Yb3+-codoped ZnF2–Al2O3–P2O5 glasses

Yb3+-codoped ZnF2–Al2O3–P2O5 glasses

Materials Chemistry and Physics 117 (2009) 29–34 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.els...

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Materials Chemistry and Physics 117 (2009) 29–34

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Concentration effect of Yb3+ on the thermal and optical properties of Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses S.W. Yung a,∗ , H.J. Lin a , Y.Y. Lin a , R.K. Brow b , Y.S. Lai a , J.S. Horng a , T. Zhang c a b c

Department of Materials Science and Engineering, National United University, Miao-Li 36003, Taiwan Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65401, USA Institute for Materials Research, Fuzhou University, Fuzhou, Fujian 350002, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 20 May 2008 Received in revised form 18 November 2008 Accepted 28 November 2008 PACS: 81.05.Pj 76.30.Kg 91.60.Mk 33.50.Dq

a b s t r a c t The effects of Yb3+ and Er3+ dopants on the optical and thermal properties and chemical durability of 40ZnF2 –10Al2 O3 –50P2 O5 glasses (nominal molar composition) have been investigated. The glass transition temperature (Tg ), dilatometric softening temperature (Ts ), crystallization temperature (Tc ), refractive index, density, and durability in water increase with increasing Er3+ and Yb3+ contents, whereas the thermal expansion coefficient decreases. Red (645 nm) and near infrared (1.535 ␮m) emissions are simultaneously observed at room temperature with excitation at 980 nm. The red up-conversion and 1.5 ␮m emission spectra are associated with the 4 F9/2 → 4 I15/2 and 4 F13/2 → 4 I15/2 transitions of Er3+ , respectively, and the up-conversion excitation processes have been analyzed. The Yb3+ ions enhance the up-conversion and emission processes. The results suggest that the Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses may be a potential material for developing optical amplifiers and up-conversion optical devices. © 2009 Published by Elsevier B.V.

Keywords: Optical glass Yb doping Phosphate glasses

1. Introduction Rare-earth doped fluorophosphate glasses have been intensively investigated for their exceptional optical properties and potential applications [1–9]. Fluorophosphate glasses are excellent materials due to their high rare-earth solubility [10], low phonon energy and non-linear refractive index [11], high gain coefficient [12], wide bandwidth capability and low up-conversion emission [8,13], and so they have found many applications, including visible and infrared lasers, optical amplifiers, devices for optical data storage, color display, under-sea optical communications, eye-safe lasers, biomedical diagnostics and sensors, etc. [8,14,15]. Fluorophosphate glasses have generally poor chemical durability and thermal stability, but these properties can be improved with the addition of rare-earth ions [9,17]. Yb3+ and Er3+ ions have already established a key role as the active dopant of fluorophosphate glasses for optical amplification at 1.53 ␮m for telecommunications devices. Li et al. [18] studied the spectroscopic properties and thermal stability of Er3+ /Yb3+ codoped Al(PO)3 based fluorophosphate glasses and indicated that

∗ Corresponding author. E-mail address: [email protected] (S.W. Yung). 0254-0584/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2008.11.060

the glasses exhibit a large thermal stability, more than 100 ◦ C, and a broadband emission at 1.53 ␮m with a FWHM over 63 nm. However, their results showed that the calculated stimulated-emission crosssection is 6.85 × 10−21 cm2 , which is smaller than that reported in the present work. Ronchin et al. [19] reported that the green-toblue up-conversion was detected on continuous-wave excitation at 514.5 nm in the erbium-activated aluminum fluoride glasses. Zhang et al. [8] studied the Er3+ /Yb3+ -codoped fluorophosphate glasses and reported that the intensity ratio of red/green light are closely related to the Yb3+ -to-Er3+ ratio and the concentration of Er3+ . dos Santos et al. [20,21] studied the effect of thermal induced threefold up-conversion emission enhancement in non-resonant excited Er3+ /Yb3+ -codoped chalcogenide glass. The mechanism of up-conversion generation is related to the excitation and energy transfer processes of erbium ions. In the present work, the red (645 nm) and near infrared (1.535 ␮m) emissions are simultaneously observed under 980 nm excitation at room temperature. The non-radiative processes include processes characteristic of the host and processes characteristic of the active ion promoted by its concentration. The high solubility of rare earth ions in fluorophosphate glasses allows a high concentration of active ion into a small volume of the glass structure [22]. However, a high concentration of Er3+ ions does not necessarily mean an increase of the quantum efficiency. The principal drawback that this ion

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presents is its low absorption cross-section in the emission range of standard laser diodes. Pump absorption and quantum efficiency have been improved by co-doping Er3+ -containing glasses with sensitizing Yb3+ ions for a wide range of compositions, including borate, phosphate and germanate glasses (e.g., refs. [44–46]), heavy metal fluorides [47], and chalcogenides [48]. In the present study, we report the effects of Yb3+ and Er3+ ions on the thermal stability, chemical durability, emission and absorption spectra, upconversion luminescence properties of a zinc fluorophosphate glass host.

2.4. Chemical durability measurement Each specimen was cut into a cubic shape and polished with 1, 0.3, and 0.05 ␮m aluminum slurries. The final size of these specimens is 0.4 m3 . Chemical durability of the ZnF2 –Al2 O3 –P2 O5 glasses was determined by measuring the weight loss after immersion in distilled water at 30 ◦ C for 3 days. The ratio of specimen surface area to the water volume is about 0.01 cm−1 . The dissolution rate was defined as the weight loss per unit surface area per unit time (g cm−2 min−1 ). 2.5. UV–vis–NIR and fluorescence spectra measurement The UV–vis–NIR absorption spectra of ZnF2 –Al2 O3 –P2 O5 glasses were measured using a Shimadzu UV-3101PC spectrophotometer (Kyoto, Japan) in the scanning range of 300–1700 nm. Fluorescence spectra were recorded using a HORIBA Jobin Yvon IRIAX 550 spectrophotometer (Edison, NJ, USA) in the wavelength range of 1400–1700 nm. Er3+ /Yb3+ -codoped specimens were excited using an adjustable diode laser (Coherent Inc., Santa Clara, CA, USA) with 300 mA current at the wavelength of 980 nm. All optical measurements were carried out at room temperature.

2. Experimental 2.1. Specimens preparation The zinc aluminum fluorophosphate glasses in this study were prepared from raw materials NH4 H2 PO4 (Mallinckrodt Baker Company), Al(OH)3 (Mallinckrodt Baker Company), ZnF2 (Aldrich Chemical Company), Er2 O3 , and Yb2 O3 (Johnson Matthey Company) with various molar ratios as listed in Table 1. All the starting chemical reagents were weighed (±0.001) on a Sartorius CP2245 digital balance. The five weighed chemical reagents were fully mixed in an alumina crucible before melting in an electric furnace, heated from room temperature to 1350 ◦ C with a heating rate of 10 ◦ C min−1 . After the melts were held for 90 min, they were cast onto a pre-heated brass plate and then immediately annealed near the glass transition temperature for 5 h to release the residual thermal stresses. After furnace cooling from 550 ◦ C to room temperature, the specimens were cut into rectangular shapes with approximate dimensions of 5 mm × 10 mm × 1.5 mm and polished in parallel with 1, 0.3, and 0.05 ␮m alumina slurries, respectively for refractive index measurement. Energy dispersive X-ray spectrometry revealed that the FP base composition was 48.4(mol%) P2 O5 –12.9Al2 O3 –38.7ZnF2 for the FP glass, which had a batch composition of 50P2 O5 –10Al2 O3 –40ZnF2 (Table 1), indicating that some Al2 O3 was picked up from the crucible during melting

3. Results and discussion The Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses with different molar composition, as listed in Table 1, were synthesized in this study. Note that when the Er2 O3 and Yb2 O3 are more than 5 mol% and 1 mol%, respectively, these compositions exceed the stable glass forming region. The color of ZnF2 –Al2 O3 –P2 O5 glasses or Yb3+ -doped ZnF2 –Al2 O3 –P2 O5 glasses is transparent but the color of Er3+ -doped ZnF2 –Al2 O3 –P2 O5 glasses is pink. These glasses contain a larger amount of Al2 O3 than expected from the batch composition, because of the dissolution of the alumina crucible used to prepare the glasses and the possible volatilization of the phosphate and fluoride components. EDX results showed that the compositions of Al2 O3 are always 2.5–3.5 mol% greater than that of batch compositions and the ZnF2 and P2 O5 contents are decreased simultaneously compared with those expected. The density of the 40ZnF2 –10Al2 O3 –50P2 O5 base glass is 2.805 ± 0.012 g cm−3 . The glass densities increase as Er2 O3 and/or Yb2 O3 replace ZnF2 in the nominal glass composition, as shown in Fig. 1. This is reasonable because the atomic weights of both of Er and Yb are greater than those of the other cations that constitute the glass composition. A decrease in molar volume with the addition of Er and Yb to phosphate glass may also contribute to the increase in density [17]. The refractive index of Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses is indicated in Fig. 2 as a linear function with Yb2 O3 . At a given quantity of Yb2 O3 , the refractive index of Er3+ /Yb3+ codoped ZnF2 –Al2 O3 –P2 O5 glasses also increases with increasing Er2 O3 contents. The refractive index of a glass is mainly dependent on individual polarizabilities of cation ions and the concentration of cation per unit volume [11]. The refractive index, generally, also increases with increasing cation size [23]. In our study, we assume that RE ions are homogeneously distributed into the host

2.2. Density and refractive index measurement Density of the specimens was measured by the Archimedes method with distilled water as the immersion fluid at 25 ◦ C. Two samples were measured for the average density. The refractive index of the specimens was measured using an ATAGO DR-A1 Abbe refractometer (ATAGO Co., Tokyo, Japan) at room temperature. The wavelength of 589 nm was used to measure index.

2.3. DTA and TMA measurement The glass transition temperature (Tg ), dilatometric softening temperature (Ts ), crystallization temperature (Tc ), and thermal expansion coefficient (˛) of the glasses were measured using a PerkinElmer DTA 7 differential thermal analyzer (DTA) (Shelton, CT, USA) and a PerkinElmer TMA 7 thermal mechanical analyzer (TMA) (Shelton, CT, USA). The DTA apparatus was calibrated by using indium (mp 156.6 ◦ C) and aluminum (mp 660.4 ◦ C) standards. The TMA also was calibrated using standards and expansion coefficient was calculated from 100 to 350 ◦ C. Specimens were heated from room temperature with a heating rate of 10 ◦ C min−1 to determine the characteristic properties. Most of the TMA and DTA measurements were made in duplicate to estimate the experimental error.

Table 1 The batch composition, glass transition temperature (Tg ), dilatometric softening temperature (Ts ), crystalline temperature (Tc ), and thermal expansion coefficient (˛) of Er3+ /Yb3+ -codoped zinc aluminum fluorophosphate glasses. Sample

FP FP1E FP1E1Y FP1E2Y FP1E3Y FP1E4Y FP3E FP3E1Y FP3E2Y FP3E3Y FP5E FP5E1Y

Batch composition (mol%) P2 O5

Al2 O3

ZnF2

Er2 O3

Yb2 O3

50 50 50 50 50 50 50 50 50 50 50 50

10 10 10 10 10 10 10 10 10 10 10 10

40 39 38 37 36 35 37 36 35 34 35 34

– 1 1 1 1 1 3 3 3 3 5 5

– – 1 2 3 4 – 1 2 3 – 1

Tg (◦ C)

Ts (◦ C)

Tc (◦ C)

˛ (×10-6 ◦ C−1 )

451.1 472.4 476.7 480.8 482.1 493.1 480.5 482.1 501.3 508.5 483.9 507.2

484.0 499.6 513.2 519.8 529.9 533.8 514.8 526.3 537.3 544.5 533.8 543.0

601.7 621.2 626.5 643.2 649.5 654.6 645.5 650.9 659.6 666.8 661.5 665.4

5.65 5.58 5.15 4.95 4.71 4.65 5.18 4.96 4.89 4.69 4.70 4.35

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Fig. 1. The density of Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses as a linear function with Yb2 O3 contents.

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Fig. 3. Typical TMA and DTA curve of Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses.

glass matrix. Glass formers, like for which the ionic polarizability is 3.6 Å3 , contribute less to the refractive index than larger, more polarizable modifiers like Er3+ (22.7 Å3 ) and Yb3+ (21 Å3 ) [24]. Hence the Er3+ and Yb3+ ions increase refractive index of the glass because of their greater ionic polarizability. Besides the effect mentioned, non-bridging oxygens are strongly polarizable and can also contribute to the refractive index [25]. The Er3+ and Yb3+ ions were added in the zinc aluminum fluorophosphate glasses as the glass modifiers which may break the P–O–P bonds and generate nonbridging oxygens (P-O− · · · Er+ , P-O− · · · Yb+ ). Therefore, increasing size of cation and ionic polarizability due to rare earth ion doped in the glass lead to an increase of the refractive index as shown in Fig. 2. The results are similar to those of Choi’s study of Yb3+ -doped fluorophosphates glasses [12]. The glass transition temperature (Tg ), dilatometric softening temperature (Ts ), crystallization temperature (Tc ), and thermal expansion coefficient (˛) are related to the cross-linking density, molecular entanglement, and properties of modifiers of the glasses [26]. Fig. 3 shows typical DTA and TMA curves for Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses. From the TMA and

DTA analyses, we can determine the thermal expansion coefficient (˛), glass transition temperature (Tg ), dilatometric softening temperature (Ts ), and crystallization temperature (Tc ) of glasses, respectively, as indicated in Fig. 3. The Tg , Ts , Tc , and ˛ of ZnF2 –Al2 O3 –P2 O5 glasses with various molar ratio of Er3+ /Yb3+ codoped are listed in Table 1. All the characteristic temperatures increase with increasing contents of Er3+ and Yb3+ in the glasses. This is due to the strong ionic bonding between Er3+ and/or Yb3+ and the O2− and/or F− anions from the glass network. These bands strengthen the network by cross-linking neighboring fluorophosphate chains [17,27]. Fig. 4 shows the effect of Yb2 O3 content on the thermal expansion coefficient (˛) of phosphate glasses doped with different levels of Er2 O3 . Both the Er3+ and Yb3+ were located in the glass network structure as the glass modifiers and provide strong ionic bonds with the neighborhood anions in the glass network [17,27]. Fig. 5 shows the dissolution rates of the Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses in distilled water at 30 ◦ C for 3 days. The dissolution rate decreases by about an order of magnitude with the addition of 1.4 × 1021 Er3+ + Yb3+ ions (6 mol% Er2 O3 + Yb2 O3 ). Comparing the FP1EXY and FP3EXY series of glasses, the dissolution rate also decreases as the concentration of Yb3+ dopant increases. The coordination numbers (CNs) of Er3+ and Yb3+ in ZnF2 –Al2 O3 –P2 O5

Fig. 2. The refractive index of Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses as a linear function with Yb2 O3 contents.

Fig. 4. Effect of Yb2 O3 concentration on the thermal expansion coefficient of Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses.

P5+

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Fig. 5. Effect of Er3+ /Yb3+ concentration on the dissolution rate of Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses.

glass network are larger than six which is preferred [26]. In the study of rare-earth local environment in ultraphosphate glasses, Karabulut et al. [29] reported that there are 7–8 oxygen nearest neighbors in the first co-ordination sphere around rare-earth ions in ultraphosphate glasses and ∼6 oxygens in the first coordination sphere around rare-earth ions in metaphosphate glasses. The results also demonstrated that the larger rare-earth CNs correlate with the larger number of non-bridging oxygens available per rare-earth ion. The high coordination numbers of Er3+ and Yb3+ in glass network implies that the Er3+ and Yb3+ are occupied at the glass modifier position. The Er3+ and Yb3+ ions offer a strong ionic bonding with its neighborhood anions in the glass matrix. It inhibits the water molecules diffusing into the glass network by increasing and strengthening the cross-linking of glasses. Shih [17] studied the chemical durability of the Er3+ /phosphate glasses system and indicated that as one Er3+ is incorporated into the glass network, three P–O− · · · Er3+ linkages will form between phosphate chains. The increase of cross-links between the phosphate chains will retard the entrance of water molecules, and lead to improved chemical durability of the erbium-doped glasses. The increase of cross-linking density and strength by increasing rare-earth doping levels improves the chemical durability of flurophosphate glasses as shown in Fig. 5. The UV–vis–NIR absorption spectra of the Er3+ -doped and Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses are shown in Fig. 6(a and b), respectively. The absorption spectra consist of the four sets of peaks for Er3+ , centered near 1535, 980, 798, and 651 nm, and corresponding to the absorptions from the ground state 4 I15/2 to the excited states 4 I13/2 , 4 I11/2 , 4 I9/2 , and 4 F9/2 , respectively. It is reasonable that the absorbance increases due to the increasing Er3+ concentration in the glass as indicated in Fig. 6(a). For the Er3+ /Yb3+ -codoped glasses, the strong absorption at 977 nm, in Fig. 6(b) corresponds to the ground state absorption of Yb3+ . The absorbance increases also with increasing the Yb3+ concentration in the glass. It should be noted that the Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses showed a much broader absorption band from 870 to 1060 nm, than for either the singly doped compositions, because of the overlap of the ground state absorption (GSA) band for 2 F5/2 excitation state of Yb3+ with the 4 I11/2 excitation state of Er3+ . The absorption cross-section of Yb3+ is greater than that of Er3+ , hence, we consider that the Yb3+ absorption dominates in this region (870–1060 nm). The fluorescence spectra of Er3+ /Yb3+ -codoped and Er3+ -doped ZnF2 –Al2 O3 –P2 O5 glasses are shown in Fig. 7(a and b), respectively.

Fig. 6. The UV–vis–NIR absorption spectra of (a) Er3+ -doped ZnF2 –Al2 O3 –P2 O5 glasses, and (b) Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses.

The intensity of the 1.535 ␮m emission increases with increasing Yb3+ concentration in these glasses (Fig. 7(a)), whereas the intensity of the 1.535 ␮m emission decreases with increasing the Er3+ concentration in the Yb3+ -free compositions (Fig. 7(b)). The spectra in Fig. 7(a) illustrate the well-known sensitizing effect of Yb3+ on Er-emission intensity [49,31], and the spectra in Fig. 7(b) indicate that concentration quenching effects occur in the Yb3+ -free glasses when Er2 O3 -contents exceed ∼1 mol%. Both effects are dependent on electronic interactions between the rare earth ions. The distance between rare earth ions in the glass matrix decreases with increasing [Er + Yb] concentrations, and the rate of the energy transfer processes is inversely proportional to r6 [30], where r is the distance between two coupled ions. Increasing the energy transfer rate between like ions (e.g., Er–Er) increases the probability that an excited ion will non-radiatively de-excite, leading to the concentration quenching effects illustrated in Fig. 7(b), but also increases the probability for energy transfer between different ions (e.g., Yb–Er), as illustrated by the sensitizing effects of Yb3+ in Fig. 7(a). The Er–Er interactions also contribute to enhanced upconversion processes that yield emission peaks at 645 nm, as shown in the inset to Fig. 8. (The glasses were excited by a diode laser with 300 mA current at the wavelength of 980 nm.) Greater Er2 O3 contents in the Yb3+ -free glasses produce greater up-conversion emission intensities, despite the reduced photoluminescence efficiency (Fig. 7(b)).

S.W. Yung et al. / Materials Chemistry and Physics 117 (2009) 29–34

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Fig. 9. Schematic diagram of the infrared emission at 1.535 ␮m and red emission at 645 nm of Er3+ .

Fig. 7. The fluorescence spectra of (a) Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses, and (b) Er3+ -doped ZnF2 –Al2 O3 –P2 O5 glasses.

Fig. 8 also shows the fluorescence emission up-conversion spectra of the Er3+ /Yb3+ -codoped glasses excited. The peak around 645 nm corresponds to the red emission from the transition of the 4 F9/2 excited state to the 4 I15/2 ground state, as shown in Fig. 9. It was found that the emission intensity increases slightly with increasing concentration of Yb3+ .

The population of Er3+ in 4 F9/2 level depends on the following process: Excited State Absorption (ESA), 4 I13/2 (Er3+ ) + a photon → 4 F9/2 (Er3+ ), Phonon-Assisted Energy Transfer (PAET) from Yb3+ , 2 F5/2 (Yb3+ ) + 4 I13/2 (Er3+ ) → 2 F7/2 (Yb3+ ) + 4 F9/2 (Er3+ ) and Energy Transfer (ET) between Er3+ ions, 3+ 4 3+ 4 3+ 4 3+ 4I [31,32]. The 13/2 (Er ) + I11/2 (Er ) → F9/2 (Er ) + I15/2 (Er ) 4S 4 3+ 3/2 level to the F9/2 level of Er , also contributes to the red emission [32,33]. The results are similar to that of Er3+ /Yb3+ codoped fluorophosphates glass [34], oxyfluoride glass [35] and Bi2 O3 –GeO2 –Ga2 O3 –Na2 O glass [36]. The intense red and relatively weak green up-conversion emission were detected in these samples. Fig. 9 illustrates the relevant transitions responsible for the 1.535 ␮m and 645 nm emissions when the Er3+ /Yb3+ -codoped glasses are excited at 980 nm. The Yb3+ ions play a role of sensitizer due to its strong absorption at 980 nm and because the 2 F5/2 energy level of Yb3+ ion is close to the 4 I11/2 energy level of Er3+ ion. In addition, the 2 F5/2 energy level of Yb3+ ion is about twice the energy of the 4 I13/2 level for the Er3+ ion, and so the energy transfer (ET) processes for populating the 4 I13/2 emitting state are markedly enhanced, leading to the greater emissions intensities for the Er3+ /Yb3+ -codoped glasses, relative to the Yb3+ -free compositions. The relevant energy transfer process describing the sensitizing effect is 2 F5/2 (Yb3+ ) + 4 I15/2 (Er3+ ) → 2 F7/2 (Yb3+ ) + 4 I11/2 (Er3+ ). According to the McCumber theory [37,38], the absorption crosssection,  a , of rare earth ions doped in glass can be calculated from the absorption spectrum: a =

1 ln N×

I  0

I

(1)

where I0 /I is absorbance,  the thickness of glass specimen, and N is the concentration of Er3+ or Yb3+ ions doped in glass. The absorption cross-sections of Er3+ -doped glasses are listed in Table 2. In the present study, the maximum absorption cross-section of Er3+ -doped ZnF2 –Al2 O3 –P2 O5 glasses is around 7.75 × 10−21 cm2 at a wavelength of 1.535 ␮m. Accordingly, the emission crosssection,  e , of rare earth ions doped glasses can be obtained using the McCumber theory [37,38]. The absorption ( a ) and stimulated emission ( e ) cross-sections are related by e = a exp

Fig. 8. The red emission ZnF2 –Al2 O3 –P2 O5 glasses.

band

of

Er3+ /Yb3+ -codoped

and

Er3+ -doped

 ε − h  kT

(2)

where  is the photon frequency, ε is the net free energy required to excite one Er3+ ion from the 4 I15/2 to 4 I13/2 states at temperature T, h is the Planck constant and k is the Boltzmann constant. Fig. 10 illustrates the measured absorption and calculated emission cross-sections of Er3+ ions and the values are listed in Table 2. The obtained data are also compared with the data reported in the literature as listed in Table 2. In our study, as indicated in Table 2, the emission cross-section,  e , of Er3+ -doped ZnF2 –Al2 O3 –P2 O5 glasses

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Table 2 Absorption and emission cross-sections of rare-earth doped glasses. Glasses

 a (×10−21 cm2 )

 e (×10−21 cm2 )

FWHM (nm)

FWHM ×  e

Reference

FP1E FP3E FP5E Germanate Phosphate

6.50 6.69 6.71 – –

7.50 7.72 7.75 5.68 6.40

30 35 40 53 37

225.0 270.5 310.0 301.0 236.8

This work This work This work Ref. [39] Ref. [40]

Silicate



5.50 5.50

45 40

247.5 220

Ref. [41] Ref. [42]

Tellurite



7.50

65

487

Ref. [42]

the greatest fluorescence intensity and this intensity increases with increasing Yb3+ concentration doped in the glass. Acknowledgement This work was supported by the Technology Development Program for Academic (project No. 95-EC-17-A-08-S1-033) of Ministry of Economic Affairs, Republic of China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] Fig. 10. The measured absorption (solid line) and calculated emission (dot line) cross-sections of Er3+ -doped in ZnF2 –Al2 O3 –P2 O5 glasses.

are greater than that reported in the literature [39–42]. The values of FWHM are close to those of phosphate and silicate glasses [40,42], are smaller than that of tellurite and germinate glasses [39,42]. The FWHM and  e are very important parameters for optical amplifier applications [43]. The gain bandwidth of an amplifier can be evaluated by the FWHM ×  e product. When the product is larger, the optical amplification is greater. Comparing the data listed in Table 2, it is obvious that tellurite glass has remarkable optical properties for a broadband amplifier, and the optical properties of 5 mol% Er3+ -doped ZnF2 –Al2 O3 –P2 O5 glass in this work are better than those of the Er3+ -doped silicate, phosphate or even the germanate glass. Consequently, the glass samples can be used as host material for potential broadband optical amplifier in WDM. 4. Conclusions The Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses were prepared and the concentration effects of Yb3+ on the Er3+ /Yb3+ codoped ZnF2 –Al2 O3 –P2 O5 glasses have been investigated. It was shown that the Yb3+ concentration affects the spectroscopic properties, thermal stability and chemical durability of Er3+ /Yb3+ -codoped ZnF2 –Al2 O3 –P2 O5 glasses. The glass transition temperature, softening temperature, crystallization temperature, refractive index, density and water durability all increase with increasing Er3+ /Yb3+ concentration, whereas the thermal expansion coefficient of the glass decreases. Intense infrared-to-visible up-conversion fluorescence in ZnF2 –Al2 O3 –P2 O5 glasses has been observed at 645 nm under 980 nm excitation at room temperature. The red upconversion emission is associated with the 4 F9/2 → 4 I15/2 transition of Er3+ ions. The 1 mol% erbium doped ZnF2 –Al2 O3 –P2 O5 glass has

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