Low Silica Calcium Aluminosilicate Glasses Research Articles

The first demonstration of wavelength-tunable Yb3+-doped low silica calcium aluminosilicate (LSCAS) glass lasers

We report on the first demonstration of diode-pumped wavelength-tunable Yb3+-doped low silica calcium aluminosilicate (LSCAS) glass lasers. Using a 976 nm diode laser as pump source, a Yb:LSCAS laser has been successfully achieved with a maximum output power of about 0.28 W. Using an MgF2 thin plate as birefringence filter, we have found that the lasing wavelength can be tuned with a range of about 60 nm. We have also analyzed the intracavity round-trip loss of the current laser cavity, and related it to the optical quality of the used Yb glass. This sheds light on the path of the development of upcoming Yb3+ glass lasers in the future since it has many advantages, and after improving the optical quality, the wide and smooth emission spectrum of Yb:LSCAS will be very conducive to obtaining femtosecond pulse lasers.

Downconversion luminescence in Ce3+, Yb2+, and Yb3+ co-doped low silica calcium aluminosilicate glasses: Potential for spectral conversion in solar cell technology

Yb–Ce co-doped LSCAS glasses melted under a vacuum atmosphere are free of OH− radicals presenting downconversion luminescence of interest for solar cell application. Absorption spectra showed an ultraviolet–visible broad band related to the 2F5/2, 2F7/2 → 5d transitions of the Ce3+. Time-resolved luminescence evidenced energy transfer from Ce3+ to Yb3+, and the contour maps spectra showed a decrease in Ce3+ emission with the increase of Yb concentration. Thermal lens results evidenced an increase in heat generation, suggesting the occurrence of nonradiative transitions due to the formation of Yb2+ or Ce4+ ions. The observed emissions are attributed to energy transfer from Ce3+ or Yb2+ to two Yb3+ ions by quantum cutting mechanism, emission from Ce3+ at 520 nm, and predominantly, a phonon-assisted energy transfer from Ce3+ or Yb2+ to one Yb3+ ion.

Thin-film of Nd3+–Yb3+ co-doped low silica calcium aluminosilicate glass grown by a laser deposition technique

Recent theory has predicted that spectral converters would be a promising pathway to improve the spectral response of silicon solar cells. As a consequence, new materials and methods of coupling these sensitizers in photovoltaic cells have been investigated. In this work, a UV pulsed laser deposition technique was used to grow thin films of Nd3+–Yb3+ co-doped low silica calcium aluminosilicate glass on silicon substrates, aiming for the development of hybrid solar cells with spectral converters. The role of both laser shots and substrate temperature was investigated, as well as the film's optical and structural properties. Our films present no crystalline order (amorphous) and present a surface with a roughness of the order of sub-nanometers. Significant flat regions between grains were observed by AFM analyses. Photoacoustic spectroscopy was successfully developed to determine the thicknesses of thin films in a range higher than the limit of the x-ray reflectivity technique (≥170 nm). From 1000 to 20 000 laser shots, the film thicknesses increased from 27 to 680 nm. The photoluminescence measurements demonstrated efficient energy transfer processes toward the Yb3+ 950–1200 nm emission, suggesting this material as a promising spectral converter for the development of hybrid solar cells.

Spectroscopic investigation and heat generation of Tm3+/Ho3+-codoped aluminosilicate glasses emitting at 2.0 µm

T m 3 + / H o 3 + -codoped low silica calcium aluminosilicate (LSCAS) glasses were prepared and characterized using optical absorption, luminescence decay, and thermal lens techniques. The emission band at 2.0 µm exhibited an enhancement when the Ho concentration was increased. The luminescence quantum efficiency values were determined for both 3 F 4 and 5 I 7 levels of T m 3 + and H o 3 + ions, respectively. The high decrease of the quantum efficiency of the 3 F 4 ( T m 3 + ) level for high H o 3 + concentration evidenced efficient energy transfer processes from T m 3 + to H o 3 + . The heat generated in the energy transfer processes were estimated via a thermal lens experiment, and a rate equation model was proposed to explain the obtained result. The experimental and estimated parameters with the proposed theoretical model are in good agreement. Finally, the results of this work suggest the T m 3 + / H o 3 + -codoped LSCAS is a promising material for applications using the emission around 2.0 µm that is of great interest for biomedical applications.

Evaluating the link between blue-green luminescence and cross-relaxation processes in Tb3+-doped glasses

In this study, luminescence properties of Tb3+-doped low silica calcium aluminosilicate glasses are analyzed as a function of Tb3+ concentration (0.04–15.0 wt%). Decay curves for the 5D3 level are found to be non-exponential for all Tb3+ concentrations due to ion–ion energy transfer through cross-relaxation process. To identify the origin of the energy transfer mechanism, the decay curves were well fitted by Inokuti–Hirayama model (s = 6), which indicates that the energy transfer process is related to dipole–dipole interaction. Trivalent Tb-doped materials normally exhibit stronger green emission compared to blue emission, a feature that increases with Tb3+ concentration and is attributed to the cross-relaxation process. In this paper, we show a remarkable blue emission in comparison to other Tb3+-doped glasses. A theoretical model for the blue and green emissions, from 5D3 and 5D4 levels respectively, was proposed to establishing a link between the color emission and the 5D3 lifetime quenching due to a cross-relaxation process. This model was successfully applied to the experimental results of this paper as well as previous data in the literature.

Two ratiometric thermometry methods based on the interplay between Eu[formula omitted] and Eu[formula omitted] and single Eu[formula omitted] emissions on OH[formula omitted]-free low-silica calcium aluminosilicate glass

The constant demand to accurately measure the temperature sheds light on luminescent materials that have dependence on their thermodynamic quantities. Several materials have been applied as sensors to measure temperature for different applications, however, very few showed high precision and sensitivity. This work reports the Eu2+/Eu3+-doped low-silica calcium aluminosilicate glass as a potential candidate for high sensitive ratiometric thermometer. The glass was analyzed close to the physiological temperature range, between 288.15 – 333.15 K. Since this glassy matrix provides the presence of two europium valences, two different approaches are presented. The first takes into account the temperature dependence of 5D0→7F4 emission for two different excitation wavelengths, and the second one is based on the interplay between Eu2+ and Eu3+ emissions. Both ratiometric approaches presented high reproducibility and relative sensitivity values, from 1.2 to 1.6 %/K and from 0.75 to 0.95 %/K, for the first and second approach, respectively.

Combination of broad emission bands of Ti3+,4+/ Eu2+,3+ co-doped OH− free low silica calcium aluminosilicate glasses as emitting phosphors for white lighting devices

In this report, Ti3+, 4+/Eu2+, 3+ co-doped OH− free low silica calcium aluminosilicate glasses (LSCAS) were synthesized and their optical properties investigated aiming applications as phosphors for white light generation. This research explores the possibility of combining the broad emission bands in the visible of these glasses co-doped with Ti3+,4+/Eu2+ ions with those in the red emitted by Eu3+, in addition to the high luminescence quantum yield they present. The excitation and emission results show that exciting the samples at 300 nm, a broad emission band is generated, embracing very well the visible region. The emission spectra were analyzed by (x-y) CIE-1931 chromaticity diagram and the color rendering index (CRI) presented Ra values up to 80, with the correlate color temperature (CCT) varying from 4995 to 6609 K. The luminescence quantum yield determined by Thermal Lens Spectroscopy showed values of 38 and 42% for the samples with (in wt%) 0.1Eu2O3-2.0TiO2 and 0.5Eu2O3-2.0TiO2, respectively. Finally, the energy transfer processes between these luminescent ions were discussed, highlighting the potential of the Eu–Ti LSCAS glasses as candidates for the development of devices for white light generation.

Elastic and structural properties of low silica calcium aluminosilicate glasses from molecular dynamics simulations

The elastic constants and the structural of low silica calcium aluminosilicate glasses with a low content of silica (5–25 mol%) and [CaO]/[Al2O3] =2, are investigated using molecular dynamics simulations. In particular, their elastic constants are calculated using two methods: a standard approach at zero temperature and another method designed to work at finite temperature. We show that while they both reproduce qualitatively the experimental behavior, only the method taking temperature into account provides a quantitative agreement with the experimental values. Therefore, taking temperature into account in the determination of elastic constants seems necessary to give a satisfactory account of the mechanical properties of these materials. In accordance with the experimental results, our molecular dynamics calculations show values of elastic constants that decrease with increasing amount of silica in the vitreous network. Then, the structure of the glasses is analyzed in term of pair distribution functions, oxygen kinds, and the parameter of short-range order, and discussed in correlation with the available experimental data. An overall good agreement is obtained with the calculated structure and the experimental data.

The effect of silica content on the luminescence properties of Tb3+-doped calcium aluminosilicate glasses

Tb3+ doped materials exhibit strong emission in the green (5D4) and weak emission in the UV-blue (5D3) regions. The blue emission can occur from direct UV excitation or by excited state absorption, through 5D4 metastable level. In this paper, we investigate the effect of silica content on the spectroscopic properties of the Tb3+ doped calcium aluminosilicate glasses. The low silica calcium aluminosilicate glass (LSCAS) and calcium aluminosilicate glass (CAS) doped with 0.5 wt% of Tb3+ were prepared and characterized through absorption, excitation, Stokes emission, upconversion emission and 5D4 lifetime decay measurements. The green luminescence in the LSCAS glass presented a fast quenching. The silica influence on this behavior was discussed and a comprehensive interpretation is presented, correlating the Tb3+ luminescence behavior with the silica concentration. Additionally, we analyze the excited state absorption effect on fluorescence transients to 5D4 energy level lifetimes.

Fluorescence line narrowing and Judd-Ofelt theory analyses of [formula omitted]-doped low-silica calcium aluminosilicate glass and glass-ceramic

Eu-doped calcium aluminosilicate glass and glass-ceramic were synthesized and studied by site-selective spectroscopy. Using both broadband excitation and fluorescence line narrowing techniques, the D05→F17 emission transition was studied under resonant excitation at different wavelengths within the F07→D05 band. In order to explore the local structure of the systems, crystal-field analysis was applied assuming C2v site symmetry. Moreover, from the emission spectra of glass and glass-ceramic systems, Judd-Ofelt parameters Ωt (t = 2, 4) were considered. Transition probability (A), stimulated emission cross-section (σ), radiative lifetime (τR) and branching ratios (β) due to the transition D05→FJ7 (J = 1, 2, 3 and 4) of Eu3+ ions were evaluated in different local fields and their relative variations were discussed. The crystallization process, which converted the glass into glass-ceramic, promoted an important enhancement in the quantum efficiency, which was improved from 45% to 60%.

UV–visible-NIR light generation through frequency upconversion in Tm3+-doped low silica calcium aluminosilicate glasses using multiple excitation around 1.2 µm

Intense ultraviolet upconversion emission was produced in single Tm3+-doped OH--free low silica calcium aluminosilicate glasses. A new excitation route based upon multi-Stokes Raman emissions generated in an optical fiber pumped at 1.064 µm, and exploiting the absorption band around 1.2 µm by means of the 3H5 thulium excited state, was used. Furthermore, the other bands of the stimulated Raman scattering spectrum resonantly enhances all the upconversion processes, resulting in efficient ultraviolet (295 nm, 360 nm), blue (456 nm, 480 nm), red (650 nm, 667 nm), and near-infrared (800 nm) emissions. The population of the 1P0, 1D2, 1G4, 3F2 and 3H4 excited-state emitting levels was accomplished through stepwise multi-photon absorption. Results indicate competing cross-relaxation processes involving Tm3+ ion-pairs producing UV emission population quenching

Pump-induced refractive index changes in Tb3+ doped glasses

It now well known in laser materials, that a refractive index change appears when the active ions are pumped from ground to excited state due to the polarizability difference between ground and excited states (metastable). In this paper this effect was investigated in Tb3+ doped glasses: calcium alumino phosphate (CAP), low-silica calcium aluminosilicate (LSCAS) and calcium aluminosilicate (CAS). The measurements were performed using the time resolved Z-scan technique, with an Ar+ laser at 488nm, close to the resonance of 7F6→5D4 absorption line, where 5D4 is a metastable state. We obtained for low-silica calcium aluminosilicate glass Δαp~10−24cm3 which is the highest value ever reported for a RE doped material.

Mechanisms of optical losses in the 5D4 and 5D3 levels in Tb3+ doped low silica calcium aluminosilicate glasses

Trivalent Tb-doped materials exhibit strong emission in the green and weak emission in the UV-blue levels. Usually, this behavior is attributed to the cross relaxation (CR) process. In this paper, the luminescence properties of Tb3+-doped low silica calcium aluminosilicate glasses are analyzed for UV (λexc = 325 nm) and visible (488 nm) excitations. Under 325 nm excitation, the intensity of green luminescence increases proportionally to Tb3+ concentration. However, the blue luminescence intensity is strongly reduced with the increase of concentration from 0.5–15.0 wt. %. In the case of 488 nm excitation, a saturation behavior of the green emission is observed at intensities two orders of magnitude smaller than expected for bleaching of the ground state population. Using a rate equation model, we showed that this behavior can be explained by an excited state absorption cross section two orders of magnitude larger than the ground state absorption. The blue emission is much weaker than expected from our rate equations (325 nm and 488 nm excitation). We concluded that only the CR process cannot explain the overall feature of measured luminescence quenching in the wide range of Tb3+ concentrations. Cooperative upconversion from a pair of excited ions (5D3:5D3 or 5D3:5D4) and other mechanisms involving upper lying states (4f5d, charge transfer, host matrix, defects, etc.) may play a significant role.

Spectroscopic studies and downconversion luminescence in OH−-free Pr3+–Yb3+ co-doped low-silica calcium aluminosilicate glasses

We report on spectroscopic studies and downconversion luminescence in OH−-free Pr3+–Yb3+ co-doped low-silica calcium aluminosilicate glasses (LSCAS). The dependencies of the visible to near-infrared emissions, photoacoustic signals, luminescence decay times and energy transfer efficiencies from Pr3+→Yb3+ on the doped Yb2O3 concentration were investigated. The results confirmed the occurrence of downconversion processes in the examined samples, followed by a back-energy transfer mechanism from the Yb3+:2F5/2 level to the Pr3+:1G4 level, from where the multiphonon relaxations through the 1G4,3F4→3H4 transitions occurred. The photoacoustic data provided evidence of the energy transfer that resulted in heat generation in the sample, confirming the observations provided by the luminescence data. The energy transfer mechanisms that are involved in the Pr3+:Yb3+ co-doped LSCAS glasses are discussed.

Near-infrared quantum cutting in OH− free Nd3+-Yb3+ co-doped low-silica calcium aluminosilicate glasses

Near-infrared (NIR) quantum cutting (QC) through downconversion (DC) in OH− free Nd3+-Yb3+ co-doped low-silica calcium aluminosilicate glasses (LSCAS) was observed. Luminescence and time-resolved measurements allowed the investigation of the origin of the Yb3+ DC emission. Our results showed that only one NIR photon is emitted by the Yb3+ ion through one-step energy transfer after excitation with a VIS photon absorbed by the Nd3+ ion. However, after excitation with an UV photon absorbed by the Nd3+ ion, two NIR photons are emitted by the Yb3+ ion through two-step energy transfer. The Yb3+ DC normalized intensities after excitation at 325 nm showed an increase of a factor of two compared to that of 514 nm excitation. In addition, the energy transfer probabilities from Nd3+ to Yb3+ were estimated by analyzing the Yb3+ DC kinetics. The overall results showed an efficient QC through DC process in Nd3+-Yb3+ co-doped LSCAS glasses, and the energy transfer mechanisms are discussed.

Spectroscopic investigation and heat generation of Yb^3+/Ho^3+ codoped aluminosilicate glasses looking for the emission at 2 μm

Energy transfer (ET) and heat generation processes in Yb3+/Ho3+-codoped low-silica calcium aluminosilicate glasses were investigated using thermal lens (TL) and photoluminescence measurements looking for the emission around 2.0 μm. Stepwise ET processes from Yb3+ to Ho3+, upon excitation at 0.976 μm, produced highly efficient emission in the mid-infrared range at around 2.0 μm, with high fluorescence quantum efficiency (η1∼0.85 and independent of Ho3+ concentration) and relatively very low thermal loading (<0.4) for concentration up to 1.5% of Ho2O3. An equation was deduced for the description of the TL results that provided the absolute value of η1 and the number of emitted photons at 2.0 μm per absorbed pump photon by the Yb3+ ions, the latter reaching 60% for the highest Ho3+ concentration. These results suggest that the studied codoped system would be a promising candidate for the construction of photonic devices, especially for medical applications.

Broad combined orange-red emissions from Eu^2+- and Eu^3+-doped low-silica calcium aluminosilicate glass

In this paper, a broad combined orange-red emission from Eu²⁺- and Eu³⁺-doped low-silica calcium aluminosilicate (LSCAS) glass is reported. Spectroscopic results demonstrate that it is possible to tune the emission wavelength by changing the excitation wavelength in the UV-Vis region. The color coordinates for the emission spectra were calculated, and using the Commission Internationale de l'Éclairage 1931 and 1976 chromatic diagrams, it is possible to note that they are dependent on the excitation wavelength. In addition, the (u', v') color coordinates for the investigated LSCAS samples are close to the Planckian spectrum in the cold region between 2000 and 2600K. Our results show that the Eu:LSCAS system can be used in a white light phosphor when mixed in aggregate with phosphors using green-yellow luminescent ions.

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.

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.