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 γ-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 H₂-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.     

Tb3+-impregnated,non-porous silica glass possessing intense green luminescence under UV and VUV excitation

A colorless transparent luminescence material was successfully prepared by impregnation of leached, porous glass with Tb³⁺ ions followed by reductive sintering in a CO atmosphere. Tb³⁺ emissions under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation clearly showed the most intense emission band to be situated at 543 nm, which corresponds to the ⁵D₄ → ⁷F₅ transition. Sintering of the Tb glass in a reducing atmosphere resulted in a significant enhancement of Tb³⁺ emission intensity in comparison with sintering in air. The presence of traces of cerium ions was verified in Tb glasses, and more Ce³⁺ ions were produced as a result of the reductive sintering. The increase in Ce³⁺ ions was believed to be mainly responsible for the enhancement of ⁵D₃ → ⁷F_j and ⁵D₄ → ⁷F_j transitions from Tb³⁺ ions owing to an energy transfer channel. A clearly defined difference in the spectral energy distribution of Tb³⁺ emissions was found for 231 nm UV and 160 nm VUV excitation of the Tb glass. The phenomenon of cross relaxation was only observed under 231 nm UV excitation. Different excitation mechanisms were taken into account. Direct excitation of Tb³⁺ ions together with Ce³⁺ ions occurred in the Tb glass under the 231 nm UV light, whereas indirect excitation consisting of host absorption of energy and transfer from host to Tb³⁺ ions occurred under the 160 nm VUV light.     

FTIR and UV–VIS optical absorption spectra of gamma-irradiated MoO3-doped lead borate glasses

Structure and optical properties of MoO₃-doped lead borate glasses which contain high PbO content (60, 70 and 80%) have been studied using Fourier transform infrared (FTIR) and ultraviolet–visible (UV–VIS) spectroscopic tools. FTIR spectra reveal absorption bands which are characteristic for various structural units of borate network, mainly BO₃ triangles and BO₄ tetrahedra, in addition to the PbO_n (where n = 3 and/or 4) structural units. UV–VIS optical absorption spectra reveal broad intense charge transfer UV bands due to Pb^2 + ions in the range 320–385 nm. Within this range, molybdenum ions, preferably Mo^3 + and Mo^5 +, can interfere at about 360–385 nm. Additionally, molybdenum ions give a weak visible band at about 850–860 nm. The optical absorption spectra of the studied glasses show marked resistance to successive gamma irradiation up to 5 Mrad. This shielding behavior can be related to the present high content of the high atomic mass Pb^2 + ions. Changes in the atomic structure before and after gamma irradiation are described and explained.     

Study of the effects of Ga3+ co-doping on the Lu0.8Sc0.2BO3:Ce scintillation crystals

Transparent Lu_0.8Sc_0.2BO₃ crystals doped with 1 at%Ce³⁺ and co-doped with 1 at% and 3 at%Ga³⁺ were grown by the Czochralski method. We applied absorption spectrum, luminescence spectra under UV and X-ray excitation, fluorescence decay curve, three dimensional thermoluminescene and X-ray absorption near edge spectroscopy to study the effect of Ga³⁺ co-doping on the Lu_0.8Sc_0.2BO₃:Ce scintillation crystals. Experimental results indicated that no positive contribution of the Ga³⁺ ion doping on the scintillation efficiency was found. The causes for the deterioration of scintillation efficiency by co-doping Ga³⁺ were revealed. The decrease of practical cerium content and the Ce³⁺/Ce⁴⁺ ratio in crystals, and the increase of the trap concentrations, although the corresponding trap types still maintained the same, played a joint influence on the degrading of scintillation efficiency of Lu_0.8Sc_0.2BO₃:Ce crystals.     

UV–visible and infrared absorption spectra of transition metals-doped lead phosphate glasses and the effect of gamma irradiation

Undoped and transition metals (TM 3d)-doped lead phosphate glasses were prepared. Ultraviolet–visible absorption spectra were measured in the range 200–1100 nm before and after successive gamma irradiation. Experimental results indicate that the undoped lead phosphate glass reveals before irradiation strong and broad ultraviolet absorption which is related to the co-sharing of absorption due to both trace iron impurities and lead ions (Pb²⁺). In the TM-doped glasses, characteristic absorption bands are obtained in both the UV and/or visible regions due to each respective TM ion in addition to that observed by the base undoped UV absorption. Gamma irradiation produces with the undoped glass a prominent induced ultraviolet broad band centered at about 300 nm originating mostly from the contribution of trace iron impurities and the visible spectra reveal markedly high shielding behavior towards successive gamma irradiation, due to the presence of both high content of heavy Pb²⁺ ions and the sharing of phosphate as a partner. With TM-doped samples, the observed induced bands are virtually varying and related to the type of the sharing TM ions. Infrared absorption spectra reveal in the undoped and TM-doped glasses characteristic structural phosphate groups mainly consisting of metaphosphate and pyrophosphate units. Transition metals are assumed to cause depolymerization of the phosphate glass network with different ratios but the changes in IR spectral data are limited due to the low doping level. Gamma irradiation of the samples is assumed to cause changes in the bond angles or bond lengths of the structural phosphate units within network as evident in the variation of the intensities of the IR bands.     

Enhanced 2.0 μm emission and energy transfer in Yb3+/Ho3+/Ce3+ triply doped tellurite glass

Ce³⁺ induced enhancement of Ho³⁺ ~ 2.0 μm emission in Yb³⁺/Ho³⁺ codoped sodium–zinc–tellurite (TNZ) glass was achieved under 980 nm LD laser excitation. The spectroscopic studies show that the upconversion is remarkably reduced by the presence of Ce³⁺. The ~ 2.0 μm fluorescence intensity is nearly triply enhanced, and the energy transfer efficiency from Yb³⁺ to Ho³⁺ is improved from 16.1% to 42.6% by increasing the Ce³⁺ concentration from 0 to 0.8 mol%. The mechanism responsible for the upconversion reduction and ~ 2.0 μm emission enhancement in Yb³⁺/Ho³⁺/Ce³⁺ triply-doped TNZ glass is also discussed. Our results indicate that the Yb³⁺/Ho³⁺/Ce³⁺ triply-doped TNZ glass is a promising candidate material for improving the Ho³⁺ 2.0 μm fiber laser performance.     

Luminescence properties and energy transfer of Eu/Tb ions codoped aluminoborosilicate glasses

Eu/Tb codoped aluminoborosilicate glasses were fabricated by high temperature melting-quenched technique and their luminescence properties were investigated by excitation and emission spectra. Under 376 nm excitation, blue, green and red emission bands were simultaneously observed at 425 nm, 485 nm, 540 nm and 611 nm, respectively. The broad blue emission band centered at 425 nm was originated from the reduced Eu²⁺ ions, which were reduced from Eu³⁺ ions at high temperature in an ambient atmosphere and the reduction process may be related with the optical basicity of glass matrix. A complex bright white light emission was obtained for 0.5 mol% Eu₂O₃, 0.5 mol% Tb₂O₃ codoped aluminoborosilicate glass with CIE-X = 0.31 and CIE-Y = 0.33. The energy transfer among Eu³⁺, Eu²⁺ and Tb³⁺ ions was also discussed.     

Spectroscopy and crystal-field analysis of Cr4+-doped transparent silicate glass-ceramics

We report transparent Cr⁴⁺-doped SiO₂–Al₂O₃–ZnO–Li₂O–K₂O glass-ceramics with broadband infrared luminescence. After heart-treatment, Li₂ZnSiO₄ crystallite was precipitated in the glasses, and its average size increased with increasing heat-treatment temperature. Racah parameters of Cr⁴⁺–Li₂ZnSiO₄ glass-ceramics have been calculated, and it was confirmed from absorption spectra that the energy levels of Cr⁴⁺-doped glass-ceramics are close to the cross point ¹E and ³T states. No infrared emission was detected in the as-made glass samples, but the broadband infrared emission centered at 1210 nm with the full width at half maximum (FWHM) of more than 250 nm was observed by exciting the glass-ceramics with excitation of an 808 nm laser diode. In order to analyze the located crystal field of Cr⁴⁺ ions, the emission spectra are fitted by multi-peak Gauss fitting. It is seen that the fluorescence spectra are fitted into two Gaussian bands at around 1195 and 1263 nm with band widths of 208 and 278 nm, respectively. The two Gaussian bands at around 1195 and 1263 nm have about the same decay rate, and hence they would probably originate from the same luminescent centers. The observed infrared emission could be attributed to Cr⁴⁺ ions at low-field sites in Li₂ZnSiO₄ glass-ceramics.     

Ultraviolet to near-infrared spectral modification in Ce3+ and Yb3+ codoped phosphate glasses

The ultraviolet to near-infrared spectral modification in Ce³⁺ and Yb³⁺ codoped phosphate glasses was realized through the energy transfer from Ce³⁺ to Yb³⁺. The absorption spectra, fluorescence excitation and emission spectra, luminescence decay curves, and time-resolved emission spectra were measured and analyzed. The energy transfer efficiency and concentration quenching efficiency were calculated based on the decay curves of Ce³⁺ 340 nm emission and Yb³⁺ 976 nm emission. The calculated and experimental NIR emission intensities on the Yb³⁺ concentrations were compared and discussed.     

Changes in Optical Properties of Ce: YAG Crystals under Annealing and Irradiation Processing

Changes of optical properties of cerium doped YAG single crystals (0.05, 0.1, 0.2 at.% Ce, and 0.2 at.% Ce "+" 0.1at.% Mg) after thermal annealing at 1400°C in air or 1200°C in N₂+H₂ mixture and subsequent g or proton irradiation were investigated. For initial Ce³⁺ contents <0.05 at.% an increase and for Ce³⁺ contents >0,05 at.% a decrease of the final Ce³⁺ concentration was observed. Appropriate changes in luminescence of Ce: YAG crystals were observed as a consequence of dopant concentration changes. They were small after gamma-irradiation of Ce: YAG crystals with Ce³⁺ content >0,05at.% and reached about 100% after gamma-irradiation of crystals with Ce³⁺ content < 0.05 at.%.     

Phase transformation and crystal growth behaviors of La3+/Ce3+-doped TiO2–20 wt% SnO2 gels

The transformation behaviors of La³⁺/Ce³⁺-doped TiO₂–SnO₂ gels were studied by using differential thermal analysis and X-ray diffraction methods so as to improve the phase transformation and decrease the granularity of crystals. Experimental results show that, anatase, rutile and SnO₂ nanocrystals can exist in the sintering products by varying La³⁺/Ce³⁺ contents and sintering temperatures. 0.8–1.1 wt% of La₂O₃ or CeO₂ doping greatly depresses the growth of anatase and rutile crystals, obtaining nanosized crystals when sintered up to 600 °C for 2 h. With La³⁺/Ce³⁺-doping and increasing their contents, the transformations of gel to anatase and anatase to rutile, as well as the growth of anatase and rutile crystals can be depressed, while the transformation temperature of anatase to rutile receives much less affect. Moreover, the La³⁺-doping has stronger effects on them than Ce³⁺ doping, but has a weaker inhibiting effect on precipitation and growth of SnO₂ crystals.     

Thermally stimulated luminescence of Ce and Tb doped SiO2 sol–gel glasses

Thermally stimulated luminescence (TSL) properties of cerium and terbium doped SiO₂ sol–gel glasses were studied after X-ray irradiation in the temperature range 10–700 K. The role of Ce³⁺ and Tb³⁺ as recombination centers was shown. The existence of a distribution of trap levels was observed; the activation energies of such a distribution were calculated to extend from about 8 × 10⁻³ eV up to 1.8 eV for both cerium and terbium doped sol–gel glasses. The effect of a post-densification thermal treatment on TSL properties was also analyzed.     

Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystal

Focused infrared femtosecond laser pulses (wavelength ∼800 nm, emission pulse duration 100 fs) were employed to fabricate optoelectronic devices such as waveguides, micro-gratings and laser active centers in LiF crystals. F₂ color centers of about 2 × 10¹⁸ cm⁻³ and refractive index change of about 1% at 633 nm were induced by the fs-laser irradiation. This technique was applied to fabricate a distributed-feedback (DFB) F₂ color center laser structure inside LiF single crystal. The LiF DFB laser exhibited laser oscillation at 707 nm at room temperature. The slope efficiency of ∼10% and beam divergence of ∼20 mrad were achieved.     

Luminescent properties and applications of Tb3+ doped silicate glasses with industrial scales

Tb³⁺ doped X-ray conversion glassy screen with an industrial scale (50 mm × 50 mm × 12 mm) was successfully fabricated, and its luminescent properties and applications in CCD imaging system were investigated. Results showed that Tb³⁺ doped silicate glasses mainly emit weak blue (400–460 nm) and strong green (480–570 nm) fluorescence. With the increase of Tb³⁺ ion concentration, the intensity of green emission increases, but that of blue emission decreases. Gd³⁺ ions can sensitize the luminescence of Tb³⁺ ions among silicate glasses. With the increase of CeO₂ concentration, the luminescent intensity of Tb³⁺ doped silicate glasses at 550 nm quickly decreases. However, the irradiation resistance of Tb³⁺ doped silicate glasses can be effectively improved by CeO₂ addition. The imaging quality of the luminescent glass screen is more excellent than that of Gd₂O₂S polycrystalline screens.     

Optical properties and valence state of Sm ions in aluminoborosilicate glass under β-irradiation

Sm-doped borosilicate glasses exposed to β-irradiation with doses from 8 × 10⁵ up to 4 × 10⁹ Gy have been studied by luminescence, Raman and electron paramagnetic resonance (EPR) spectroscopies. The luminescence spectra for pristine and irradiated glasses reveal that the β-irradiation process affects valence state of samarium ions. Intense emission at 684 and 727 nm excited by Ar+ laser (514.5 nm) due to the transition of Sm²⁺ ion was observed after irradiation. Relative proportion of Sm²⁺ ions estimated as a function of both Sm₂O₃ content and irradiation dose has the tendency to increase with increasing irradiation dose. In contrast, the EPR spectra of the studied samples reveal a decrease of the defect content, which are mostly hole defects, produced during irradiation, as a function of Sm₂O₃ content. Finally, the addition of Sm₂O₃ leads to a decrease of the Si–O–Si bending vibration modes shift and polymerisation changes under irradiation.     

Novel luminescence behaviors of Ce3+/Tb3+ co-doped phosphate glasses

In the present work, Ce³⁺/Tb³⁺ co-doped 60P₂O₅-30BaO-10B₂O₃ phosphate glasses are prepared and their luminescence properties are presented. Under excitation at 303 nm, the Ce³⁺ ions singly doped sample show a novel red emission besides the UV one. The co-doped samples show enhanced Tb³⁺ ions emission with the increasing of Tb³⁺ ions concentration at the cost of Ce³⁺ emission. The mechanism of this luminescent behavior is discussed with respect to the relevant energy transfer process.     

Spectroscopic studies of gamma irradiated Nd doped phosphate glasses

Ultra violet-visible (UV-Vis) and Fourier transform infrared (FT-IR) spectra of Nd doped phosphate glasses have been studied before and after gamma irradiation in order to understand the changes in the optical properties of glasses as well as to find the characteristics frequencies of the vibrational modes of chemical bonds, which decide the structural and spectral changes. UV, Vis, IR absorption and photoluminescence spectra of these glasses show changes depending on the composition of glass matrix. These changes are correlated on the basis of oxygen (O) and neodymium (Nd) concentration ratio obtained from energy dispersive X-ray spectroscopic (EDX) measurement. Gamma irradiation shows decrease in transmission below 700 nm for all the Nd³⁺ absorption lines from all the samples. Differential absorption spectra (UV-vis) of the samples before and after gamma irradiation show generation of some new bands below 700 nm along with dips (decrease) in the spectrum at the location of main Nd³⁺ absorption lines. This is attributed to the generation of different types of defects in the glass matrix along with possibility of change in the valence state of Nd³⁺ to Nd²⁺. IR absorption spectra of these glasses are found dominated mainly by the characteristics phosphate groups and water (OH) present in the glass network. The effects of gamma irradiation on IR absorption are observed in the form of bond breaking and possible re-arrangement of bonding. EDX and X-ray photoelectron spectroscopic (XPS) measurements indicate decrease in the relative concentration of oxygen in the glass samples after γ-irradiation.     

Rare-earth doped photonic crystal microcavities prepared by sol–gel

Rare-earth doped photonic materials and structures have been prepared by sol–gel processing, in the form of 1D photonic bandgap multilayer stacks of silica and titania. A significant enhancement of the Er³⁺ emission at ca. 1530 nm occurred when these ions were inserted into Bragg mirrors and microcavities. In Er³⁺/Yb³⁺ co-doped structures, an efficient energy transfer at 980 nm was observed from Yb³⁺ to Er³⁺ when these ions were in close proximity and especially when they were simultaneously present, in the same defect layer, with a 1530 nm photoluminescence enhancement of up to ∼25 times being observed for excitation at 980 nm, compared to the excitation of the same microcavities samples at 514.5 nm.     

Silica nano-rings and nano-hollows: Preparation and UV photoluminescence emission

By annealing fused silica coated with ultra-thin Ag film, silica nano-rings and nano-hollows were prepared on the substrate. The Ag nano-particles attached on the wall of nano-hollows or embedded in silica were confirmed with energy dispersive spectroscopy and transmission electron microscopy. Besides the well-known characteristic stretching bands of silica, three novel stretching bands around 1579, 1320 and 270 cm^?1 were found in the annealed Ag-coated silica by Raman scattering spectroscopy, which have been attributed to the O₂ in ground state, O–O and metal–oxygen stretching bands, respectively. The formation mechanism of nano-rings and nano-hollows has been discussed based on the experimental results. An ultraviolet photoluminescence emission of 360–370 nm from annealed Ag-coated silica was found when the excitations were 230 nm and 280 nm or longer. The possible photoluminescence emission mechanism has been discussed, which suggests that oxygen excess defects are responsible for the photoluminescence emission, and photoexcitation occurs in the silica as well as in Ag⁺ ions.     

Visible up-conversion photoluminescence from IR diode-pumped SiO2–TiO2 nano-composite films heavily doped with Er3+–Yb3+ and Nd3+–Yb3+

Efficient infrared-to-visible conversion is reported in thin film nano-composites, with composition 90% SiO₂–10% TiO₂, fabricated by a spin-coating sol–gel route and co-doped with Er³⁺ Yb³⁺ and with Nd³⁺:Yb³⁺ ions. The conversion process is observed under 808 nm laser diode excitation and results in the generation of green (526 and 550 nm) and red (650 nm) emissions: from the former, and blue (478 nm) and green (513 and 580 nm) emissions from the latter. The main mechanism that allows for up-conversion is ascribed to energy transfer among Er³⁺ and Yb³⁺ ions in their excited states. Up-conversion efficiency for red emission predominates in samples doped with Er³⁺:Yb³. The results illustrate the large potential of this class of materials for photonic applications in optoelectronics devices.