Visible up-conversion and NIR luminescence studies of LiAl<Subscript>5</Subscript>O<Subscript>8</Subscript>:Er phosphor co-doped with Yb<Superscript>3+</Superscript> and Zn<Superscript>2+</Superscript>

The frequency up-conversion, an efficient laser emission and amplification in Er³⁺:LiAl₅O₈ phosphors co-doped with Yb³⁺ and Zn²⁺ phosphor powders in the 520–560, 640–680 nm regions and at ∼1.5 μm, respectively, have been reported. The emission corresponds to the ²H_11/2, ⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions upon direct excitation into the intermediated ⁴I_11/2 level using ∼980 nm radiation from a CW laser. Possible mechanisms involved for the up-conversion processes based on the energy level matching scheme, the pump-power dependence and the dynamical behaviour have been discussed. The effect of the addition of Yb³⁺ and Zn²⁺ for the amplification in the 1.5 μm eye-safe telecommunication window has been elaborated and characterized in detail.     

Infrared, visible and upconversion emission of CaAl12O19 powders doped with Er3+, Yb3+ and Mg2+ ions

CaAl₁₂O₁₉ powders doped with Er³⁺, Yb³⁺, and Mg²⁺ ions have been prepared by a low-temperature combustion synthesis technique. Formation and chemical compositions were analysed by powder X-ray diffraction and energy-dispersive spectroscopy. The visible luminescence spectra of the doped phosphor upon excitation with ∼378 nm radiation from a Xenon lamp have been studied. A broad band emission in the range of 1400–1700 nm with a peak around 1.5 μm and FWHM of about ∼80 nm responsible for the eye-safe telecommunication window has been observed upon direct excitation with a NIR laser into the ⁴I_11/2 level of Er³⁺. The effect of co-doping with Yb³⁺ and Mg²⁺ ions in the CaAl₁₂O₁₉:Er³⁺ matrix on the photoluminescence intensity corresponding to the ²H_11/2,⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions of Er³⁺ is elaborated and discussed in detail.     

Up-conversion emissions of Er<Superscript>3+</Superscript>-Yb<Superscript>3+</Superscript> codoped Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles by the arc discharge synthesis method

The Er³⁺-Yb³⁺ codoped Al₂O₃ nanoparticles with an average particle size of about 50 nm have been synthesized by an arc discharge synthesis method. The green and red up-conversion emissions centered at about 526, 547 and 677 nm, corresponding respectively to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺, were detected by a 978-nm semiconductor laser diode excitation. The Annealing has evident effect on the up-conversion emissions of the samples: The red up-conversion emission is noticeable before annealing; however, the green up-conversion emission becomes predominant after annealing. The mixture of (Er,Yb)₃Al₅O₁₂ and α-(Al,Er,Yb)₂O₃ phases is more favorable for green up-conversion emissions due to an enhancement of the ESA (I) of ⁴I_11/2+a photon→⁴F_7/2 and ET (III) of ²F_5/2(Yb³⁺)+⁴I_11/2(Er³⁺)→²F_7/2(Yb³⁺)+⁴F_7/2(Er³⁺) processes. The two-photon absorption up-conversion process is involved in the green and red up-conversion emissions. The results have proved that arc discharge synthesis is a new promising preparation technology for optical materials. Supported by National Natural Science Foundation of China (Grant No. 10804015), the Scientific Research Foundation for Doctor of Liaoning Province (Grant No. 20071095), and the Educational Committee Foundation of Liaoning Province (Grant No. 2008123)     

Infrared emission and defect centres in Er and Yb codoped Y3Al5O12 phosphors

YAG phosphor powders doped/codoped with Er³⁺/(Er³⁺ + Yb³⁺) have been synthesised by using the solution combustion method. The effect of direct pumping into the ⁴I_11/2 level under 980 nm excitation of doped/codoped Er³⁺/Yb³⁺−Er³⁺ in Y₃Al₅O₁₂ (YAG) phosphor responsible for an infrared (IR) emission peaking at ∼1.53 μm corresponding to the ⁴I_13/2→⁴I_15/2 transition has been studied. YAG exhibits three thermally-stimulated luminescence (TSL) peaks at around 140°C, 210°C and 445°C. Electron spin resonance (ESR) studies were carried out to identify the centres responsible for the TSL peaks. The room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0176 is identified as O⁻ ion, while centre II with an isotropic g-factor 2.0020 is assigned to an F⁺ centre (singly ionised oxygen vacancy). An additional defect centre is observed during thermal-annealing experiments and this centre (assigned to F⁺ centre) seems to originate from an F-centre (oxygen vacancy with two electrons) and these two centres appear to correlate with the observed high-temperature TSL peak in YAG phosphor.     

Visible upconversion and infrared luminescence investigations of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powders doped with Er<Superscript>3+</Superscript>, Yb<Superscript>3+</Superscript> and Zn<Superscript>2+</Superscript> ions

Alumina (Al₂O₃) powders doped with Er³⁺, Yb³⁺ and Zn²⁺ ions have been prepared by a low-temperature combustion synthesis technique. The phase purity and crystalline structure of the combustion products are confirmed by powder X-ray diffraction. An efficient frequency upconversion in the visible region and the emission in the infrared (IR) region respectively corresponding to the ²H_11/2, ⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions upon direct excitation with a CW laser lasing at ∼980 nm are discussed. The enhancement observed in the intensity of the upconversion emission bands in the visible region and the emission band in the IR region due to the presence of Yb³⁺ and Zn²⁺ in Er³⁺:Al₂O₃ powders is reported and explained in detail.     

Infrared-to-visible up-conversion emissions and thermometric applications of Er 3+-doped Al 2 O 3

Er³⁺-doped Al₂O₃ has been prepared by a sol–gel method, using aluminium isopropoxide [Al(OC₃H₇)₃]-derived Al₂O₃ sols with addition of erbium nitrate [Er(NO₃)₃·5H₂O]. The phase structure of θ-(Al,Er)₂O₃ phase was obtained for the 0.1 mol. % Er³⁺-doped Al₂O₃ at the sintering temperature of 1273 K. The green and red up-conversion emissions centered at about 523, 545 and 660 nm, corresponding respectively to the ² H _11/2, ⁴ S _3/2→⁴ I _15/2 and ⁴ F _9/2→⁴ I _15/2 transitions of Er³⁺, were detected by a 978-nm semiconductor laser diode excitation. The temperature dependence of green up-conversion emissions was studied over a wide temperature range of 295–725 K, and reasonable agreement between the calculated temperatures obtained by the fluorescence intensity ratio theory and the measured temperature was obtained, which proved that Er³⁺-doped Al₂O₃ has a good potential for the development of high-temperature sensors. It has some advantages compared to glasses due to its higher thermal and mechanical resistance and allows measurements in a large temperature range. PACS 78.55.Hx; 81.40.Tv     

Infrared and visible emission of Er in combustion-synthesized CaAl2O4 phosphors

New near-infrared luminescent, monoclinic CaAl₂O₄:Er³⁺ phosphor was prepared by using the combustion route at furnace temperatures as low as 500 °C in a few minutes. Combustion synthesized phosphor has been well characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive analysis of X-ray (EDAX) mapping studies. The luminescence spectra of Er³⁺-doped calcium aluminate were studied at UV (380 nm), vis (488 nm) and IR (980 nm) excitation. Upon UV and vis excitation, the CaAl₂O₄:Er³⁺ phosphor exhibits emission bands at ~523 nm and at ~547 nm, corresponding to transitions from the ²H_11/2 and ⁴S_3/2 erbium levels to the ⁴I_15/2 ground state. A strong luminescence at 1.55 μm in the infrared (IR) region due to ⁴I_13/2→⁴I_15/2 transition has been observed in CaAl₂O₄:Er³⁺ phosphor upon 980 nm CW pumping. In the spectrum of IR-excited up-conversion luminescence, green (~523 and ~547 nm) and red (662 nm) luminescence bands were present, the latter associated with the ⁴F_9/2→⁴I_15/2 transitions of Er³⁺ ions. Both excited state absorption and energy transfer may be proposed as processes responsible for the population of the ⁴S_3/2 and ⁴F_9/2 erbium levels upon IR excitation. The mechanisms responsible for the up-conversion luminescence are discussed.     

Diode laser pumped Gd2O3:Er3+/Yb3+ phosphor as optical nano-heater

An intense green upconversion (UC) emission (λ _exc=976 nm) followed by the heating effect in Yb³⁺/Er³⁺ co-doped Gd₂O₃ nanoparticles has been detected. A temperature rise up to 504 K has been observed (on a noteworthy low laser excitation of 290 mW) using fluorescence intensity ratio (FIR) method of the thermalized UC luminescence bands ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 of Er³⁺ ion. The reported controlled optical heating of nanoparticles and its nano-volume has potential applications in biomedicines and in the creation of holes in soft materials.     

Up-Conversion Luminescence of the NaRF<Subscript>4</Subscript>-NaR′F<Subscript>4</Subscript> (R: Y,Yb, Er) Core-Shell Nanomaterials

Up-converting NaRF₄-NaR′F₄ (R: Y, Yb, Er) nanomaterials with different core-shell combinations were prepared with the co-precipitation method. The X-ray powder diffraction (XPD) measurements revealed the presence of both the cubic and hexagonal NaRF₄ phases. The crystallite sizes calculated with the Scherrer formula were 100 and 150 nm for the cubic and hexagonal phases, respectively. The FT-IR spectra showed water impurities. The up-conversion luminescence and luminescence decays were studied with NIR laser excitation at 970 nm. The up-conversion luminescence spectra showed strong red (640–685 nm) (⁴F_9/2 → ⁴I_15/2) and moderate green (515–560 nm) (²H_11/2, ⁴S_3/2 → ⁴I_15/2) Er³⁺ luminescence. The strongest up-conversion luminescence and longest red luminescence decay was obtained from the Na(Y,Yb)F₄-NaErF₄ core-shell combination.     

Multifunctional Er3+–Yb3+ codoped Gd2O3 nanocrystalline phosphor synthesized through optimized combustion route

This paper reports the synthesis of high upconversion luminescent Gd₂O₃: Er³⁺, Yb³⁺ nanophosphor through optimized combustion route using urea as a reducing agent. The paper also reports the first observation of upconversion emission bands extending upto the UV region (335, 366 and 380 nm) in Er³⁺–Yb³⁺ co-doped phosphor materials. The fuel to oxidizer ratio has been varied to obtain the maximum upconversion luminescence. Three high intensity bands are found at 408, 523–548 and 667 nm due to the ⁴G_11/2 → ⁴I_15/2, ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions, respectively, along with the other bands. Input excitation power dependence has been studied for different transitions, and the saturation effect and decrease in the slope of different transitions at higher input pump power has been explained. Heat treatments of the samples show change in crystallite phase/size and relative upconversion luminescence intensities of blue, green and red bands. The color of the phosphor emission has shown to be tunable with change in the crystal structure as well as on excitation laser power and Er³⁺–Yb³⁺ concentration. The property of color tunability of the phosphor material has been used to record the fingerprint in different colors. Also, the future prospect of the nanocrystalline phosphor material as a sensor for temperature, using FIR method, has been explored.     

Photoluminescence from germanate glasses containing silicon nanocrystals and erbium ions

We report the first observation of photoluminescence enhancement in Er³⁺ doped GeO₂–Bi₂O₃ glasses containing silicon nanocrystals (Si-NCs) excited by a laser operating at 980 nm. The growth of ≈200% in the intensity of the Er³⁺ transition ⁴S_3/2→⁴I_15/2 (545 nm) and of ≈100% for transitions ²H_11/2→⁴I_15/2 (525 nm), ⁴F_9/2→⁴I_15/2 (660 nm), and ⁴I_5/2→⁴I_13/2 (1530 nm) was observed in comparison with a reference sample that does not contain Si-NCs. The results open a new road for obtaining efficient Stokes and anti-Stokes emissions in germanate composites doped with rare-earth ions.     

Avidin conjugation to up-conversion phosphor NaYF4:Yb3+, Er3+ by the oxidation of the oligosaccharide chains

NaYF₄:Yb³⁺, Er³⁺ nanoparticles were successfully prepared by a polyol process using diethyleneglycol (DEG) as solvent. After being functionalized with SiO₂–NH₂ layer, these NaYF₄:Yb³⁺, Er³⁺ nanoparticles can conjugate with activated avidin molecules (activated by the oxidation of the oligosaccharide chain). The as-formed NaYF₄:Yb³⁺, Er³⁺ nanoparticles, NaYF₄:Yb³⁺, Er³⁺ nanoparticles functionalized with amino groups, avidin conjugated amino-functionalized NaYF₄:Yb³⁺, Er³⁺ nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), Fourier transform infrared (FT-IR), UV/Vis absorption spectra, and up-conversion luminescence spectra, respectively. The biofunctionalization of the NaYF₄:Yb³⁺, Er³⁺ nanoparticles has less effect on their luminescence properties, i.e., they still show the up-conversion emission (from Er³⁺, with ⁴S_3/2 → ⁴I_15/2 at ~540 nm and ⁴F_9/2 → ⁴I_15/2 at ~653 nm), indicative of the great potential for these NaYF₄:Yb³⁺, Er³⁺ nanoparticles to be used as fluorescence probes for biological system.     

Intense Upconversion Luminescence of Yb<Superscript>3+</Superscript>-Er<Superscript>3+</Superscript> in Li<Subscript>2</Subscript>O Content Tungsten-tellurite Glasses

The Er³⁺ -Yb³⁺ codoped in Li₂O content tungsten -tellurite (TWL) transparent glasses are synthesized and measured the absorption, Raman and upconversion luminescence (UPL) spectra. At room temperature intense green emission peak at 560 nm ( ⁴S_3/2→⁴I_15/2) and red emission peak at 670 nm ( ⁴F_9/2→⁴I_15/2) of Er³⁺ observed even at minimum 86 mW pumping power of infrared 980 nm excitation. For structure of the TWL glass, Raman spectrum result revealed that an important role of WO₃ in the formation of glass network linkage with Li₂O. Under this influence estimated lifetime of the ⁴I_11/2 of Er³⁺ was 1.89 μs and due to lower phonon energy of the glass produce strong upconversion signal. The effect of Er₂O₃ concentration on emission intensity result indicated that green emission intensity initially increase in compare to red emission. Under the 980 nm pump power variation measured the relatively increases the red emission to the green emission intensity and analyze the possible upconversion mechanism and process.     

Defect Structure and Up-conversion Luminescence Properties of ZrO<Subscript>2</Subscript>:Yb<Superscript>3+</Superscript>,Er<Superscript>3+</Superscript> Nanomaterials

The up-converting ZrO₂:Yb³⁺,Er³⁺ nanomaterials were prepared with the combustion and sol–gel methods. FT-IR spectroscopy was used for analyzing the impurities. The crystal structures were characterized with X-ray powder diffraction and the mean crystallite sizes were estimated with the Scherrer formula. Up-conversion luminescence measurements were made at room temperature with IR-laser excitation at 977 nm. The IR spectra revealed the conventional and OH⁻ impurities for the combustion synthesis products. The structure of the ZrO₂:Yb³⁺, Er³⁺ nanomaterials was cubic except for the minor monoclinic and tetragonal impurities obtained with the sol–gel method. The materials showed red (650–700 nm) and green (520–560 nm) up-conversion luminescence due to the ⁴F_9/2→⁴I_15/2 and (²H_11/2, ⁴S_3/2)→⁴I_15/2 transitions of Er³⁺, respectively. The products obtained with the combustion synthesis exhibited the most intense luminescence intensity and showed considerable afterglow.     

Er3+-doped sol–gel SnO2 for optical laser and amplifier applications

Erbium-doped tin dioxide (SnO₂:Er³⁺) was obtained by the sol–gel method. Spectroscopic properties of the SnO₂:Er³⁺ are analyzed from the Judd–Ofelt (JO) theory. The JO model has been applied to absorption intensities of Er³⁺ (4f¹¹) transitions to establish the so-called Judd–Ofelt intensity parameters: Ω₂, Ω₄, and Ω₆. With the weak spectroscopic quality factors Ω₄/Ω₆, we expect a relatively prominent infrared laser emission. The intensity parameters are used to determine the spontaneous emission probabilities of some relevant transitions, the branching ratios, and the radiative lifetimes of several excited states of Er³⁺. The emission cross section (1.31×10^-20 cm²) is evaluated at 1.54 μm and was found to be relatively high compared to that of erbium in other systems. Efficient green and red up-conversion luminescence were observed, at room temperature, using a 798-nm excitation wavelength. The green up-conversion emission is mainly due to the excited state absorption from ⁴ I _11/2, which populates the ⁴ F _3/2,5/2 states. The red up-conversion emission is due to the energy transfer process described by Er³⁺ (⁴I_13/2)+Er³⁺(⁴I_11/2)→Er³⁺(⁴F_9/2)+Er³⁺ (⁴ I _15/2) and the cross-relaxation process. The efficient visible up-conversion and infrared luminescence indicate that Er³⁺-doped sol–gel SnO₂ is a promising laser and amplifier material. PACS 71.20.Eh; 74.25.Gz; 78.55.-m     

Structural changes induced on strontium barium niobate glass by femtosecond laser irradiation

Localized modification of the optical properties of erbium doped strontium barium niobate (SBN) glass has been performed using femtosecond laser irradiation. The samples, with composition SrO–BaO–Nb₂O₅–B₂O₅ and doped with 5%mol of Er³⁺, were fabricated using a melt-quenching method. The samples were irradiated with different number of pulses per spot (1–50 pulses) at two different laser fluences (2.6 and 5.6 J/cm²) by using an fs laser amplifier operating at 800 nm and generating pulses with a duration of 120 fs. Micro-luminescent microscopy, using an Ar⁺ laser as excitation source, has been used to analyze the modifications of the luminescent properties of the sample upon fs laser exposure. The emissions of the Er³⁺: ⁴I_11/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions allow appreciating the structural modifications caused by femtosecond laser exposure. The lifetimes of the levels involved in these transitions were measured inside and outside the laser irradiated region. These measurements have been compared with those obtained in bulk glass ceramic sample, which is obtained from the glass precursor by a thermal treatment in order to estimate the optimal conditions to produce nanocrystals in a localized region by ultrafast laser irradiation.     

Polarized spectral properties of Er<Superscript>3+</Superscript> ions in NaGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

Polarized spectral properties of Er³⁺:NaGd(WO₄)₂ single crystal are reported. The crystal was grown by the Czochralski method. The Judd–Ofelt theory was applied to analyze the polarized absorption spectra and then calculate the spontaneous emission probabilities, radiative lifetimes, and branching ratios. Fluorescence decay curves of the ⁴ I _13/2, ⁴ I _11/2, and ⁴ S _3/2 multiplets for the Er³⁺ ions were measured. Stimulated emission cross-sections of the ⁴ I _13/2→⁴ I _15/2 transition obtained by the Fuchtbauer–Ladenberg formula and the reciprocity method were compared. Multi-phonon relaxation rates of the crystal were estimated. Green up-conversion fluorescence around 531 and 552 nm was observed, and the possible up-conversion mechanisms were proposed. PACS 78.20.-e; 42.70.Hj     

Green and red up-conversion emissions of Er-Yb-codoped Al2O3 powders prepared by the nonaqueous sol-gel method

The 1 mol% Er³⁺- and 0-20 mol% Yb³⁺-codoped Al₂O₃ powders have been prepared by the nonaqueous sol-gel process using aluminum isopropoxide as precursor, acetylacetone as chelating agent, nitric acid as catalyzer, and hydrated erbium and ytterbium nitrate as dopant under isopropanol environment. The two crystalline types of doped Al₂O₃, γ and θ, and a stoichiometric compound, (Yb,Er)₃Al₅O₁₂, were obtained for all the Er³⁺-Yb³⁺-codoped Al₂O₃ powders at the sintering temperature of 1000 °C. The maximal intensity of both the green and red up-conversion emissions centered at about 523, 545, and 660 nm was observed for the 1 mol% Er³⁺- and 10 mol% Yb³⁺-codoped Al₂O₃ powders. The intensity ratio of the red to green up-conversion emission (I_red/I_green) increased with increasing the Yb³⁺ doping concentration for the Er³⁺-Yb³⁺-codoped Al₂O₃ powders. Furthermore, the intensity ratio of the green up-conversion emission at about 523 to 545 nm (I₅₂₃/I₅₄₅) was proportional to the Yb³⁺ doping concentration and pump electric current, which was associated with the elevated temperature of powders.     

Solvent effect on the optimization of 1.54 μm emission in Er-doped Y2O3–Al2O3–SiO2 powders synthesized by a modified Pechini method

In order to find a new Er-doped host for near infrared (NIR) optical amplifiers, a study on the optimization of the erbium emission ions in the Y₂O₃–Al₂O₃–SiO₂ system was performed. (100 ? x) Y₃Al₅O₁₂ ? (x) SiO₂ powders (x varies from 0 to 70, in mol%) with a fixed Er₂O₃ concentration of 1.0 mol% were synthesized by a modified Pechini method and heat-treated at 900 and 1000 °C. The photoluminescence (PL) spectra at 1540 nm of the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions and the up-conversion spectra at visible region (²H_11/2 + ⁴S_3/2 + ⁴F_9/2 → ⁴I_15/2) upon 980 nm excitation were evaluated. Different techniques, such as thermogravimetry (TG), differential scanning calorimetry (DSC), X-ray powder diffractometry (XRD) and Fourier transform infrared spectroscopy (FT-IR) were considered to evaluate crystallization and phase-evolution of the powders as a function of the silica content (x) and annealing temperature. The analyses were based on the comparison between two different solvents used in the preparation of the polymeric resins: ethanol and water. The optimal conditions for ethanol are quite different than the conditions for water used as solvent, confirming that the PL properties at the NIR region are highly sensitive to the changes in the host stoichiometry and processing conditions. The highest emission intensity at 1540 nm was observed for x = 30 for ethanol and x = 70 for water, treated at 900 and 1000 °C, respectively. This result could be attributed to the combination of low symmetry and good dispersion of the Er³⁺ions in these hosts.     

Comparative study of the spectroscopic properties of Yb3+/Er3+ codoped tellurite glasses modified with R2O (R=Li,Na and K)

Spectroscopic characterization of Yb³⁺/Er³⁺ codoped TeO₂–R₂O–ZnO–Ln₂O₃ glasses as a function of network modifiers (R=Li, Na and K) has been investigated. The Judd–Ofelt parameters (Ω_t), quantum efficiency in near infrared (1.55 μm) and visible up-conversion (546 and 660 nm) and quality factor spectroscopy (χ) were calculated. Three up-conversion emission bands centered at 525, 546 and 660 nm were observed as maxima for glasses containing potassium. The measured lifetime of ⁴I_13/2, ⁴F_9/2 and ⁴S_3/2 from Er³⁺ and ⁴F_5/2 from Yb³⁺ levels increased when potassium (K) replaced lithium (Li) and Na. The maximum emission cross-section (ECS) for ⁴I_13/2→⁴I_15/2 transition of Er³⁺ was calculated to be 1.02×10^?20 cm² for TeO₂–Li₂O–ZnO–Ln₂O₃ glasses. The energy transfer efficiency (ET) from Yb³⁺ to Er³⁺, (⁴F_5/2)+(⁴I_15/2)→(⁴F_7/2)+(⁴I_13/2), was calculated using the measured lifetimes of Yb³⁺ with and without the presence of acceptor (Er³⁺). The maximum calculated ET was 58% for 0.25 mol% of Er³⁺ and 3 mol% of Yb³⁺ for TeO₂–K₂O–ZnO–Ln₂O₃ glass composition.