Doped Calcium Aluminosilicate Research Articles

Spectroscopic investigation and interest of Pr3+-doped calcium aluminosilicate glass

Abstract In this report, Pr3+-doped calcium aluminosilicate (CAS) glasses were synthesized and their spectroscopic properties investigated. The present subject was motivated by the red emission of the Pr3+ ion that can be combined with the green-yellow emission of other luminescent ions like Ce3+ or Eu2+ for white light generation. This combination of materials has been required since commercial diode devices for white lighting exhibit low color rendering index (CRI) and high correlated color temperature (CCT) due to their low emission intensity in the red region. The results showed that when Pr3+-doped CAS glasses are excited at 445 nm, red and green broadband emissions (FWHM ∼ 1300 and 1800 cm−1, respectively) with a strong dependence on the ion concentration can be observed. The (x, y) coordinates in the CIE-1931 chromaticity diagram changed from (0.61, 0.36) to (0.47, 0.41) when the Pr3+ concentration varied from 0.2 to 2.0 wt.%, what was explained in terms of cross-relaxation mechanisms between Pr3+ ions. The luminescence quantum yield (QY), in the visible range from 450 to 800 nm, of the Pr3+-doped CAS glass was determined by an integrating sphere as 0.37, which is a relatively high value for Pr3+-doped glasses. At high Pr3+ concentrations, the studied system is appropriate to be combined with other green-yellow phosphor material for white light generation.

Standard and modified Judd-Ofelt theories in Pr3+-doped calcium aluminosilicate glasses: A comparative analysis

The small energy difference between the fundamental level and the first opposite parity configuration of Pr3+-doped hosts is particularly challenging for the characterization of radiative transitions using the Judd-Ofelt theory, although modified versions of the theory have been proposed in the past for the investigation of praseodymium doped materials. Here, we present a detailed spectroscopic investigation on two sets of calcium aluminosilicate glasses, with 34 wt% of SiO2 (CAS) and with 7 wt% of SiO2 (LSCAS), doped with different concentrations of Pr3+ (0.5, 1.0 and 2.0 wt%). We use the standard Judd-Ofelt theory to characterize the glasses and the results and derived spectroscopic quantities — such as transition probabilities, radiative lifetimes and branching ratios — are compared to results obtained by the modified Judd-Ofelt theories. The analysis showed that the modified theories could lead to smaller values of root mean square deviations. However, a better agreement between experimental data and the standard theory was achieved when the derived spectroscopic quantities are taken into account. Moreover, the branching ratios of the P03→H43 and D21→H43 transitions were over 60% for both glass hosts, suggesting its potential use as solid-state laser devices.

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.

Energy transfer mechanism for generation of white light in Tb3+-doped calcium aluminosilicate amorphous powder

Calcium aluminosilicate (Ca3Al2Si6O18) powders containing different doping concentrations of luminescent trivalent terbium ions (Tb3+) were prepared by combustion synthesis. Non-crystalline samples were obtained with heat treatment at 850°C for 4h. Luminescence corresponding to 4f‐4f electronic transitions originating from states 5D3 and 5D4 of Tb3+ were observed under UV excitation at λ = 355nm. Tunable green-to-white emission was obtained by adjusting the Tb3+ doping concentration. The origin of the color change phenomenon was attributed to the presence of energy transfer among Tb3+ ions. The energy transfer mechanism was found to be predominantly of exchange type. The potential use of this phosphor for solid state white lighting was carefully assessed by estimating its chromaticity coordinates, correlated color temperature and luminescence quantum yield.

Fracto-mechanoluminescence and thermoluminescence properties of UV and γ-irradiated Ca₂Al₂SiO₇:Ce³⁺ phosphor.

Ce(3+)-doped calcium aluminosilicate phosphor was prepared by a combustion-assisted method at an initiating temperature of 600°C. Structural characterization was carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The absorption spectra of Ca2Al2SiO7:Ce(3+) showed an absorption edge at 230 nm. The optical characterization of Ca2Al2SiO7:Ce(3+) phosphor was investigated in a fracto-mechanoluminescence (FML) and thermoluminescence (TL) study. The peak of ML intensity increased as the height of impact of the moving piston increased. The TL intensity of Ca2Al2SiO7:Ce(3+) was recorded for different exposure times of UV and γ-irradiation and it was observed that TL intensity was maximum for a UV irradiation time of 30 min and for a γ-dose of 1180 Gy. The TL intensity had three peaks for UV irradiation at temperatures 82°C, 125°C and 203°C. Also the TL intensity had a single peak at 152°C for γ-irradiation. The TL and ML emission spectra of Ca2Al2SiO7:Ce(3+) phosphor showed maximum emission at 400 nm. The possible mechanisms involved in the TL and ML processes of the Ca2Al2SiO7:Ce(3+) phosphor are also explained.

Nd3+ doped CAS glasses: A thermo-optical and spectroscopic investigation

Previous works have showed that calcium aluminosilicate (CAS) glasses, when prepared under vacuum conditions, are good candidates for solid state laser medium hosts and optical devices due to their appropriated thermal, optical and mechanical properties. These promising results led us to investigate the thermo-optical properties and emission spectra as a function of temperature in Nd3+ doped CAS glasses. Temperature changes in optical systems can cause structural modifications to the host, as well as other effects, such as emission quenching, or self-focalization. In this work, two series of CAS glasses, doped up to 5wt.% Nd2O3, were prepared and characterized. Measurements of thermal coefficient of optical path length (dS/dT) and emission were performed on both series of Nd3+ doped CAS. In addition, measurements of optical absorption coefficient and emission lifetime were carried out. The results are discussed in terms of temperature dependence of these properties and Nd2O3 content. Comparisons with other glasses, such as LSCAS (low-silica calcium aluminosilicate) are also presented.

Resonant excited state absorption and relaxation mechanisms in Tb^3+-doped calcium aluminosilicate glasses: an investigation by thermal mirror spectroscopy

Resonant excited state absorption (ESA) and relaxation processes in Tb(3+)-doped aluminosilicate glasses are quantitatively evaluated. A model describing the excitation steps and upconversion emission is developed and applied to interpret the results from laser-induced surface deformation using thermal mirror spectroscopy. The fluorescence quantum efficiency of level (5)D(4) was found to be close to unity and concentration independent while, for the level (5)D(3), it decreases with Tb(3+) concentration. Emission spectroscopy measurements supported these results. ESA cross sections are found to be more than three orders of magnitude higher than the ground state absorption cross section.

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.

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 toward smart white lighting: Combination of glass phosphor and light emitting diodes

We present our recent achievements of a glass able to produce smart white light combining a glass phosphor with light emitting diodes (LEDs). The combined emissions of Ce3+-doped calcium aluminosilicate glass and the 405 nm LED using the Commission Internationale de l’Éclairage 1931 chromatic diagram showed that this system presents an emission close to the ideal white light and allows tunability. In addition, the glass blue emission overlaps with the spectral range of the retinal photoreceptors involved in circadian responses. This glass combined with UV LED emission is suitable for circadian lights and therefore may contribute to improve environmental lighting and human well being.

Concentration-dependent luminescence of Tb 3+ ions in high calcium aluminosilicate glasses

This study deals with the results on the concentration-dependent fluorescence properties of Tb 3+-doped calcium aluminosilicate (CAS) glasses of composition (100− x)(58SiO 2–23CaO–5Al 2O 3–4MgO–10NaF in mol%)- x Tb 2O 3 ( x=0, 0.25, 0.5, 1, 2, 4, 8, 16, 24, 32, 40 in wt%). The FTIR reflectance spectra suggested the role of dopant ions as network modifiers in the glass network. The fluorescence spectra of low Tb 3+-doped glasses have revealed prominent blue and green emissions from 5D 3 and 5D 4 excited levels to 7F j ground state multiplet, respectively. The glass with 2 wt% of Tb 2O 3 has exhibited maximum intensity of blue emission from 5D 3 level, while green emission from 5D 4 level has increased linearly up to 24 wt% and showed reduction in the rate of increase for higher Tb 2O 3 concentrations. The concentration quenching of blue emission ( 5D 3→ 7F j) is attributed mainly to the resonant energy transfer (RET) assisted cross-relaxation (CR) among the excited and nearest neighbour unexcited Tb 3+ ions in the glass matrix. The decline in rate of increase of green emission ( 5D 4→ 7F j) at higher concentrations has been explained due to a possible occurrence of cooperative energy transfers leading to 4f 8→4f 75d transition interactions. The blue and green emission decay kinetics have been recorded to compute the excited level ( 5D 3 and 5D 4) lifetimes, which confirmed the Tb 3+ concentration quenching of the blue emission in these glasses.

Compositional Dependence of Fluorescence Spectra of Cr3+/Cr4+-Doped Calcium Alumino-Silicate Glasses for Broadband Telecommunication

The compositional dependence of the optical properties of Cr-doped glasses in a calcium alumino-silicate ternary system were investigated. By 792 nm excitation, two broad emission bands centered at 1000 and 1350 nm, which were attributed to Cr3+ and Cr4+ ions, respectively, were observed. The intensity ratio of these bands changed continuously as the host composition was varied; a positive correlation between the relative intensity of the Cr4+ band and the optical basicity Λ of the host glass was observed. In glasses with Λ=0.68–0.70, the two bands became comparable in their intensities and formed an extremely broad emission band with a bandwidth of more than 500 nm as their superposition.

Laser emission at 1077 nm in Nd3+-doped calcium aluminosilicate glass

Laser emission at 1077 nm (4F3/2→4I11/2) was demonstrated in Nd3+-doped low silica content calcium aluminosilicate (LSCA) glass under 810-nm pumping by a Ti:sapphire laser. The slope efficiency of the laser emission was found to be 34% with a 20-mW threshold power. The excited-state absorption (ESA) and stimulated emission were measured between 850 and 1520 nm. The results showed two ESA bands from 931 to 1070 nm (4F3/2→2D3/2,2G9/2,4G11/2,2K15/2) and from 1160 to 1414 nm (4F3/2,4G9/2 → 4G7/2,2K13/2). The stimulated emission for the laser transition was found to be 45-nm broad with a peak value of 1.8×10-20 cm2. Thermal and mechanical properties of the LSCA glass were compared to the ED-2 and LHG-8 glasses. While the mechanical properties of the LSCA glass are superior to those of the commercial glasses, its temperature coefficient of optical path length is the highest among the three hosts.