Emission Spectra Of Er3 Research Articles

Up-conversion luminescence and multimodal temperature sensing behaviors in the visible and infrared region of Er3+ and (Er3+, Yb3+) ions in Ca3Ta2Ga3O12 phosphors

Ca3Ta2Ga3O12 - CTGG ceramic phosphors doped with xEr3+ ions, x = (0.1, 1, 3, 5, 7, and 9 at.%) and (5Er, yYb), y = (0.1, 1, 3, 5, and 7 at.%) were investigated. The emission spectra of Er3+ ions were measured under 973 nm and 1550 nm excitations and the optimal concentrations of Er3+ and (Er3+, Yb3+) ions allowing to obtain the maximum emission intensities in the visible range were determined to be 5Er and (5Er, 5Yb), respectively. The best results in terms of correlated color temperature (CCT) and color purity (CRI) were obtained under λex = 973 nm. High values of CCT (between 5000 K and 6000 K) and CRI (in the range of 80 - 92) parameters indicate their potential as efficient materials for white light-emitting diode devices. The temperature dependence of Er3+ luminescence in 5Er:CTGG and (5Er, 5Yb):CTGG samples was investigated by the luminescence intensity ratio for different thermally coupled (TCLs) and non-thermally coupled (NTCLs) levels and the fluorescence lifetime. The highest value of the relative thermal sensitivity parameter (Sr) for 5Er:CTGG sample, Sr = 1.41 % K-1 at 325 K, was obtained from TCLs (2H11/2/4S3/2) under λex = 973 nm. For (5Er, 5Yb):CTGG sample, the highest values of Sr were obtained as follows: Sr = 1.77 % K-1 at 325 K from TCLs (2H11/2/4S3/2), Sr = 1.19 % K-1 at 150 K from TCLs (4I13/2(2)/4I13/2(3)), and Sr = 0.62 % K−1 at 450 K from 4S3/2 lifetime, under λex = 973 nm, and Sr = 1.23 % K-1 at 325 K from TCLs (2H11/2/4S3/2) and Sr = 1.21 % K-1 at 325 K from NTCLs (2H11/2/4F9/2) under λex = 1550 nm. These results indicate that 5Er:CTGG and (5Er, 5Yb):CTGG ceramic phosphors have high potential for applications in the field of optical temperature sensors.

Mid-infrared laser operation of (Er0.07La0.10Y0.83)2O3 sesquioxide ceramic

A transparent sesquioxide ceramic (Er0.07La0.10Y0.83)2O3 was fabricated by vacuum sintering at 1780 °C for 3 h with lanthana acting as a sintering additive. It was of single-phase nature (sp. gr. Ia ) with a close-packed microstructure (average grain size: 17 μm, pore content: 5 ppmv) and high transparency of 80% at 1.1 μm. The La3+ ions induced additional inhomogeneous broadening in the absorption and emission spectra of Er3+ ions. For the 4I11/2→4I13/2 transition, the stimulated-emission cross-section σ SE was 1.06 × 10−20 cm2 at 2719 nm and the luminescence lifetimes of the 4I11/2/4I13/2 states amounted to 2.73/5.91 ms, respectively. A (Er0.07La0.10Y0.83)2O3 ceramic laser generated 300 mW at 2840 nm with a slope efficiency of 27.7% and a laser threshold of only 17 mW.

Effect of Bi2O3 on Physical and Luminescence Properties of Unconventional Bi/Er Co-Doped Sb2O3-WO3-Li2O Glasses

New glasses in the system (80-x) Sb2O3-10WO3-10Li2O co-doped with Bi3+/Er3+ were synthesized using the melt quenching method. The Er3+ concentration was kept constant at 0.25 (mol. %) and the Bi2O3 content takes the values 0.5, 1, 1.5 and 2 (mol. %) in order to elucidate the effect of the Bi2O3 concentration on the measured physical and optical properties of the glasses studied. The pulse echo method of the propagation of ultrasound in these glasses has been used to determine the elastic modules (E, G, K and L) and Poisson ratio. The introduction of Bi2O3 in these glasses increases the thermal stability factor of the glass from 152 to 183°C with an increase in Tg from 275 to 285°C. The UV-Vis cutoff of these glasses redshifts from 408 to 413 nm with a reduction of the indirect band gap from 2.72 to 2.62 ev and a small Urbach energy close to 0.125 nm resulting in small electronic defects in these glasses. The refraction index was found high with small variations in the range [1.965-1.975]. The emission spectra of Er3+ ions in these glasses gave an intense green laser line at 544 nm for an excitation at 488 nm. Moreover, the addition of more Bi2O3, considerably improves this laser line. However, the OH content in these glasses increases with the increase in Bismuth content related probably to the introduction of atmosphere moisture during the synthesis of these glasses. Therefore, the addition of Bi2O3 improves the various physical properties of these glasses making them good candidates for optical applications.

Improvement in 1.53-μm-band fluorescence of Er3+/Yb3+-codoped borate glasses based on infrared energy transfer

Er3+/Yb3+-codoped borate glass consisting of B2O3–WO3–ZnO–Y2O3 was synthesized as a potential gain medium for an Er3+-doped fiber amplifier (EDFA). X-ray diffraction (XRD) patterns and Raman spectra were characterized to investigate the structural behavior of the glass host, and absorption spectra were recorded to investigate the spectroscopic properties of the Er3+/Yb3+-codoped borate glasses according to the Judd–Ofelt theory. The 1.53-μm-band emission spectra of Er3+ under 980-nm excitation were measured to verify the process of energy transfer (ET) from Yb3+ to Er3+, and the decay lifetimes of Yb3+:1020-nm emission were determined to evaluate the ET efficiency (ETE) between Yb3+ and Er3+. The results indicate that a broad 1.53-μm-band emission occurs for Er3+, with a full width at half maximum (FWHM) of approximately 82 nm. Moreover, the emission intensity increases by approximately 66.7% in Er3+/Yb3+-codoped borate glass containing 10.0 mol% Yb3+, which is ascribed to the efficiency ETE of Yb3+:2F5/2 + 4I15/2 → Yb3+:2F7/2 + Er3+:4I11/2. Compared with the previously reported glass systems, the prepared borate glasses have the typical features, including high gain and broad emission at 1.53-μm, which provide a new route to improve the signal gain in the C-band and further develop the short-length optical amplifiers.

Cross section spectral distribution of Er3+ electronic transitions in SrGdGa3O7 single-crystal

Abstract Visible and infrared absorption and emission spectra of Er3+-doped strontium gadolinium gallate (SrGdGa3O7) single-crystal were measured at room temperature. From the measured fluorescence spectra, the Er3+ emission cross section spectra were calculated at first. Then, by utilizing McCumber relation, the absorption cross section spectra were further calculated. In parallel, the absorption cross section spectra were also calculated from the measured absorption spectra of the crystal, and compared with those spectra calculated from the McCumber relation. The comparison shows that the absorption cross section spectra obtained using the McCumber relation are in reasonable agreement with those deduced from the measured absorption spectrum.

Crystal Structure and Judd–Ofelt Analysis of Er3+ Doped LuAl3(BO3)4 Crystal**Supported by the National Natural Science Foundation of China under Grant No 51772171.

Single crystal Er:LuAl3(BO3) (Er:LuAB) is successfully grown using the top-seeded solution growth method with a K2Mo3O10 flux. The cell parameters of the grown crystal are estimated by an x-ray single crystal diffactometor and x-ray powder diffraction analysis. The result indicates that it still belongs to the space group R32. The obtained unit-cell parameters are a = 9.2793(19) Å, c = 7.210(3) Å, V = 537.65(27) Å3, and Z = 3. The absorption spectrum is measured at room temperature. The spectroscopy properties are investigated based on the Judd–Ofelt (J-O) theory, and the effective J-O parameters were calculated to be , , and . The emission spectra of Er:LuAB crystal at room temperature are also studied and the fluorescence around 3170 nm is observed. The emission cross section calculated by the F-L formula is 8.6 × 10−20 cm2. These results suggest that the Er:LuAB crystal may be a promising ∼3 μm laser material.

Li-impurity effect in optical spectra of KTaO3:Er3+ crystals

The f–f absorption and emission spectra of Er3+ impurities were observed and identified in KTaO3 and K1 – xLixTaO3 single crystals. It was shown that Li off-centers related random fields markedly influence intensity, width and position of zero-phonon lines. The detail analysis of the absorption spectra obtained allowed one to determine energies of the Stark sublevels of excited states for Er3+ dominant centers of noncubic symmetry.

Er3+ doped germanate–tellurite glass for mid-infrared 2.7 μm fiber laser material

2.7μm fluorescence has been achieved in the different concentration Er3+ doped germanate–tellurite glasses. The germanate–tellurite glass shows a good thermal stability and Fourier transform infrared spectra indicates that the mid-infrared transmission spectra performance is good. Based on the measured absorption spectra, the Judd–Ofelt parameters were calculated and discussed. Moreover, the emission spectra of Er3+ doped glasses show that the emission intensity at ~2.7μm reaches a maximal value and no obvious concentration quenching phenomenon happens even if the ErF3-doping concentration is 1.5mol%. In addition, the 2.7μm radiative transition probability and emission cross section is 35.57s−1 and 13.87×10−21cm2 corresponding to the Er3+:4I11/2→4I13/2 transition and superior gain performance was also obtained from the prepared glass. Meanwhile, energy transfer mechanism has been investigated in detail. Hence, the spectroscopic characteristics as well as the good thermal property indicate that this kind of glass is an attractive host for developing mid-infrared fiber laser.

Glass structure and NIR emission of Er3+ at 1.5 μm in oxyfluoride BaF2–Al2O3–B2O3 glasses

The glass structure, photoluminescence properties of Eu3+, Judd–Ofelt analysis, and near infrared emissions of Er3+ at 1.5μm in the oxyfluoride glasses and glass–ceramics of 1Eu2O3- or 1Er2O3-doped 50BaF2–xAl2O3–(50−x)B2O3 (x=0–25mol%) were investigated. It was clarified on the ground of Raman scattering spectroscopy and F1s and O1s XPS measurements that the glass with no Al2O3 (1Er2O3–50BaF2–50B2O3) is composed of BO3, BO2F and BO3F units with F–Ba bonds. The glasses with 25Al2O3 (1Er2O3–50BaF2–25Al2O3–25B2O3) is mainly composed of BO3− and Al(O,F)x units. Existence of non-bridging oxygen was not detected by O1s-XPS spectra. It was proposed that these structures are largely affected on crystallization behavior, e.g., the glass with no Al2O3 forms BaF2 and β-BaB2O4 due to Ba–F bonds and the glass with 25Al2O3 forms BaAlBO3F2 because the glass structure composed of BO3 and Al(O,F) units is similar to the BaAlBO3F2 crystal structure. Judd–Ofelt parameters of Er3+ and Eu3+ in the glasses showed almost the same values in Ω4 and Ω6 for each glass, on the other hand Ω2 decreased with addition of Al2O3. The emission spectra of Er3+ at 1.5μm in the glasses and glass–ceramics with BaAlBO3F2 crystals showed broad peaks. It is proposed that oxyfluoride glasses and glass–ceramics based on the BaF2–Al2O3–B2O3 system have a high potential for optical device applications such as broadband optical amplifiers.

Highly efficient resonantly pumped Er^3+:LuVO_4 laser

Absorption and emission spectra of Er3+:LuVO4 for the 4I15/2 ↔ 4I13/2 transitions have been investigated at room and cryogenic temperatures. A corrected energy level diagram was built. An efficient, resonantly pumped Er3+:LuVO4 laser operating at 1609 nm (π-polarization) or 1597.5 nm (σ-polarization) was demonstrated. For CW pumping into the 1532 nm absorption line by a narrow-bandwidth Er-fiber laser, the maximum output power was ~4.6 W and the maximum slope efficiency was 64% - the best reported efficiency of Er-doped vanadate lasers at room temperature. Resonant pumping into the 1529 nm absorption band with a fiber-coupled diode laser yielded a 62% slope efficiency. To the best of our knowledge, this is the first report on an Er:LuVO4 laser.

Spectroscopic Properties of Er<sup>3+</sup>-Doped Ga<sub>2</sub>O<sub>3</sub>-GeO<sub>2</sub>-Bi<sub>2</sub>O<sub>3</sub>-PbO Glass

Spectroscopic properties in Er3+-doped Ga2O3-GeO2-Bi2O3-PbO（BP）glass have been investigated and their promising application as Er3+-doped fiber amplifiers (EDFA) has also been evaluated under the excitation of 980 nm. The important roles of Bi2O3 substitution for PbO on spectroscopic properties, such as Judd-Ofelt intensity parameters (t=2, 4, 6) and emission spectra of Er3+, have been studied. The calculated peak emission cross-section of the 4I13/2→4I15/2 transition of Er3+ ions are 16.03×10-21 cm2 when the substituted PbO content by Bi2O3 is about 25 mol. The product of the peak emission cross-section ( ) and the full width at half-maximum (FWHM) of the sample is~689.29×10-28 cm3, which is higher than that of Er3+-doped silicate, phosphate and ZBLAN glasses. It was found that the substitution of Bi2O3 for PbO provides two potentials: broadening the full width at half-maximum of 1.53 μm fluorescence and improving the values of FWHM× of Er3+. The results strongly support that the Er3+-doped BP2 glass could be identified as a promising material for achieving 1.53 μm fiber amplifier.

Luminescence Properties of Er<sup>3+</sup>/Tm<sup>3+</sup>-Doped Ga<sub>2</sub>O<sub>3</sub>-Bi<sub>2</sub>O<sub>3</sub>-PbO-GeO<sub>2</sub> System Glasses

The visible and near infrared emission spectra of Er3+/Tm3+-doped Ga2O3-Bi2O3-PbO-GeO2(GBPG) glasses excited at 808 nm are experimentally investigated. The results reveal that 1.53 µm emission were enhanced with an increase of Er3+concentration. Furthermore, the incorporation of Er3+into Tm3+-doped systems has also resulted in intense 522, 545 and 693nm upconversion emission intensity and an weak 660 nm red emission. The possible mechanism and related discussions on this phenomenon have been presented. The results show that Er3+/Tm3+-codoped GBPG glass may be a promising materials for developing laser and fiber optical devices.

Emission and absorption cross section spectra of Er3+ in LiNbO3 crystals codoped with indium

Abstract

Study of Er3+ fluorescence on tellurite glasses containing Ag nanoparticles

Optical characteristics of tellurite glasses containing silver nanoparticles (NPs) and the influence on the emission spectrum of Er3+ ions were studied. The transitions 4f ↔ 4f from erbium ions, mainly the 4I13/2 → 4I15/2 transition that involve upconversion energy process, have a strongly dependence with the chemical structure of the rare earth ion. In the present work, silver nanparticles (NPs) embedded in the host vitreous material, show a significant enhance (or quenching) on the erbium fluorescence due the long-range electromagnetic interaction between the plasmon surface energy of the Ag NPs (Localized Surface Plasmon Resonance -LSPR) and the Er3+ ions.

Emission and Absorption Cross-Sections of Mg/Er-Codoped Near-Stoichiometric ${\rm LiNbO}_{3}$ Crystals

Near-stoichiometric (NS) (Mg:) Er:LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> crystals were grown from melts ([Li <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O]:[Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> ] = 1:1) containing 0.0/0.5, 0.5/0.5, and 1.0/0.5 mol%/mol% MgO/Er <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . OH absorption study shows that the optical damage threshold Mg concentration of the Er/Mg-codoped NS crystal is just near 1.0 mol%. Polarized visible and near infrared, and unpolarized mid-infrared (2.7 μm, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11/2</sub> → <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13/2</sub> ) emission spectra of Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ion in these crystals were measured. From measured emission spectra, the emission and absorption cross-section spectra were calculated based upon McCumber theory, and compared with the data of congruent Er:LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> bulk material and/or Ti-diffused Er:LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> strip waveguide and the results obtained from the direct unpolarized optical absorption measurements. The 550, 980, and 1530 nm emission lifetimes were also measured, and compared with the Judd-Ofelt theoretical values and the measured values of the congruent material. The results show that the polarized absorption cross-section data derived from the emission spectra are comparable to the unpolarized cross-section data from direct absorption measurements. The emission and absorption cross-sections, their polarization dependences as well as the lifetime all change definitely as the crystal composition shifts from the congruent point to the near-stoichiometry. In the NS crystal, the Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ion has smaller emission and absorption cross-sections, and longer lifetimes. In contrast, MgO codoping has less effect on the emission and absorption cross-sections, and lifetime.

NIR Emission Analysis of Er&lt;sup&gt;3+&lt;/sup&gt;/Yb&lt;sup&gt;3+&lt;/sup&gt; and Er&lt;sup&gt;3+&lt;/sup&gt;/ Tm&lt;sup&gt;3+&lt;/sup&gt; Ions Doped Zinc lithium Boro Tellurite (ZLiBT) Glasses for Optical Fiber Amplification

This paper reports on the development and the near infra-red emission(NIR) analysis of Er3+/Yb3+ and Er3+/ Tm3+ ions doped TeO2 - B2O3 – ZnO - Li2O glasses for tunable laser and optical fiber amplification. From the measured near infra-red emission spectra of Er3+/Yb3+, Er3+/Tm3+: ZLiBT glasses have revealed the prominent near infra-red emission bands at 1532 nm (4I13/2 → 4I15/2), with an excitation wavelength of λexci = 515 nm (Ar+ ion laser). The full width at half-maxima (FWHM) values are also estimated from the measured near infra-red emission spectra of the glasses. Based on spectral results, broad near infra-red emissions should have potential applications in broadly tunable laser sources and broad band optical amplification at low-loss telecommunication windows.

Spectral analysis of RE3+ (RE = Er, Nd, Prand Ho):GeO2–B2O3–ZnO–LiF glasses

This paper reports on the absorption, visible and near-infrared luminescence properties ofEr3+,Nd3+,Pr3+ and Ho3+-doped alkali fluoride zinc boro germanate glasses. The x-ray diffraction(XRD) and differential thermal analysis (DTA) profiles of the host glasshave been carried out, to confirm its structure and thermal stability. Fromthe measured absorption spectra, Judd–Ofelt (JO) intensity parameters(Ω2,Ω4 and Ω6) have been calculated for all the studied ions. ForPr3+ and Ho3+-doped glasses, upconversion emission spectra have also beenmeasured. Decay curves were measured for the visible emissions ofEr3+,Pr3+ andHo3+ ions at 547 nm(Er3+: green), 604 nm and641 nm (Pr3+: orange andred) and 658 nm (Ho3+: red), respectively. The near-infrared emission spectra ofEr3+/Yb3+ and Nd3+-doped glasses have shown full width at half maximum (FWHM) around 95 nm (for the transition) and 63 nm (for the transition), respectively. The measured maximum decay times of transition (at wavelength 1.51 µm) are about 5.72 ms and 6.14 ms for1.0Er3+ and 1.0Er3+/ 2Yb3+ (mol%)-codoped glasses, respectively. The maximum stimulated emission crosssections for transition of Er3+ and Er3+/Yb3+ are 5.447 × 10−21 cm2 and 4.540 × 10−21 cm2. These glasses with better thermal stability, bright visible emissions and broadnear-infrared emissions should have potential applications in broadly tunable laser sources,interesting optical luminescent materials and broadband optical amplification in low-losstelecommunication windows.

Optical spectroscopy and local structure of Er3+ luminescence centres in СaO–Ga2O3–GeO2 glasses

Optical absorption, luminescence excitation and emission spectra of Er3+ centres in Ca3Ga2Ge3O12:Er glass with Er content of 1.46wt% are presented and analysed. Luminescence kinetics for the main Er3+ transitions was satisfactorily described by single exponential decays with characteristic lifetimes. Oscillator strengths, phenomenological Judd–Ofelt intensity parameters, radiative decay rates (emission probabilities of transitions), branching ratios and radiative lifetimes for Er3+ centres in Ca3Ga2Ge3O12:Er glass are calculated and compared with the corresponding parameters of the Ca3Sc2Ge3O12:Er3+ garnet and other crystals and glasses. Quantum efficiency, η, of the 4I13/2→4I15/2 Er3+ transition is determined. Incorporation peculiarities and local structure of Er3+ luminescence centres in Ca3Ga2Ge3O12:Er3+ glass are discussed in comparison with garnet crystals and oxide glasses. On the basis of the presented results and referenced EXAFS data for Er, Eu and Ho impurities (L3-edge) it has been shown that Er3+ centres in Ca3Ga2Ge3O12 glass occupy network sites with the coordination number to oxygen of N=6.

Temperature-dependent fluorescence characteristics of an ytterbium-sensitized erbium-doped tellurite glass

In this article, the 1.5 μm emission spectra corresponding to the 4I 13/2→ 4I 15/2 transition of Er 3+ in tellurite glass are studied within the temperature from 8 to 300 K. The emission spectra of Er 3+: 4I 13/2→ 4I 15/2 transition are also analyzed using a peak-fit routine, and an equivalent four-level system is proposed to estimate the stark splitting for the 4I 15/2 and 4I 13/2 levels of Er 3+ in the tellurite glass. The results indicate that the 4I 13/2→ 4I 15/2 emission of Er 3+ can exhibit a considerable broadening due to a significant enhance the peak a′, and b′ change, respectively, and the peaks of which are located at about 1507 and 1556 nm. A detailed study of temperature-dependent 1.5 μm emission spectra involving the change of the corresponding sub-bands shows that as the temperature decreases from 300 to 8 K, its line-shape becomes sharper and more intense (the full-width at half-maximum decreases from 59 to 38 nm). Temperature-dependent fluorescence intensities and the experimentally determined lifetimes are investigated; the results show that a decrease behavior of fluorescence intensities and lifetimes are observed for temperature from 8 to 300 K.

Low temperature emission characteristics of an ytterbium sensitized erbium-doped tellurite glass

The 15 μm emission spectrum corresponding to the 4I13/2→4I15/2 transition and upconversion of Er3+ in tellurite glass in the temperature range from 8 to 300K is studied. The emission spectrum of Er3+: 4I13/2→4I15/2 transition is also analyzed using a peak-fit routine, and an equivalent four-level system is proposed to estimate the Stark splitting for the 4I15/2 and 4I13/2 levels of Er3+ in the tellurite glass. The results indicate that the 4I13/2→4I15/2 emission of Er3+ has considerable broadening due to a significant enhancement of the a' and b' emission peaks. Temperature-dependent FWHM are investigated, the results show that a monotonic increase of FWHM is observed for temperatures from 8 to 300K. Intense upconversion emission signals around 529, 545 and 669nm corresponding to the 2H11/2, 4S3/2, and 4F9/2 transitions, respectively, to the 4I15/2 ground state are generated and are measured as functions of temperature in the 8 to 300K range. The most remarkable result is observed in the green upconversion signal around 546 and 669nm which presented a maximum intensity enhancement of 2198 and 1556 times around 80K compared with that around 300K. Monotonic decrease is observed at temperatures above 80K. In the same temperature range the signal at 529nm diminishes to zero with lowering temperature.