Low Silica Calcium Aluminosilicate Research Articles

Tunable color temperature of Ce^3+/Eu^2+, 3+ co-doped low silica aluminosilicate glasses for white lighting

In this paper we report results of tunable lighting in Ce(3+)/Eu(2+,3+) doped low silica calcium aluminosilicate glass. Optical spectroscopy experiments indicate that there is a red color compensation from Eu(2+) and Eu(3+) to the green emission from Ce(3+), resulting in a broad and tunable emission spectra depending on the excitation wavelength. This result analysed in the CIE 1976 color diagram shows a close distance from the Plank emission and a correlated color temperature, varying from 5200 to 3500K. This indicates that our system can be easily excited by GaN based blue LEDs, being an interesting phosphor for white lighting devices.

Analysis of energy transfer processes in Yb3+-Tb3+ co-doped, low-silica calcium aluminosilicate glasses

Energy transfer processes in Yb3+-Tb3+ co-doped, low-silica calcium aluminosilicate glasses were analyzed. Luminescence and time-resolved measurements were used to study upconversion processes, such as Yb-Tb cooperative sensitization, Yb-Yb cooperative luminescence, and Yb-Tb cross relaxation. The quantum cross relaxation efficiency was evaluated as a function on the Yb3+ concentration, and the maximum estimated value was approximately 51%. In addition, the intensity of the upconversion luminescence from the Tb3+:5D4 level decreased by two orders of magnitude comparing the value at room temperature with that at 123 K. As a consequence, Yb-Yb cooperative luminescence around 500 nm became comparable with the intensity of upconversion from the Tb3+:5D4 level. Furthermore, a dependence of the upconversion kinetics luminescence on temperature was observed. The upconversion rise time was constant and equal to 65 μs for temperatures between 296 to 473 K and decreased from 65 to 19 μs, without variation in the decay part, when the temperature was lowered from 296 to 123 K. These results were explained by a phonon-assisted cooperative sensitization process for the population of the Tb3+:5D4 level.

Spectroscopic properties, concentration quenching, and laser investigations of Yb^3+-doped calcium aluminosilicate glasses

“Calcium aluminosilicate” (CAS) and “low silica calcium aluminosilicate” (LSCAS) OH− free glasses have demonstrated a good potential for solid-state laser media and luminophore hosts due to good thermal, optical, and mechanical properties. Some examples have already been shown with Nd3+, Er3+, Ce3+, and Ti3+ luminescent ions. In this work, Yb3+ rare-earth laser ions have been introduced into the two series of CAS and LSCAS glasses melted at 1600 °C in a vacuum furnace and have been optically characterized and compared. Special attention has been devoted to both structural characterization in relationship with spectroscopic properties and concentration quenching phenomenon in relationship with laser parameters. Laser measurements at 1037 nm have been successfully performed under laser diode pumping.

Tunable light emission and similarities with garnet structure of Ce-doped LSCAS glass for white-light devices

In this paper, we report results concerning tunable light emission and color temperature in cerium-doped low-silica-calcium-alumino-silicate (LSCAS) glass for smart white-light devices. Spectroscopic results, analyzed using the CIE 1931 x– y chromatic diagram, show that this glass presents two broad emission bands centered at 475 and 540 nm, whose intensities can be tuned by the excitation wavelength. Moreover, the same emission can be achieved from a color temperature range from 3200 to 10,000 K, with a color-rendering index (CRI) of around 75% obtained by changing the optical path length of the sample. Our new phosphor LSCAS glass, which is a unique system that exhibits tunable yellow emission, combines all qualities for white-light devices.

Assessment of nonradiative relaxation time and characteristic diffusion time of neodymium, erbium and cobalt doped low silica calcium aluminosilicate glasses

Nonradiative relaxation time ( τ) and characteristic diffusion time ( τ β ) of Nd, Er and Co doped low silica calcium aluminosilicate (LSCA) glasses were assessed by means of photoacoustic spectroscopy. The results are interpreted in terms of frequency dependence predicted by the theory of Rosencwaig and Gersho. Whereas τ varied between 7.2 ms and 36 ms, the values of τ β lied between 0.25 ms and 0.7 ms. For the neodymium doped LSCA glass, a decrease in τ β was observed as the ion concentration was increased. The results indicates that rare earth ion acts as network modifier, disrupting the glass lattice.

Color tunability with temperature and pump intensity in Yb3+/Tm3+ codoped aluminosilicate glass under anti-Stokes excitation

Pump and thermally induced color tunabilities were demonstrated in Yb(3+)/Tm(3+) codoped low silica calcium aluminosilicate (LSCAS) glass under anti-Stokes excitation at 1.064 microm. The effects of pump intensity and sample's temperature on the upconversion emissions and mainly on the color tunabilities (from 800 to 480 nm) were investigated. The results revealed a 20- and a threefold reductions at 800/480 nm ratio as, respectively, the pump intensity and sample's temperature were increased from 27 to 700 kW/cm(2) and from 296 to 577 K. These behaviors with pump intensity and temperature (a strong increase of the 480 nm emission in comparison with the 800 nm one) were attributed to the several efficient processes occurring in the LSCAS system (Yb(3+)-->Tm(3+) energy-transfer processes, easy saturations of the Yb(3+) and Tm(3+) excited states, and radiative emissions). Besides these assigns, the temperature dependence is mainly assigned to the temperature-dependent effective absorption cross section of the ytterbium sensitizer through the so-called multiphonon-assisted anti-Stokes excitation process. Theoretical analyses and fits of the experimental data provided quantitative information.

High values of gain cross section and luminescence quantum efficiency in OH^−-free Ti^3+-doped low-silica calcium aluminosilicate glass

We recently reported that Ti(3+)-doped low-silica calcium aluminosilicate glass presents long luminescence lifetime (170 micros) and broad emission band (190 nm) shifted toward the visible region when compared with those from Ti(3+):sapphire single crystal and Ti(3+)-doped glasses [Phys. Rev. Lett.100, 027402 (2008)]. Here we demonstrate that this glass also shows high values of both gain cross section (approximately 4.7 x 10(-19) cm(2)) and luminescence quantum efficiency (approximately 70%). By comparing these values with those for Ti(3+):sapphire crystal, we can conclude that the studied Ti(3+)-doped glass is a promising system for tunable solid-state lasers.

Study on the observation of Eu2+ and Eu3+ valence states in low silica calcium aluminosilicate glasses

The optical, magnetic and structural properties of Eu doped low silica calcium aluminosilicateglasses were investigated. The optical absorption coefficient presented two bands at 39 246 and29 416 cm−1, which were assigned respectively to the , and transitions of Eu2+. The fluorescence measured at 300 K on a sample doped with 0.5 wt% ofEu2O3 exhibited a broadband centered at 17 350 cm−1, which is attributed to the transition of Eu2+, whereas the additional peaks are due to the transitions of Eu3+. From magnetization and XANES data it was possible to evaluate the fractions ofEu2+ andEu3+ for the sample doped with 0.5 and 5.0 wt% ofEu2O3, the values of which were approximately 30 and 70%, respectively.

Investigation of doped calcium aluminosilicate glass: A coupling between thermal-expansion and thermal-diffusion models for assessment of nonradiative relaxation time and characteristic diffusion time

This paper discusses the use of photoacoustic models to obtain the nonradiative relaxation time (τ) and characteristic diffusion time (τβ) for a sample showing visible absorption bands from fluorescent ion-doped low-silica calcium aluminosilicate glass. Two models allowing phase shift analyses, the thermal-expansion and thermal-diffusion models, are briefly reviewed. These models have limitations when the photoacoustic signal depends on both factors, in a coupling mechanism. An alternative model is proposed to take both thermal expansion and thermal diffusion into account with a single temperature solution for the heat-coupled differential equation. This model is simulated for absorbing samples near the thermally thick region. The model is applied to Eu–V codoped glass showing intermediate signal dependence from ω−1.0 to ω−3/2. The nonradiative time and characteristic diffusion time are derived with 33<τ(ms)<39, and τβ(ms)∼70 ms for the Eu-ion and 340<τβ(ms)<710 for the V-ion. Four absorption bands were analyzed (280, 350, 420, and 600 nm), which showed a signal dependence from ω−1.1 to ω−1.52. Absorption coefficients were derived from τβ in the range of 15<β(cm−1)<51, which agreed fairly well with spectrophotometer data for the same ions.

A Step Forward Towards Smart White Lighting: Combination of Glass Phosphor and Blue LEDs

We report a broad and intense yellow emission band from Ce3+-doped Low Silica Calcium Aluminosilicate (LSCAS) glass obtained under blue light excitation. The CIE 1931 x-y chromatic diagram shows that the association of the Ce3+:LSCAS glass with 405 nm blue LED presents emission very close to the ideal white light position.

Spectroscopic assignments ofTi3+andTi4+in titanium-dopedOH−free low-silica calcium aluminosilicate glass and role of structural defects on the observed long lifetime and high fluorescence ofTi3+ions

In this work we present the spectroscopic assignments of ${\text{Ti}}^{3+}$ and ${\text{Ti}}^{4+}$ in titanium-doped ${\text{OH}}^{\ensuremath{-}}$ free low-silica calcium aluminosilicate glass and the influence of structural defects on the observed long lifetime and high fluorescence intensity of ${\text{Ti}}^{3+}$ ions. Measurements were performed with electron-spin resonance (ESR), time resolved luminescence, ultraviolet-visible (UV-VIS) optical excitation and emission spectra, and conventional optical absorption and photoconductivity. The ESR data showed that the ${\text{Ti}}^{3+}/{\text{Ti}}^{4+}$ ratio increases with the doping concentration and that the ${\text{Ti}}^{3+}$ ions are in distorted octahedral sites. The assignment of the ${\text{Ti}}^{3+}$ and ${\text{Ti}}^{4+}$ emission bands derived from the spectroscopic results allowed us to propose a model explaining the mechanisms involved in the luminescence processes. The long lifetime of the ${\text{Ti}}^{3+}$ emission around 650 nm (on the order of $170\text{ }\ensuremath{\mu}\text{s}$) is about two orders of magnitude higher than the values found in the literature and was associated to the trapping of the excited electrons by the glass defects followed by detrapping via defect recombination. In conclusion, the combination of several techniques permitted a comprehensive characterization of the Ti ions in this ${\text{OH}}^{\ensuremath{-}}$ free glass.

Nanoscale surface displacement detection in high absorbing solids by time-resolved thermal mirror

Nanoscale surface displacement is used to determine thermo-optical–mechanical properties of high absorbing solids by means of the time-resolved thermal mirror method. The thermoelastic equation for the surface displacement and an expression for the probe beam intensity at the detector plane were derived. Experiments were performed in a high absorbing TiO2-doped low silica calcium aluminosilicate glass, and obtained the valuable values of the fluorescence quantum efficiency and thermal properties. The results indicate that this method is reliable for the characterization of semitransparent, high absorbing, and opaque materials.

Pressure dependence of elastic properties of low-silica calcium alumino-silicate glasses

The hydrostatic and uni-axial pressure dependence of elastic properties of a low-silica calcium alumino-silicate glass (LSCAS) is determined by ultrasonic pulse-echo techniques at room temperature. The experimental results are used to obtain the third-order elastic constants (TOECs) of these glasses. The pressure dependence of fractal bond connectivity of these glasses is discussed. The normal behavior of positive pressure dependence of ultrasonic velocities was observed for the glass. The pressure dependence of both shear modulus and bulk modulus are positive for these glasses.

Time resolved thermal lens measurements of the thermo-optical properties of Nd2O3-doped low silica calcium aluminosilicate glasses down to 4.3 K

In this work, the thermal lens spectrometry was applied to measure the thermo-optical properties of Nd2O3-doped low silica calcium aluminosilicate glasses as a function of temperature, between 4.3 and 300K. The thermal relaxation calorimetry was used to determine the specific heat, cp. The results showed a decrease of the thermal diffusivity of about one order of magnitude from 4.3K up to 300K, with a T−1 dependence in the interval between 20 and 70K and a T−0.35 between 4.3 and 20K. The fluorescence quantum efficiencies of the doped samples were calculated down to 50K, showing a variation of the order of 12% and 25% for the samples with 0.6 and 1.04mol% of Nd2O3, respectively. In addition, the temperature corresponding to the maximum in cp/T3, the so-called boson peak, was observed at about 17K for the undoped sample and at lower temperatures for the doped glasses. In conclusion, our results showed the ability of the time resolved thermal lens to determine the thermo-optical properties of glasses at temperatures lower than 300K, bringing new possibilities for experiments in a wide range of optical materials.

Highly efficient upconversion emission and luminescence switching from Yb 3+/Tm 3+ co-doped water-free low silica calcium aluminosilicate glass

We present highly efficient 480 and 800 nm upconversion emissions in Tm 3+/Yb 3+ co-doped water-free low silica calcium aluminosilicate glass under excitation at 976 nm. As a result of this efficient upconversion process, a luminescent switching with the excitation intensity has been observed. The switching is explained and discussed using rate equations analysis and saturation effects. By means of fitting of the experimental data point, it was possible to obtain the value of the energy transfer parameter related to the transition 2F 5/2, 3H 4→ 2F 7/2, 1G 4. The value of this parameter is higher than that of materials like YLF. This switching mechanism could be used in the development of sensors and networks for optical processing and optical communications.

Pump-power-controlled luminescence switching in Yb3+∕Tm3+ codoped water-free low silica calcium aluminosilicate glasses

Intense infrared-to-visible upconversion emissions in Tm3+∕Yb3+ codoped water-free low silica calcium aluminosilicate glasses have been obtained under excitation at 976nm. The results showed that as the pump power/intensity is increased, a reduction of up to one order of magnitude at the 800∕480nm emitted intensity ratio is observed; characterizing what can be denominated as luminescent switching. The physical origin of this switching is discussed and explained in terms of the tailoring of luminescent switchers to operate in a large range of pump powers, what could be used in the development of sensors and networks for optical processing and optical communications.

Low temperature specific heat of doped and undoped glasses

In this work we have measured the specific heat, cp, of several glasses between 2 and 160K. The experiments were performed in low silica calcium aluminosilicate (prepared under vacuum and room atmosphere conditions), in silicate and in fluoride glasses. The influence of neodymium, iron and cobalt in cp values at low temperature was also investigated. The scaling proposed by Liu and Löhneysen was used to analyze the experimental data. The temperatures in which the maxima in cp/T3, the so called boson peak, occur are discussed in terms of the Hrubý coefficient, which provides information about the glass forming ability.

Luminescence quantum efficiency investigation of low silica calcium aluminosilicate glasses doped with Eu2O3 by thermal lens spectrometry

The thermal diffusivity and luminescence quantum efficiency of Eu in low silica calcium aluminosilicate glasses have been measured using the mode mismatch Thermal Lens technique (TL). Two different concentrations were studied: 0.5 and 5(wt%) of Eu2O3. Our results show that the thermal diffusivity is independent of Eu2O3 concentration while the luminescence quantum efficiency decreased with the increasing in the doping concentration.

Time-resolved thermal lens measurements of the thermo-optical properties of glasses at low temperature down to 20 K

In this work the time resolved thermal lens spectrometry was applied to measure the absolute values of the thermo-optical properties of low silica calcium aluminosilicate and soda lime glasses at low temperatures, in the range between 20 and 300 K. The thermal relaxation calorimetry was used as a complementary technique to determine the specific heat. The results showed a marked decrease of the thermal diffusivity with the temperature rise, with a dependence similar to that of the mean free path $(\ensuremath{\sim}{T}^{\ensuremath{-}1})$ in the interval between 20 and 70 K, while in the range between 70 and 300 K the dependence was ${T}^{\ensuremath{-}(0.33\ifmmode\pm\else\textpm\fi{}0.02)}$. The marked variation of the temperature coefficient of the optical path length change with the temperature rise was attributed to the increase in the coefficient of the electronic polarizability. The results also showed that for the aluminosilicate glass the excess in the specific heat correlated to the so-called boson peak occurred at about 17 K, higher than that of soda lime, which occurs at about 12 K. In conclusion, our results showed the ability of the time resolved thermal lens to determine the thermo-optical properties of glasses at low temperatures, bringing possibilities for experiments in a wide range of optical materials.

Relationship Between Structure and Glass Transition Temperature in Low‐silica Calcium Aluminosilicate Glasses: the Origin of the Anomaly at Low Silica Content

The anomalous behavior of the glass transition temperature (Tg) in low silica calcium aluminosilicate glasses has been related to the structural modifications observed by neutron and X‐ray diffraction. The diffraction data indicate that Al and Si are in tetrahedral sites and that Ca atoms are in distorted octahedral sites. By subtracting the correlation functions for glasses at constant SiO2 or constant Al2O3 content, we have shown that Si and Al atoms are introduced in a different way within the glass structure. Si is present in various Qn sites, while Al resides in Q3 and Q4 sites for glasses with high CaO content and enters fully polymerized Q4 sites with increasing SiO2 or Al2O3 content. The higher proportion of Al in Q3 positions at high CaO content yields a depolymerization of the network. The lower connectivity will contribute to a decrease of the viscosity, which may be at the origin of the decrease of Tg for glasses at low silica content.