Y3Al5O12 ceramic absorbers for the suppression of parasitic oscillation in high-power Nd:YAG lasers

The objective of this study was to identify a material suitable to absorb radiation at the wavelength of neodymium-doped Yttrium Aluminum Garnet (Y₃Al₅O₁₂:YAG), 1064 nm. M-(M= Sm³⁺, Co²⁺, Co³⁺, Cr³⁺, and Cr⁴⁺) doped highly transparent YAG ceramics were fabricated, and their absorption spectra were measured. Unlike Co²⁺ and Cr³⁺-doped ceramic samples, Co³⁺ and Cr⁴⁺ and Sm³⁺-doped:YAG ceramics were found to have significant absorption at 1064 nm. However, the Sm³⁺-doped YAG clearly emerged as the best candidate because it is also transparent at 808 nm, the pumping wavelength laser diode (LD), and also at most absorption bands used for flash-lamp pumping.     

Solution-combustion synthesis of Tb-doped Y3Al5O12 nanoparticles

Nano-sized YAG:Tb powder phosphors were prepared by a solution-combustion method, using the general inorganic salts as starting materials. The X-ray diffraction (XRD) measurements showed that the precursor can be well-crystallized at 900 °C. As-prepared particles have sizes mostly in the range between 30 and 100 nm as obtained by scanning electron microscope (SEM) and transition electron microscope (TEM). Selected area electron diffraction (SAED) patterns proved that the larger particles are monocrystalline. The effects of annealing temperature and Tb-doping concentration on the luminescence intensity were studied.     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

Luminescence and Tb-Ce-Eu ion energy transfer in single-crystalline films of Tb3Al5O12:Ce,Eu garnet

The paper is devoted to investigation of the processes of excitation energy transfer between the host cations (Tb³⁺ ions) and the activators (Ce³⁺ and Eu³⁺ ions) in single-crystalline films of Tb₃Al₅O₁₂:Ce,Eu (TbAG:Ce,Eu) garnet which is considered as a promising luminescent material for the conversion of LED's radiation. The cascade process of excitation energy transfer is shown to be realized in TbAG:Ce,Eu: (i) from Tb³⁺ ions to Ce³⁺ and Eu³⁺ ions; (ii) from Ce³⁺ ions to Eu³⁺ ions by means of dipole-dipole interaction and through Tb³⁺ ion sublattice.     

Positron-trap centers in neutron-irradiated silicon containing hydrogen

The defect evolution as a function of the annealing temperature has been studied in monocrystalline silicon grown in a hydrogen atmosphere and irradiated with 3.6×10¹⁷ neutrons/cm². Positron lifetime spectroscopy has been used and the results compared with infrated absorption measurements. Vacancy-H, vacancy-2H, vacancy-O–H and divacancy complexes withm hydrogen atoms (m<6) have been identified for the first time as possible positron traps.     

Radiation and thermally induced effects in YAlO3:Mn crystals

We have studied effects induced by γ-radiation and temperature in Mn-doped YAlO₃ crystals. The studies have been performed by means of optical spectroscopy that include measuring of optical absorption changes induced by γ-radiation and elevated temperature as well as thermally stimulated luminescence (TSL). It has been shown that under γ-irradiation of YAlO₃:Mn crystals, along with the ionization of Mn_Al⁴⁺ ions (Mn_Al⁴⁺→Mn_Al⁵⁺+e⁻), some additional coloration processes take place. This additional coloration is characterized by a wide intense band centered at 26,000- that is ascribed to color centers intrinsic to YAlO₃ lattice. This coloration is removed by the way of crystal warming at , while the coloration caused by Mn_Al⁵⁺ ions is removed at higher temperature . The observed TSL glow of irradiated crystals reveals three peaks near 360, 400 and that correspond to three types of traps. Parameters of the traps have been determined. The TSL emission corresponds to intra-center luminescence of Mn_Al⁴⁺ and Mn_Y²⁺ ions. The possible ionization and trapping mechanisms in YAlO₃:Mn crystals are discussed.     

Al2O3/SiO2 films prepared by electron-beam evaporation as UV antireflection coatings on 4H-SiC

Al₂O₃/SiO₂ films have been deposited as UV antireflection coatings on 4H-SiC by electron-beam evaporation and characterized by reflection spectrum, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The reflectance of the Al₂O₃/SiO₂ films is 0.33% and 10 times lower than that of a thermally grown SiO₂ single layer at 276 nm. The films are amorphous in microstructure and characterize good adhesion to 4H-SiC substrate. XPS results indicate an abrupt interface between evaporated SiO₂ and 4H-SiC substrate free of Si-suboxides. These results make the possibility for 4H-SiC based high performance UV optoelectronic devices with Al₂O₃/SiO₂ films as antireflection coatings.     

ESR spectra of Eu centers in defect Ga2S3 single crystals

In this paper, the author presents the results of measurements of the low-temperature and angular dependences of the ESR spectra of Eu²⁺ centers in defect Ga₂S₃ single crystals in the temperature range 8–29 K and for 0–180° orientations of the static magnetic field. The electron structure of impurity ¹⁵¹Eu atoms in Ga₂S₃:Eu single crystals has been studied by using the ESR method at different doping proportions of Eu atoms. Ga₂S₃ single crystals were grown from the melt using the Bridgman method. The Eu concentration was determined by atomic absorption analysis and X–ray fluorescence analysis (XRFA). By investigation on the ESR spectra, the author has first determined the values of charge states for Eu, which have turned out to be a Eu²⁺(4f⁷) ion with spin S=7/2, g=4.18±0.02 and concentration of the states of Eu N=6.3×10¹⁴ cm⁻³.     

Anisotropic properties of self-flux grown LiB3O5 single crystals

In this paper, we report the existence of anisotropic behavior along the crystallographic axes in optical, electrical and thermal properties of lithium tri borate, a recently developed vacuum UV-NLO material. The variation of refractive index with the wavelength along the crystallographic axes was investigated by prism coupling method. The results of impedance spectroscopy measurement reveal the presence of a strong anisotropy in ionic conductivity and dielectric constant along the axes and also show the super-ionic conduction behavior along the c-axis with the activation energy of Δ∼0.20 eV. A thermo-mechanical study in the temperature range of 300-900 K indicates the existence of a strong variation in the linear thermal expansion coefficient (positive value along the a-axis, and negative value along the c-axis) of LiB₃O₅ crystals.     

Photophysical analysis of the organic complex [Eu(C12H8N2)2](NO3)3

We have determined the photophysical properties of [Eu(C₁₂H₈N₂)₂](NO₃)₃, (EuPhen), a complex which is very promising for photonic and optoelectronic applications, because of its easy synthetic procedure and high thermal stability (up to 300 °C) combined with large sensitization efficiency and good emission quantum yield. Available experimental absorption and emission data have been analyzed by using Judd-Ofelt analysis. Moreover, semi-empirical calculations have been used to determine the structure of the complex and to interpret the convoluted shape of the absorption spectrum.     

Enhancement of red spectral emission intensity of Y3Al5O12:Ce phosphor via Pr co-doping and Tb substitution for the application to white LEDs

We have enhanced color-rendering property of a blue light emitting diode (LED) pumped white LED with yellow emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) phosphor using addition of Pr and Tb as a co-activator and host lattice element, respectively. Pr³⁺ addition to YAG:Ce phosphor resulted in sharp emission peak at about 610 nm through ¹D₂→³H₄ transition. And when Tb³⁺ substituted Y³⁺ sites, Ce³⁺ emission band shifted to a longer wavelength due to larger crystal field splitting. Y₃Al₅O₁₂:Ce³⁺, Pr³⁺ and (Y_1−xTb_x)₃Al₅O₁₂:Ce³⁺ phosphors were coated on blue LEDs to fabricate white LEDs, respectively, and their color-rendering indices (CRIs, R_a) were measured. As a consequence of the addition of Pr³⁺ or Tb³⁺, CRI of the white LEDs improved to be R_a=83 and 80, respectively. Especially, blue LED pumped (Y_0.2Tb_0.8)₃Al₅O₁₂:Ce³⁺ white LED showed both strong luminescence and high color-rendering property.     

Origin of green luminescence in PbWO4 crystals

Characteristics of two green emission bands, G(I) and G(II), and their origin were investigated within 0.4-300 K under photoexcitation in the 3.4-6.0 eV energy range for undoped and Mo⁶⁺-, Mo⁶⁺ , Y³⁺-, Mo⁶⁺, Nb⁵⁺-, Mo⁶⁺, Ce³⁺-, Cr⁶⁺-, La³⁺-, Ba²⁺- and Cd²⁺-doped PbWO₄ crystals with different concentrations of impurity and intrinsic defects, grown by different methods and annealed at different conditions. The G(I) emission band, observed at low temperatures, located around 2.3-2.4 eV and excited around 3.9 eV, is usually a superposition of many closely positioned bands. The G(I) emission of undoped crystals is assumed to arise from the WO₄²⁻ groups located in the crystal regions of lead-deficient structure. In Mo⁶⁺-doped crystals, this emission arises mainly from the MoO₄²⁻ groups themselves. The G(II) emission band located at 2.5 eV is observed only in the crystals, containing the isolated oxygen vacancies — WO₃ groups. This emission appears at T>160 K under excitation around 4.07 eV as a result of the photo-thermally stimulated disintegration of localized exciton states and subsequent recombination of the produced electron and hole centres near WO₃ groups. The G(II) emission accompanies also thermally stimulated recombination processes in PbWO₄ crystals above 150 K. Mainly the G(II) emission is responsible for the slow decay of the green luminescence in PbWO₄ crystals.     

Phase coexistence in annealed CaMn7O12

The structural phase transition in annealed CaMn₇O₁₂ has been investigated by using high resolution synchrotron radiation powder diffraction. There is a phase coexistence phenomenon: two different crystallographic phases coexist in the material between 410 and 458 K. The first one is trigonal and it has a charge ordering (CO) of the Mn³⁺ and Mn⁴⁺ ions, while the second one is cubic and charge delocalized (CD). The volume fraction of the CD phase increases with temperature from 22% at 418 K up to 100% at 468 K. Both phases have domains of at least 150 nm at each temperature. The annealing of CaMn₇O₁₂ relaxed a part of the strains in the lattice, but did not influence the phase coexistence phenomenon.     

Cooperative emission study in ytterbium-doped Y2SiO5

Ytterbium ions infrared and visible cooperative luminescences, resulting from YAG laser and selective site excitations, in (6%) Yb-doped Y₂SiO₅ thin film are analyzed. Magnetically coupled Yb-Yb ion pairs seem to play a major role in energy transfer and cooperative emission, confirming the prevalence of superexchange mechanisms.     

First-principles study of color centers in PbMoO4 crystals

Electronic structures of PbMoO₄ crystals containing Mn ion impurities located at Pb²⁺ sites are studied within the framework of the fully relativistic self-consistent Direc-Slater theory, using a numerically discrete variational (DV-Xα) method. The calculated results show that Mn²⁺ ions have donor energy levers in the forbidden band, which may correspond to the yellowish color absorption band of PbMoO₄ as-grown in air. The new-formed Mn³⁺ ions have acceptor energy levers in the forbidden band, which may relate to the photochromic effect in PbMoO₄ crystal.     

X-ray and thermoluminescence of LiKB4O7 single crystals

The thermoluminescent (TL) and X-ray luminescent (XL) spectra of undoped LiKB₄O₇ (LKBO) single crystals had been investigated in the temperature range 80-300 K. It was found that in LKBO crystals, there are two intensive TL peaks at 112 and 132 K. The only one band emission spectra of sharply defined Gaussian shape, confirming the same mechanism of XL and TL by the radiation annihilation of the strongly localized self-trapped excitons (STE), had been observed in the TL and XL spectra. The possible models of these localization centers STE have been discussed.     

Growth and spectral characterization of β-Ga2O3 single crystals

Beta gallium oxide (β-Ga₂O₃) single crystals were grown by the floating zone technique. The absorption spectra and the luminescence of the crystals were measured. The absorption spectra showed an intrinsic short cutoff edge around 260 nm with two shoulders at 270 and 300 nm. Not only the characteristic UV (395 nm), blue (471 nm) and green (559 nm) lights, but also the red (692 nm) light can be seen in the emission spectra. The deep UV light was attributed to the existing of quantum wells above the valence band and the red light was owed to the electron-hole recombination via the vicinity donors and acceptors.     

First-principles study on electronic structures of BaWO4 crystals containing F-type color centers

The electronic structures of BaWO₄ crystals containing F-type color centers are studied within the framework of the fully relativistic self-consistent Dirac-Slater theory, using a numerically discrete variational (DV-Xα) method. It is concluded that F and F⁺ color centers have donor energy level in the forbidden band. The optical transition energies are 2.449 and 3.101 eV, which correspond to the 507 and 400 nm absorption bands, respectively. It is predicted that 400-550 nm absorption bands originate from the F and F⁺ color centers in BaWO₄ crystals.     

Thermoluminescence glow curves analysis of pure and CeO2-doped Li2O-Al2O3-SiO2 glass ceramics

A comparative study of the thermoluminescence (TL) emission between beta-irradiated lithium aluminosilicates, Li₂O-Al₂O₃-SiO₂ (LAS), and beta-irradiated CeO₂-doped lithium aluminosilicates, Li₂O-Al₂O₃-SiO₂-CeO₂ (LAS:Ce), grown by sol-gel technique and preannealed at 1250 °C is presented. It is found that doping reinforces the result of preannealing the sample; the incorporation of CeO₂ at low concentrations shifts the TL curves towards higher temperatures and increases the total intensity with respect to samples without the dopant. This behavior, together with the fact that CeO₂ is a good densification agent, suggests the possibility of using CeO₂ to improve the technological properties of LAS. Deconvolution analysis of the thermoluminescence glow curves of doped materials under general order kinetics shows that the relevant temperatures at which the component signals appear are virtually the same as for pure samples. This suggests that CeO₂ does not introduce new types of traps, but only changes the population and distribution of impurities responsible for the TL in the original matrix of LAS. Finally, the kinetic parameters for the deconvoluted curves are reported. They corroborate that changes in CeO₂ concentration only vary the depth and distribution of the same kind of original traps.