Defects in U3+: CaF2 single crystals grown under different conditions by the temperature gradient technique

Defects in as-grown U3+ : CaF2 crystals grown with or without PbF2 as an oxygen scavenger were studied using Raman spectra, thermoluminescence glow curves, and additional absorption (AA) spectra induced by heating and gamma-irradiation. The effects of heating and irradiation on as-grown U3+: CaF2 crystals are similar, accompanied by the elimination of H-type centers and production of F-type centers. U3+ is demonstrated to act as an electron donor in the CaF2 lattice, which is oxidized to the tetravalent form by thermal activation or gamma-irradiation. In the absence of PbF(2)as an oxygen scavenger, the as-grown U3+:CaF2 crystals contain many more lattice defects in terms of both quantity and type, due to the presence of O2- impurities. Some of these defects can recombine with each other in the process of heating and gamma-irradiation. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Growth and spectral characterization of beta-Ga2O3 single crystals

Beta gallium oxide (beta-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. (c) 2006 Elsevier Ltd. All rights reserved.

Effects of gamma irradiation and air annealing on Yb-doped YAlO3 single crystals

The effects of gamma irradiation on as-grown 5 at% Yb:YAlO3 (YAP) and air annealing on gamma-irradiated 5 at% Yb: YAP have been studied by the difference in the absorption spectra before and after treatment. The gamma irradiation and air annealing led to opposite changes of the absorption properties of the Yb: YAP crystal. After air annealing, the gamma-irradiation effects were totally removed over the wavelength range 390-800 nm and the concentrations of Fe3+ and Yb3+ were slightly increased. For the first time, the gamma-irradiation-induced valence changes between Yb3+ and Yb2+ ions in Yb: YAP crystals have been observed. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Effect of air annealing on the spectral properties of Ce : Y3Al5O12 single crystals grown by the temperature gradient technique

Five absorption hands, at 227, 300 340, 370 and 457nm, were observed in the optical absorption spectrum of Ce:Y3Al5O12 (Ce:YAG) crystals grown by the temperature gradient technique (TGT). The absorption bands at 227, 340, and 457 nm were identified Lis belonging to the Ce3+ -ion in the YAG crystal. A near UV optical emission band at 398nm was observed. with an excitation spectrum containing two bands, at 235 and 370nm. No fluorescence was detected under 300 nm excitation. The pair of absorption bands at 235 and 370 nm and the absorption band at 300 nm were attributed to the F- and F+-type color centers, respectively. The color centers model was also applied to explain the spectral changes in the Ce:YAG (TGT) crystal, including the reduction in the Ce 31 -ion absorption intensity, after annealing in an oxidizing atmosphere (air). (C) 2004 Elsevier B.V. All rights reserved.

Simultaneous three-photon-excited violet upconversion luminescence of Ce3+: Lu2Si2O7 single crystals by femtosecond laser irradiation

We found that Ce3+:Lu2Si2O7 single crystals could be excited at 800 nm by using a femtosecond Ti:sapphire laser. The emission spectra of Ce3+:Lu2Si2O7 crystals were the same for one-photon excitation at 267 nm as for excitation at 800 nm. The emission intensity of Ce3+: Lu2Si2O7 crystals was found to depend on the cube of the laser power at 800 nm, consistent with simultaneous absorption of three 800 nm photons. The measured value of the three-photon absorption cross section is sigma'(3) = 2.44 x 10(-77) cm(6) s(2). (c) 2006 Optical Society of America.

Blue upconversion luminescence generation in Ce3+: Gd2SiO5 crystals by infrared femtosecond laser irradiation

Blue frequency-upconversion fluorescence emission has been observed in Ce3+-doped Gd2SiO5 single crystals, pumped with 120-fs 800 nm IR laser pulses. The observed fluorescence emission peaks at about 440nm is due to 5d -> 4f transition of Ce3+ ions. The intensity dependence of the blue fluorescence emission on the IR excitation laser power obeys the cubic law, demonstrating three-photon absorption process. Analysis suggested that three-photon simultaneous absorption induced population inversion should be the predominant frequency upconversion mechanism. (c) 2006 Optical Society of America.

Thermal, spectroscopic and laser properties of Yb3+Na+: CaF2 single crystals

The effects of Na+ doping level on the thermal conductivities, absorption and emission spectra, and fluorescence lifetimes of Yb3+ ,Na+ :CaF2 crystals were systematically studied. Sites structure, covalent force, and crystal field strength of Yb3+ :CaF2 crystals were markedly varied by codoping Na+ as charge compensator. The 2.0at% Yb3+ and 3.0at% Na+-codoped CaF2 crystal was demonstrated to operate in diode-pumped passively mode-locking scheme. Transform-limited 1 ps laser pulses were obtained, showing the crystal capable of producing ultra-short laser pulses. (c) 2006 Elsevier B.V. All rights reserved.

Color centers and charge state change in Ce : YAG crystals grown by temperature gradient techniques

Color centers and impurity defects of Ce:YAG crystals grown in reduction atmosphere by temperature gradient techniques have been investigated by means of gamma irradiation and thermal treatments. Four absorption bands associated with color centers or impurity defects at 235, 255, 294 and 370 nm were observed in as-grown crystals. Changes in optical intensity of the 235 and 370 nm bands after gamma irradiation indicate that they are associated with F+-type color center. Charge state change processes of Fe3+ impurity and Ce3+ ions take place in the irradiation process. The variations of Ce3+ ions concentration clearly indicate that Ce4+ ions exist in Ce:YAG crystals and gamma irradiations could increase the concentration of Ce3+ ions. Annealing treatments and the changes in optical density suggest that a heterovalent impurity ion associated with the 294 nm band seems to be present in the crystals. (c) 2005 Elsevier B.V. All rights reserved.

Color centers in Yb : YAG crystals grown by temperature-gradient techniques

Yb: YAG (Yb: Y3Al5O12) crystals have been grown by temperature-gradient techniques (TGT) and their color centers and impurity defects were investigated by means of gamma irradiations and thermal treatment. Two color centers located at 255 and 290 nm were observed in the as-grown TGT-Yb: YAG. Analysis shows that the 255 nm band may be associated with Fe3+ ions. Absorption intensity changes of the 290 nm band after gamma irradiation and thermal treatment indicate that this band may be associated with oxygen-vacancy defects. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Effect of annealing and gamma irradiation on undoped and Eu3+-doped Y2SiO5 single crystals

The absorption spectra of the undoped Y2SiO5 and Eu3+-doped Y2SiO5 crystals grown by the Czochralski technique were compared before and after annealing and, similarly, the unannealed and annealed crystals after gamma-ray irradiation. The absorption bands of Eu2+ ions with peaks at 300 and 390 nm were observed in the as-grown Y2SiO5:Eu3+ crystal. These peaks were more intense in H-2-annealed and irradiated Y2SiO5:Eu3+ crystals. The additional absorption peaks at 260 and 320-330 nm which were attributed to F color centers and O- hole centers were observed in irradiated undoped Y2SiO5 and Y2SiO5:Eu3+ crystals, respectively. (c) 2005 Elsevier B.V. All rights reserved.

Sites structure and spectroscopic properties of Yb-doped and Yb, Na-codoped CaF2 laser crystals

For the first time, the effect of Na+ on crystal structure, valence state of Yb ions, spectroscopic properties of YbF3-doped CaF2 system was systematically studied. Na+ can greatly suppress the deoxidization of Yb3+ to Yb2+. Absorption and emission spectra showed codoping Na+ with different Na:Yb ratios can modulate the spectroscopy and photoluminescence properties of Yb3+ ions in CaF2 lattice in a large scope. The emission lifetime and quantum efficiency of Yb3+ in CaF2 were greatly enhanced by the codopant of Na+. The potential laser performances of the new Yb, Na-codoped CaF2 crystals were predicted. (c) 2005 Elsevier B.V. All rights reserved.

Effect of gamma-irradiation on spectral properties of undoped Y2SiO5 crystals

The absorption spectra of undoped Y2SiO5 crystals were studied before and after gamma-irradiation. After gamma-irradiation, the additional absorption peaks at 260-270 and 320nm were observed in as-grown and H,annealed Y2SiO5 crystal, but it did not occur in air-annealed Y2SiO5 crystal. These absorption peaks were attributed to F color centers and O- hole centers, respectively. Owing to more oxygen vacancies and color centers in H-2-annealed Y2SiO5 crystal than that in as-grown Y2SiO5 crystal after gamma-irradiation, the additional absorption peaks were more intense in the former than that in the latter. With the irradiation dose increasing from 20 to 220kGy, the intensity of additional absorption peaks increased.

Raman spectroscopic investigation of pure and ytterbium-doped rare earth silicate crystals

Raman spectroscopy was used to study the molecular structure of a series of selected rare earth (RE) silicate crystals including Y2SiO5 (YSO), LU2SiO5 (LSO), (Lu0.5Y0.5)(2)SiO5 (LYSO) and their ytterbium-doped samples. Raman spectra show resolved bands below 500 cm(-1) region assigned to the modes of SiO4 and oxygen vibrations. Multiple bands indicate the nonequivalence of the RE-O bonds and the lifting of the degeneracy of the RE ion vibration. Low intensity bands below 500 cm(-1) are an indication of impurities. The (SiO4)(4-) tetrahedra are characterized by bands near 200 cm(-1) which show a separation of the components of nu(4) and nu(2), in the 500-700 cm(-1) region which are attributed to the distorting bending vibration and in the 880-1000 cm(-1) region which are attributed to the symmetric and antisymmetric stretching vibrational modes. The majority of the bands in the 300-610 cm(-1) region of Re2SiO5 were found to arise from vibrations involving both Si and RE ions, indicating that there is considerable mixing of Si displacements with Si-O bending modes and RE-0 stretching modes. The Raman spectra of RE silicate crystals were analyzed in terms of the molecular structure of the crystals, which enabled separation of the bands attributed to distinct vibrational units. Copyright (C) 2007 John Wiley & Sons, Ltd.

Comparison of fluorescence spectra of Yb : Y3Al5O12 and Yb : YALO(3) single crystals

Yb:Y3Al5O12 (Yb:YAG) single crystals with Yb doping concentration 0.5 at.%, 5 at.%, 15 at.%, 25 at.%, 50 at.%, 100 at.% and Yb:YAlO3 (Yb:YAP) single crystals with Yb doping concentration 0.5 at.%, 5 at.%, 15 at.%, 30 at.% were grown by the Czochralski process. The fluorescence spectra of these crystals and the effects of self-absorption on the shape of the fluorescence spectra were studied. Through comparing the fluorescence spectra of Yb:YAG and Yb:YAP, all results indicate that the effects of self-absorption on the fluorescence spectra of Yb:YAP are remarkably stronger than that of Yb:YAG at the same Yb concentration. (c) 2006 Elsevier B.V. All rights reserved.