating zone method and their fluorescence properties were investigated. The as-grown crystals did not contain any macroscopic defects such as cracks and inclusions for any Dy-concentration of up to 4 at%. Every crystal showed optical homogeneity under observation with a polarizing microscope; that is, no low-angle grain boundaries and growth striations were detected. In the visible region, two distinct fluorescence bands were observed around 480 and 575 nm, corresponding to ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions, respectively. The excitation spectrum for the emission of 573 nm indicates the possibility to use a commercially available laser diode of 450 nm as a pumping source for solid-state yellow laser.     

Rod-like nano-composite of inorganic nanowires encapsulated by mesoporous silica solid

A novel rod-like praseodymium-containing inorganic nanowire encapsulated by mesoporous silica, was synthesized via a self-assembly method. The specific surface area of the composite was 1198m²g^− 1. The nanowires with 4-5 nm in diameter and 100 nm in length were encapsulated at the center of mesoporous solid showed by TEM images. The quantum size effects of the sample can be noted. The controversial absorption band at 960 cm^− 1 in Fourier-transform infrared (FT-IR) spectra was proved to be not the evidence of the introduction of heteroatom. On the basis of the absorption spectra and select area electron diffraction (SEAD) pattern, the nano-composite was known as an amorphous phase containing praseodymium element, in which the praseodymium was trivalent. The absorption spectra indicated the 4f² electronic configuration of Pr³⁺ ions. Four energy bands were attributed to the transitions from the ground state ³H₄ to the excited levels ³P₂, ³P₁, ³P₀ and ¹D₂, respectively.     

Composition dependent upconversion of Er-doped PbF2-TeO2 glasses

The upconversion properties of Er³⁺ ions were studied for heavy metal oxyfluoride tellurite glass hosts xPbF₂-(100−x)TeO₂ under 975 nm excitation. The intense green (529 and 545 nm) and relative weak red (657 nm) emissions corresponding to the transitions ⁴S_3/2 → ⁴I_15/2, ²H_11/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2, respectively, were simultaneously observed at room temperature. The PbF₂ content has an important influence on upconversion luminescence emission. With increasing PbF₂ content, the intensities of green (529 nm) and red (657 nm) emissions increase slightly, while the green (545 nm) emission increases significantly. These results indicate that PbF₂ has more influence on the green (545 nm) emission than the green (529 nm) and red (657 nm) emissions. The intense green emission observed suggest that Er³⁺-doped heavy metal oxyfluoride tellurite glasses can become candidates for developing upconversion optical devices.     

Crystal growth and spectroscopic properties of erbium doped Lu2SiO5

A high optical quality erbium doped Lu₂SiO₅ single crystal has been grown by the Czochralski method. The distribution coefficient of Er³⁺ was measured to be ∼0.926. The absorption and emission spectra as well as the fluorescence decay curve of the excited state ⁴I_13/2 were measured at room temperature. The spectroscopic parameters were calculated using the Judd–Ofelt theory, and the J–O parameters Ω₂, Ω₄ and Ω₆ were found to be 4.451×10^-20, 1.614×10^-20 and 1.158×10^-20 cm², respectively. The room-temperature fluorescence lifetime of the Er³⁺⁴I_13/2→⁴I_15/2 transition was measured to be 7.74 ms. The absorption and emission cross-section as well as the gain cross-section in the eye-safe regime of 1400–1700 nm were also determined and discussed.     

Effect of Bi2O3 proportion on physical, structural and electrical properties of zinc bismuth phosphate glasses

The variation in physical, structural and electrical properties has been studied as a function of Bi₂O₃ content in 20ZnF₂-(10 + x) Bi₂O₃-(70-x) P₂O₅, 0 ≤ x ≤ 10 mol% glasses, which were prepared by melt quenching technique and characterized by differential thermal analysis (DTA). Colorless samples, which have no absorption peaks, are obtained for 10 and 12 mol% of Bi₂O₃ and the glasses are slowly becoming brownish from 15 to 20 mol% of Bi₂O₃ which exhibit two absorption peaks at ~ 370 nm, ~ 450 nm correspond to Bi° transitions ⁴S_3/2 → ²P_3/2 and ⁴S_3/2 → ²P_1/2 respectively. The decrease in ³P₁ → ¹S₀ transition of Bi³⁺ photo luminescence emission for 18 and 20 mol% of Bi₂O₃ and increase in optical absorption area shows the reduction of Bi³⁺ to Bi°. From FTIR studies it is observed that an addition of Bi₂O₃ decreases the P―O―P covalent bond by forming P―O―Bi bonds due to high polarizing nature of Bi³⁺ ions. Dielectric parameters like ε', tan δ and a.c. conductivity σ_ac are found to increase and activation energy for a.c. conduction is found to decrease with the increase in the concentration of Bi₂O₃. Density of defect energy states is found to increase for higher concentration of Bi₂O₃ and is discussed according to quantum mechanical tunneling (QMT) model.     

Energy transfer up-conversion in Tm-doped silica fiber

A study of the mechanisms responsible for the infra-red to near infra-red up-conversion in Tm³⁺-doped silica fibers is presented. Up-conversion luminescence was observed from the ³H₄ level of Tm³⁺ under 1586 nm pumping into the ³F₄ level. The quadratic dependence of the up-conversion luminescence at 800 nm on the 1800 nm luminescence from the ³F₄ level confirms that the ³H₄ level is populated by a two photon process. Two possible processes are proposed as mechanisms responsible for the up-conversion: excited state absorption and energy transfer up-conversion. The decay characteristics of the luminescence from the ³H₄ level were studied under direct and indirect pumping at 786 and 1586 nm, respectively. By comparing the decay waveforms to the solution of a simple set of rate equations, the energy transfer up-conversion process (³F₄, ³F₄ → ³H₄, ³H₆) was established at Tm₂O₃ concentrations greater than 200 ppm.     

Luminescence of borogermanate glasses activated by Er and Yb ions

Glasses of the 25Ln₂O₃-25B₂O₃-50GeO₂ composition (mol%) where Ln = (1 − x − y) La, xEr, yYb, with an addition of Al₂O₃ have been obtained and their luminescent characteristics examined. Probabilities of spontaneous emission, peak sections of the induced radiation and quantum yields of luminescence corresponding to the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions and the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions have been defined. Quantum yield of Yb³⁺ luminescence for glasses with low Yb₂O₃ concentration reaches values closed to 100%. The luminescence spectrum of Er³⁺ ions exhibits a broad peak at about 1530 nm with effective width more than 80 nm when excited by irradiation at λ = 977 nm. Spontaneous emission probability and peak stimulated radiation section for Er³⁺ luminescence band ⁴I_13/2 → ⁴I_15/2 were determined to be equal to 175 s⁻¹ and 4.9 × 10⁻²¹ cm² respectively. Effective quenching of both rare-earth activators by oscillations with ν ≈ 2630 and 2270 cm⁻¹ was found. These oscillators, most likely, represent OH-groups connected by a hydrogen bond with non-bridging oxygen atoms in the borogermanate matrix.     

Studies of Er doped germanate-oxyfluoride and tellurium-germanate-oxyfluoride transparent glass-ceramics

An erbium doped germanate-oxyfluoride glass 60GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (GPOF) and a tellurium-germanate-oxyfluoride glass 30TeO₂ · 30GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (TGPOF) were prepared in the bulk form. By appropriate heat treatment of the as-prepared glasses above, transparent glass-ceramics were obtained with the formation of β-PbF₂ nanocrystals in the glass matrix confirmed by X-ray diffraction. Optical absorption and photoluminescence measurements were performed on as-prepared glass and glass-ceramics. The luminescence of Er³⁺ ions in transparent glass-ceramics revealed sub-band splitting generally seen in a crystal host. The intensity of red and near infrared luminescence significantly increased in transparent glass-ceramic compared to that in as-prepared glass. Two luminescence bands at 758 nm from ⁴F_7/2 → ⁴I_13/2 and at 817 nm from ²H_11/2 → ⁴I_13/2 transitions were observed from transparent glass-ceramic but cannot be seen from the corresponding as-prepared glass. These results are attributed to the change of ligand field of Er³⁺ ions and the decrease of effective phonon energy when Er³⁺ ions were incorporated into the precipitated β-PbF₂ nanocrystals.     

Cathodoluminescence of epitaxial GaN and ZnO thin films for scintillator applications

A comparative study of cathodoluminescence ultraviolet photon yields and decay times of large area GaN and zinc oxide (ZnO) layers grown for scintillator applications by metalorganic vapor phase epitaxy is presented. Silicon-doped GaN and non-intentionally-doped ZnO yield up to 1.4±0.2 photons/kVe⁻ and 1.3±0.2 photons/kVe^– at room-temperature, respectively. For GaN the decay times scatter between 0.4 and 0.9 ns, and for ZnO between 2.5 and 3.0 ns. The GaN and the ZnO absorption coefficients, α, internal efficiencies, η_i, and radiative constants, B, are determined. The characteristics of thin-film scintillators based on these materials are compared with commercially available granular scintillators.     

Variation of luminescence properties of Na2O-CaO-SiO2: Nd glass with crystallinity

The optical absorption and emission intensities of Nd³⁺-doped transparent glass-ceramics with high crystallinity in Na₂O-CaO-SiO₂ (NSC) system were studied. The transmittance of NCS decreases with increasing degree of crystallinity, however it still remains 65.5% at λ = 710 nm when the crystallization is almost completed. Judd-Ofelt theory is performed to evaluate the radiative transition probability as well as quality factor, branching ratio and emission cross section. The maximum values of Ω₂ and emission cross section (⁴F_3/2 → ⁴I_11/2) of NCS are obtained after nucleating at 630 °C for 10 h. The quality factor increases with increasing crystallinity, while branching ratio for ⁴F_3/2 → ⁴I_11/2 is opposite. The results show that transparent glass-ceramic with high crystallinity is a potential laser host for 1.06 μm emission.     

Concentration quenching of fluorescence from the (P0 + P1) manifold in heavily-doped Pr: ZBAN glasses

Fluorescence waveforms from the (³P₀ + ³P₁) manifold in Pr³⁺ doped ZBAN glass at wavelengths of 520, 635 and 695 nm were measured for Pr³⁺ concentrations from 4 to 12 mol%. The waveforms were found to be non-exponential with decay rates rapidly increasing with Pr³⁺ concentration and independent of whether the ³P₀ or the ³P₁ level was excited. The multipolar energy transfer model was used to analyse the waveforms and this showed that concentration quenching was due to cross-relaxation by dipole-dipole interaction. The critical concentration, at which the cross-relaxation rate equals the intrinsic decay rate, was found to be of 2.06 × 10²⁶ m⁻³ (1.20 mol%). There was no evidence of excitation diffusion for Pr³⁺ concentrations of up to 12 mol%.     

The effect of PbF2 content on the microstructure and upconversion luminescence of Er-doped SiO2-PbF2-PbO glass ceramics

The Er³⁺ doped transparent oxyfluoride glass ceramics were obtained by appropriate heat treatment of the precursor glasses with composition (mol%) 50SiO₂-xPbF₂-(50 − x)PbO-0.5ErF₃. The microstructure and optical properties of the glasses and glass ceramics were determined by differential scanning calorimetry (DSC), X-ray diffraction (XRD), absorption spectra and luminescence spectra. The intensity of upconversion luminescence significantly increased in glass ceramics compared to that in precursor glass. The emission bands centered around 660 nm (⁴F_9/2 → ⁴I_15/2) and 410 nm (²H_9/2 → ⁴I_15/2) were simultaneously observed in glass ceramics but cannot be seen in the corresponding precursor glass. The influence of different PbF₂ content on the microstructure and upconversion luminescence of the samples was analyzed in detail. The results indicated that with the increase of PbF₂ content, the Ω₂ was almost the same and the ratios of red to green upconversion luminescence decreased in glass ceramics.     

Crystal growth and luminescent properties of Pr-doped K(Y,Lu)3F10 single crystal for scintillator application

Pr1%:K(Y_1−xLu_x)₃F₁₀ (x=0, 0.2, 0.4) single crystals were grown by the μ-PD method. All the grown crystals were greenish and perfectly transparent without any inclusions or cracks. Radioluminescence spectra and decay kinetics of the Pr1%:K(Y,Lu)₃F₁₀ crystals were measured. Emission from the Pr³⁺ 5d–4f transition, peaking around 260 nm and of the decay time of around 22 ns were observed. The 5d–4f emission intensities of the Pr1%:K(Y,Lu)₃F₁₀ crystals were higher than that of the standard BGO scintillator.     

Spectroscopic properties of Er-doped xGeO2-(80 − x)TeO2-10ZnO-10BaO glasses

Erbium-doped glasses with composition xGeO₂-(80 − x)TeO₂-10ZnO-10BaO were prepared by melt-quenching technique. The phonon sideband spectra and the optical absorption band edges for the host matrix were confirmed by means of the spectral measurements. Standard Judd-Ofelt calculations have been completed to these glasses. The dependence of up-conversion and infrared emission under 980 nm excitation on the glass composition was studied. The quantum efficiencies for the ⁴I_13/2 → ⁴I_15/2 transition of trivalent erbium in the glasses were estimated.     

Population dynamics of the F4 and H4 levels in highly-doped Tm:ZB(L)AN glasses

Population dynamics of the ³F₄ and ³H₄ levels in Tm³⁺ doped ZB(L)AN glasses was studied for Tm³⁺ concentrations from 0.5 to 12 mol%. Fluorescence waveforms from these levels were measured at 1.8 μm (³F₄) and 800 nm (³H₄) with both direct and indirect pumping. Decay from the ³F₄ level was found to be exponential with non-radiative decay rates proportional to the square of the Tm concentration. This indicated a process of energy migration by diffusion within the excited Tm³⁺ ions followed by quenching at sites to which the ions could migrate. The decay of the directly pumped ³H₄ level exhibited both exponential and non-exponential behavior depending on the concentration. For the lowest concentration (0.5 mol%) the decay was exponential, but at concentrations of 1, 2, 4 and 6 mol% the decay waveforms were distinctly non-exponential. The non-exponential waveforms could be fitted by the Yokota-Tanimoto model for diffusion of excited donors and dipole-dipole interactions with acceptors. This model produced values for C_DD and C_DA, the donor-donor and donor-acceptor energy transfer parameters, respectively. At the higher concentrations (8, 10, 12 mol%) the waveforms were exponential with decay rates from which the cross-relaxation parameter for the process ³H₄, ³H₆ → ³F₄, ³F₄ was obtained. When the ³F₄ level is pumped at 1660 nm, the decay of the ³H₄ level confirmed the influence of the up-conversion energy transfer process ³F₄, ³F₄ → ³H₄, ³H₆.     

Preparation and optical properties of nanoscale MgAl2O4 powders doped with Co ions

Co²⁺-doped MgAl₂O₄ nanocrystalline powders were prepared by co-precipitation method. The gels and/or calcined samples were characterized by means of thermogravimetry and differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared (FTIR) spectrum and near-infrared absorption spectrum. MgAl₂O₄ nanocrystals were produced by calcining the gel above 800 °C, with the crystallite size of 10-30 nm in the temperature range of 800-1100 °C. The influence of pH value of precipitant solution on the dispersing of powders was studied and the result showed that Co:MgAl₂O₄ nanocrystalline powders exhibited good dispersion when pH = 11. The absorption spectrum of Co²⁺-doped MgAl₂O₄ exhibited a broad absorption band in the wavelength range of 1200-1600 nm, which indicated that Co²⁺ ions substituted for the tetrahedrally coordinated Mg²⁺ ions in the MgAl₂O₄ lattice.     

Broadband infrared luminescence of Ni-doped silicate glass-ceramics containing lithium aluminate spinel nanocrystals

Transparent Ni²⁺-doped SiO₂-Al₂O₃-Ga₂O₃-Li₂O (LGAS) glass-ceramics embedding lithium aluminate spinel nanocrystals was prepared. After heat treatment, LiAl₅O₈ crystallite was precipitated in the glasses, and its size was about 3 nm. It was confirmed from the absorption spectra that the ligand environment of Ni²⁺ ions changed from the trigonal bi-pyramid fivefold sites in the as-made glass to the octahedral sites in the glass-ceramics. Upon excitation at 980 nm, broadband infrared luminescence centered at around 1250 nm with full width at half maximum (FWHM) more than 250 nm was observed originating from the ³T₂(³F) → ³A₂(³F) transition of Ni²⁺ in octahedral sites. The broadband near-infrared (NIR) emission from Ni²⁺-doped glass-ceramics can be as host materials for broadband optical amplifier.