Enhanced luminescence at 2.88 and 2.04 μm from Ho3+/Yb3+ codoped low phonon energy TeO2–TiO2–La2O3 glass

The high phonon energy and short infrared cut-off wavelength of conventional oxide glass (or crystal) hosts are the limitations to achieve mid-infrared (MIR, λ≥2.5μm) luminescence. In present study, the luminescence performance of low phonon and non-conventional TeO2-TiO2-La2O3-based glass (TTL) host doped with Ho3+ and Ho3+/Yb3+ has been investigated, for visible to MIR range. The MIR emission band with peak at 2.88μm (Ho3+:5I6→5I7) and NIR band at 2.04μm (Ho3+:5I7→5I8) has been realized from Ho3+ singly doped TTL glass due to low phonon energy and extended transmission window of the host. Intensity of MIR and NIR emission bands have enhanced significantly in Ho3+/Yb3+: TTL glass under Yb3+ excitation, signifying an efficient Yb3+→Ho3+ energy transfer. The Judd-Ofelt analysis, on Ho3+ absorption characteristics reveals relatively better radiative transition probability (34.4s−1) and branching ratio (10.5%), which is associated to Ho3+:5I6→5I7 transition. The effective bandwidth of 2.88μm emission band is 180nm, with stimulated emission cross-section is 4.26×10-21cm2 and its gain bandwidth has been evaluated as 7.67×10-26cm3. For 2.04μm (Ho3+:5I7→5I8) emission band, the effective bandwidth of 160.5nm and gain bandwidth of 7.26×10-26cm3 have been accomplished. The non-resonant Förster-Dexter method has been applied to Ho3+/Yb3+: TTL glass on emission (donor, Yb3+) and absorption (acceptor, Ho3+) cross sections. The evaluated donor-donor (CDD) and donor-acceptor (CDA) energy transfer micro-parameters are 1.02×10-38 and 5.88×10-41cm6/s respectively while, maximum energy transfer efficiency has been 80%. In concise, Ho3+/Yb3+ codoped TeO2–TiO2–La2O3 glass host has revealed its potential for MIR to NIR photonic applications

Influence of Ho2O3 on Optimizing Nanostructured Ln2Te6O15 Anti‐Glass Phases to Attain Transparent TeO2‐Based Glass‐Ceramics for Mid‐IR Photonic Applications

The transparent TeO2‐based glass‐ceramics (GCs) have yet to achieve the breakthrough in photonic technologies, because of poor understanding in optimizing the growth of nanostructured crystalline phases. In the present investigation, the size effect of phase‐separation‐induced, nanostructured Ln2Te6O15‐based (Ln: Gd, Ho) “anti‐glass” phase in Ho2O3‐modified TeO2‐based TTLG (in mol%, 80TeO210TiO25La2O35Gd2O3) glass has considered to achieve transparent GCs. Raman study of TTLG glass reveals the presence of TeO3, TeO3 + 1, and TeO4 units with average TeO coordination number as 3.49. The formation of nanostructured Ln2Te6O15 phases in GCs is confirmed by X‐ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. Furthermore, TEM analysis confirms that an increase of Ho2O3 concentration has reduced the size of phase‐separated domains in nanoscale with superstructure formation to attain transparent GCs. The superiority of this obtained transparent GCs as photonic material for near‐IR (NIR) to mid‐IR (MIR) range has been established by the realization of enhanced luminescence intensities and bandwidth at ≈2900 nm (Ho3+: 5I6 → 5I7) and ≈2050 nm (Ho3+: 5I7 → 5I8). This study offers an opportunity to fabricate the various accessible lanthanide ions‐doped and/or co‐doped TTLG glass with control over nanostructure, to design a series of GCs which are transparent from visible to MIR range

Mid‐IR transparent TeO2‐TiO2‐La2O3 glass and its crystallization behavior for photonic applications

In this report, effect of enhanced rare earth (La2O3) concentration on substitution of TeO2 within ternary TeO2‐TiO2‐La2O3 (TTL) glass system has been studied with respect to its thermal, structural, mechanical, optical, and crystallization properties with an aim to achieve glass and glass‐ceramics having rare‐earth‐rich crystalline phase for nonlinear optical and infrared photonic applications. DSC analysis (10°C/min) demonstrates a progressive increase in glass‐transition temperature (Tg) from 359 to 452°C with the increase in La2O3 content. Continuous glass network modification with transformation of [TeO4] to [TeO3/TeO3+1] units is evidenced from Raman spectra which is corroborated with XPS studies. While mechanical properties demonstrate enhancement of cross‐linking density in the network. These glasses exhibit optical transmission window extended from 0.4 to 6 μm with calculated zero dispersion wavelength (λZDW) varying from 2.41 to 2.28 μm depending upon La2O3 content. Crystallization kinetics of TTL10 (80TeO2‐10TiO2‐10La2O3 in mol%) glass has been studied via established models. Activation energy (Ea) has been evaluated and dimensionality of crystal growth (m) suggests formation of surface crystals. Glass‐ceramic with crystalline phase of La2Te6O15 has been realized in heat‐treated TTL10 glass samples (at 450°C). As predicted from DSC analysis, FESEM study unveils the formation of surface crystallized glass‐ceramics

Frequency upconversion mechanism in Ho3+/Yb3+-codoped TeO2–TiO2–La2O3 glasses

Frequency upconversion from Ho3+/Yb3+-codoped glass or crystal under Yb3+ sensitization is a known phenomenon. However, inconsistencies are prevalent in the understanding of double energy transfer mechanisms for Ho3+/Yb3+-codoped systems. In this context, rate equations are proposed for Ho3+/Yb3+-codoped low-phonon TeO2–TiO2–La2O3 glass under Yb3+ sensitization with continuous and pulsed excitations. The proposed rate equations are validated with experimental results to elucidate the mechanisms responsible for populating 5(S2, F4) and 5F5 energy levels of Ho3+ ion. The solutions of rate equations with experimental results are substantiating the occurrence of both excited state absorption (ESA) and energy transfer upconversion (ETU) mechanisms in populating Ho3+:5(S2, F4) level, though higher concentration of Ho3+ ion would decrease the probability of ETU and increase of ESA. In contrast, Ho3+:5F5 level has been populated via ETU only. Numerical solutions to the rate equations are also proposed to elucidate the mechanics for populating 5(S2, F4) and 5F5 levels of Ho3+ ion. The proposed rate equation for pulsed excitation explains the characteristics of respective decay curves, which are further used to quantify energy transfer coefficient (W02) as (1.77 ± 0.12) × 10− 17cm3 s−1 for Ho3+/Yb3+-codoped TTL glass host

Experimental evidence for quantum cutting co-operative energy transfer process in Pr3+/Yb3+ ions co-doped fluorotellurite glass: dispute over energy transfer mechanism

Pr3+/Yb3+ doped materials have been widely reported as quantum-cutting materials in recent times. However, the question of the energy transfer mechanism in the Pr3+/Yb3+ pair in light of the quantum-cutting phenomenon still remains unanswered. In view of that, we explored a series of Pr3+/Yb3+ co-doped low phonon fluorotellurite glass systems to estimate the probability of different energy transfer mechanisms. Indeed, a novel and simple way to predict the probability of the proper energy transfer mechanism in the Pr3+/Yb3+ pair is possible by considering the donor Pr3+ ion emission intensities and the relative ratio dependence in the presence of acceptor Yb3+ ions. Moreover, the observed results are very much in accordance with other estimated results that support the quantum-cutting phenomena in Pr3+/Yb3+ pairs, such as sub-linear power dependence of Yb3+ NIR emission upon visible ∼450 nm laser excitation, integrated area of the donor Pr3+ ion's visible excitation spectrum recorded by monitoring the acceptor Yb3+ ion's NIR emission, and the experimentally obtained absolute quantum yield values using an integrating sphere setup. Our results give a simple way of estimating the probability of an energy transfer mechanism and the factors to be considered, particularly for the Pr3+/Yb3+ pair

An insight into the thermal processability of highly bioactive borosilicate glasses through kinetic approach

The paucity of crystallization resistant bioactive glasses with desired biological functions stands as a bottleneck toward the fabrication of various biomedical constructs such as amorphous coatings, scaffolds, and fibers for advanced tissue engineering applications. In this context, a series of borosilicate-based bioactive glasses with a range of compositions: (53.88 - x)SiO2-21.7Na(2)O-21.7CaO-1.7P(2)O(5)-xB(2)O(3) (mol%) where x = 0, 13.47, 22.45, 31.43, and 40.41 were prepared to address such limitation. The glasses were primarily investigated for their potential to be processed into amorphous scaffolds through evaluation of crystallization kinetics, sintering behavior, and viscosity-temperature dependence. The inclusion of B2O3 gradually reduces the activation energy of crystallization (E-a), according to the prediction from different kinetic models, whereas Friedman's model-free method unraveled the variation in E-a as crystallization progresses. The crystallization event is further elucidated by obtaining the Avrami parameter (n) and dimensionality (m) through Matusita-Sakka equation. The optimization of the sintering schedule for amorphous scaffold preparation was accomplished by exploiting isothermal prediction from Avrami-Erofeev model. Moreover, viscosity-temperature relationship for the studied glasses was established to identify the processing window for drawing and sintering. This study proposes a comprehensive approach adopting theoretical models to elucidate suitable high-temperature process parameters of bioactive glasses avoiding devitrification

Enhanced 1.8μm emission in Yb3+/Tm3+ co-doped tellurite glass: Effects of Yb3+↔Tm3+ energy transfer and back transfer

The ~1.8 μm emission characteristics of Tm3+ by a direct excitation and through an energy transfer process upon sensitization with Yb3+ ions in tellurite glass are reported. The spectroscopic properties of Tm3+ ions have been evaluated by applying Judd–Ofelt theory on the measured absorption spectrum. The obtained intensity parameters, Ω2=7.155×10−20 cm2, Ω4=3.325×10−20 cm2, Ω6=1.278×10−20 cm2 are used to estimate the radiative properties of Tm3+ ions in the present glass host. A ~10 fold enhancement in the Tm3+ 1.8 μm emission observed with 16 fold reduced emission of Yb3+ ions (1008 nm) in co-doped sample on Yb3+ ions excitation illustrates the efficient energy transfer from Yb3+: 2F5/2→Tm3+: 3H5. The energy transfer process assisted by host phonon energy has been discussed by using relevant theoretical models and estimated the energy transfer micro-parameters. Effect of energy back transfer Tm3+→Yb3+ on NIR and upconversion emissions have been discussed. An efficient ~1.8 μm with comparatively higher emission cross-section 1.115×10−20 cm2 on account of reduced upconversion emissions has been achieved in the present tellurite glass

Insights into Er3+ ↔Yb3+ energy transfer dynamics upon infrared ~1550 nm excitation in a low phonon fluoro-tellurite glass system

Highly upconverting, monolithic transparent inorganic glass co-doped with Er3+/Yb3+ ions have been explored in view of their easy integration with Si-PV cell. A ~4 fold enhancement in the photo-current of mc-Si-PV cell has been observed using a Er3+/Yb3+ co-doped sample compared to Er3+ ions singly doped glass. The role of Yb3+ ions on the enhancement of photo-current has been discussed in light of the Er3+↔Yb3+ energy transfer mechanism involved from IR to NIR and VIS upconversion process upon IR ~1550 nm excitation. The influence of excitation pump power and donor Er3+ ion concentration on the energy transfer upconversion (ETU) as well as excited state absorption (ESA) energy transfer mechanisms and its effect on the upconversion emission properties have been described in detail. The prominence of ETU or ESA process were elaborated considering the decay dynamics of NIR upconversion emission upon ~1550 nm excitation

Insights into Er3+ <-> Yb3+ energy transfer dynamics upon infrared similar to 1550 nm excitation in a low phonon fluoro-tellurite glass system

Highly upconverting, monolithic transparent inorganic glass co-doped with Er3+/Yb3+ ions have been explored in view of their easy integration with Si-PV cell. A 4 fold enhancement in the photo-current of mc-Si-PV cell has been observed using a Er3+/Yb3+ co-doped sample compared to Er3+ ions singly doped glass. The role of Yb3+ ions on the enhancement of photo-current has been discussed in light of the Er3+ Yb3+ energy transfer mechanism involved from IR to NIR and VIS upconversion process upon IR similar to 1550 nm excitation. The influence of excitation pump power and donor Er3+ ion concentration on the energy transfer upconversion (ETU) as well as excited state absorption (ESA) energy transfer mechanisms and its effect on the upconversion emission properties have been described in detail. The prominence of ETU or ESA process were elaborated considering the decay dynamics of NIR upconversion emission upon similar to 1550 nm excitation. (C) 2017 Elsevier B.V. All rights reserved

Bandwidth enhancement of MIR emission in Yb3+/Er3+/Dy3+ triply doped fluoro-tellurite glass

Enhanced bandwidth of MIR emission from Yb3+/Er3+/Dy3+ triply doped low phonon oxide glass system has been reported in this work. With considerable gain cross-section, the MIR emission bandwidth can be stretched from ~2600 to 3100 nm (~500 nm) which is practically not possible to obtain from Er3+ or Dy3+ ions singly doped systems. Co-doping of Dy3+ ions not only quenches the unfavourable visible up-converted emissions from Er3+ ions but also mitigates the prominent ~1.5 µm emission. A broad MIR emission on superimposition of Er3+ ~2.76 µm and Dy3+ ~2.95 µm emissions was obtained owing to the efficient energy transfer (ET) Yb3+  →  Er3+  →  Dy3+ upon ~980 nm excitation. The present glasses can be fiberized to develop compact and tunable MIR solid state fiber laser sources