Yb3+ codoped fluorophosphate glasses

Yb3+ codoped fluorophosphate glasses

ARTICLE IN PRESS Journal of Luminescence 126 (2007) 139–144 www.elsevier.com/locate/jlumin Spectroscopic properties of Er3+/Yb3+ codoped fluorophosph...

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ARTICLE IN PRESS

Journal of Luminescence 126 (2007) 139–144 www.elsevier.com/locate/jlumin

Spectroscopic properties of Er3+/Yb3+ codoped fluorophosphate glasses Meisong Liao, Zhongchao Duan, Lili Hu, Yongzheng Fang, Lei Wen Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China Received 12 February 2006; received in revised form 13 May 2006; accepted 26 June 2006 Available online 8 August 2006

Abstract Er3+/Yb3+ codoped fluorophosphate glasses were prepared and their thermal stabilities, Raman spectra, absorption spectra, and fluorescence spectra were measured. It is found that proper content of NaF or PbF2 is helpful for the increase of stability against crystallization. The variation of Al(PO3)3 or NaF content in the composition affects not the maximum phonon energy but the phonon density. The introduction of PbF2 decreases the phonon energy slightly. Intense green and red upconversion luminescence was observed for the fluorophosphate glass with low phosphate content. A glass matrix for upconversion luminescence requiring neither expensive raw material nor special atmospheric conditioned preparation is provided. Infrared luminescence around 1530 nm was researched. Fluorophosphate glasses with bandwidth properties and stimulated-emission cross sections better than tellurite, germanate and silicate glasses are obtained. Through the introduction of NaF, the bandwidth properties are decreased. Through the introduction of PbF2 the gain properties are increased. On the whole, it is difficult to obtain a material with the best gain properties and bandwidth properties simultaneously. There should be a compromise between them according to the demand. r 2006 Elsevier B.V. All rights reserved. PACS: 73.43.Fj; 74.25.Gz; 73.61.Jc; 76.30.Kg Keywords: Er3+; Fluorophosphate glasses; Spectroscopic properties

1. Introduction Among the luminescence materials of rare earth ions doped glasses, Er3+ doped glasses emit infrared fluorescence around 1530 nm through transition of 4I13/2-4I15/2, and exhibit extensive applications in fiber communication for amplification. Also green and red upconversion luminescence can be obtained through transitions of 4 S3/2-4I15/2 and 4F9/2-4I15/2, respectively, which are useful in color displays, fluorescence guard against forge, image, sensor, etc. [1,2]. Spectroscopic properties of Er3+ doped glasses have become a focus of research in recent years. It is significant to adjust the ingredients and composition of the glasses to optimize the spectroscopic properties according to the requirements of application. Corresponding author. Tel.: +86 21 59911204; fax: +86 21 59914516.

E-mail address: [email protected] (M. Liao). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.06.009

Fluorophosphate glasses possess the merits of both fluoride and phosphate glasses. They characterized with high transmittance in infrared and ultraviolet regions, low nonlinear refractive index and high Abbe number [3–5]. The positions of rare earth ions in these glasses are flexible and they can be introduced in a high concentration without quenching. The OH group, which is difficult to be controlled under a very low concentration in phosphate glasses, can be kept in a very low concentration in fluorophosphate glasses without protection in special atmosphere. In latest years, many researches were done on the Er3+ doped silicate [6], phosphate [7], fluoride [8] and sulfide glasses [9], but the studies about the Er3+ doped fluorophosphate glasses are comparatively limited. However, the ingredients of fluorophosphate glasses are very extensive and their structures are considerably complicated. The relationships between composition, structure and optical properties are far from disclosed completely.

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In this work, Er3+/Yb3+ codoped fluorophosphate glasses with various content of metaphosphate Al(PO3)3, alkaline earth fluoride NaF, and heavy metal fluoride PbF2 were prepared by conventional melting and quenching method. Their spectroscopic properties were researched by using Raman spectra, absorption spectra, emission spectra. The Yb3+ ions are introduced to utilize the 970 nm laser diode effectively as pump source. The dependences of spectroscopic properties on compositions and structure were discussed. 2. Experiments The compositions of the fluorophosphate glasses are as follows: (60x)Mg0.17Ca0.49Sr0.17Ba0.17F2–40AlF3–xAl (PO3)3–0.8ErF3–2.4YbF3 (where x ¼ 3, 5, 7, 9, 11 mol%); xNaF–(64x)Mg0.17Ca0.49Sr0.17Ba0.17F2–31AlF3–5Al(PO3)3– 0.8ErF3–2.4YbF3 (where x ¼ 4, 12, 20 mol%), xPbF2– (62x)Mg 0.17 Ca 0.49 Sr 0.17 Ba 0.17 F 2 –33AlF 3 –5Al(PO 3 ) 3 – 0.8ErF3–2.4YbF3 (where x ¼ 2, 6, 14 mol%). Samples are named FP3, FP5, FP7, FP9, FP11, FP5Na4, FP5Na12, FP5Na20, FP5Pb2, FP5Pb6 and FP5Pb14 in turn. All starting materials are of analytical grade. Batches of 30 g were weighed and mixed, then were placed into crucible and melt at 950–1050 1C. After completely melting and fining, the glass liquids were cast into graphite moulds, then transformed in oven at transition temperature and annealed to room temperature. Samples were cut and polished carefully to meet the requirements for optical measurements. Densities were measured by Archimedes method in distilled water. The Er3+ concentrations were calculated from the measured densities and the initial compositions. Refractive indices were measured on prism minimum deviation method. Characteristic temperatures (The temperatures for glass transition Tg, for the onset of the crystallization peak Tx) were determined by differential scanning calorimetry (DSC) using NETZSCH STA 409PC. Raman spectra were measured using Micro Raman Spectra Lab Ram-1B.

UV/VIS/NIR absorption spectra were recorded using a spectrophotometer. The fluorescence spectra were measured with a Triax 550 spectrofluorimeter on excitation at 970 nm. The measures of upconversion luminescence were conducted in the same conditions in order to compare the intensity of fluorescence: the position and power of the pumping beam and the width of the slit of the spectroscopy to collect the luminescence signal were fixed at the same condition, and the sample was set at the same place in the experiment setup. The lifetime of 4I13/2 energy level was measured with pulsed signal of 970 nm laser diode and a HP546800B 100-MHz oscilloscope. 3. Results 3.1. Densities, refractive indices and characteristic temperatures Densities, concentrations of Er3+ and refractive indices of samples were listed in Table 1. With the increment of Al(PO3)3 content, densities decrease. The radius of F is 1.33 A˚ and the radius of O2 ion is 1.32 A˚. The radius of P5+ is 0.35 A˚. The volume of PO 3 is about three times of that of F ion. When more PO 3 is introduced to replace the location of F, the structure of glass becomes less compact, so the density decreases. The refractive index increases with the increase of PbF2 content and decrease with the increase of NaF content, since the polarizability of Na+ is less and Pb2+ is higher than the average polarizability of alkaline earth ions. Characteristic temperatures increase with the increment of phosphate content and decrease with the increment of NaF or PbF2 content. The temperature gap between Tg and Tx can be used to estimated the INC indirectly. The higher TxTg, the better is the stability against crystallization. By the comparison between FP5 and the glasses containing NaF or PbF2 it can be found that proper content of NaF or PbF2 is helpful for the increase of stability against crystallization.

Table 1 Density, concentration of Er3+ ions, refractive index nD and characteristic temperatures of all samples Samples no.

Density (g/cm3)

Er3+ concentration (  1020 ions/cm3)

Refractive index nD

Tg (1C)

Tx (1C)

TxTg (1C)

FP3 FP5 FP7 FP9 FP11 FP5Na4 FP5Na12 FP5Na20 FP5Pb2 FP5Pb6 FP5Pb14

3.63 3.61 3.59 3.59 3.58 3.50 3.49 3.35 3.62 3.90 4.31

1.658 1.599 1.542 1.497 1.452 1.558 1.628 1.634 1.490 1.527 1.544

1.463 1.469 1.475 1.480 1.487 1.466 1.459 1.454 1.484 1.498 1.521

425 439 447 459 474 415 398 373 423 402 386

530 548 562 596 605 534 536 476 533 520 479

105 108 115 137 131 119 138 130 110 118 93

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3.2. Raman spectra analyses The Raman spectra of undoped glasses are showed in Figs. 1 and 2. Peak a is ascribed to M–F (M representing bivalent cations including Mg2+, Ca2+, Sr2+ and Ba2+) vibration [10]. Peak b is ascribed to [AlF4] vibration [11–12]. Peak c is ascribed to P–O–P stretching vibration [13]. Peak d is due to F–P–F stretching vibration [14], and peak e is due to O–P–F stretching vibration [11]. Peak f is due to O–P–O stretching vibration [14]. In Fig. 1 the wavenumber corresponding to peak e is about 1000 cm1, and nearly keeps constant with the variation of composition, which suggests that the maximum phonon energy of these series glasses is about 1000 cm1. In Fig. 2 it can be found that the introduction of alkaline earth fluoride shows no influence on the maximum phonon energy, and the

introduction of heavy metal fluoride decreases the maximum phonon energy a little. 3.3. Spectroscopic properties Absorption spectra of samples with different phosphate contents are shown in Fig. 3. The wavelengths of absorption peak remain constant with the variation of composition, which indicates that Er3+ energy levels remain constant in these glasses. With the increment of Al(PO3)3 content, the cutoff band shifts to a longer wavelength. Generally the absorption of glasses in ultraviolet region is due to the transition absorption of valence electron of anions from the valence band to the conduction band. The cutoff wavelength is resolved by the excited energy required for the transition. Formula (1) can be used to calculate the excited energy [15]: Hv ¼ E þ M  j,

Fig. 1. Raman spectra of samples with various content of Al(PO3)3.

Fig. 2. Raman spectra of partial samples with various content of NaF or PbF2.

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(1)

in formula (1), Hv is the excited energy; M is the power overcoming the Coulomb force between anion and cation; j is the energy required for the polarizability of anions; E is the electron affinities. For O2 the E is 3.80 eV, and for F the E is 4.03 eV. With the increment of Al(PO3)3 content, Hv decreases and the cutoff bands exhibit red shift. Upconversion spectra of samples with various content of Al(PO3)3 are shown in Fig. 4. The green (543 nm) and red (653 nm) upconversion emissions are due to the 4S3/24 I15/2 and 4F9/2-4I15/2 transitions, respectively. Their mechanisms have already been discussed in the previous research [16]. With the increment of phosphate content, the intensity of the luminescence decreases sharply. The measured lifetimes of 4I13/2 and full width at half maximum (FWHM) are listed in Table 2. For the samples containing phosphate, it can be found that FWHM is the minimum for the samples with 7–9 mol% Al(PO3)3 content. FWHM changes with the change of the coordinate

Fig. 3. Absorption spectra of samples with various content of Al(PO3)3.

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environment of rare earth ions. The greater the disorder in the rare earth environment, the greater the differences in Stark levels, leading to a wider fluorescence. In the fluoride glasses the environment of rare earth ions has much larger disorder than in the phosphate glasses, so the fluorescence is much wider for the fluoride glasses. The location of rare earth ions in fluorophosphate is considerable free. Er3+ in these glasses can be both in and out of network positions [17–18]. Here Raman spectra cannot be used to analyze structure of the Er3+/Yb3+ codoped fluorophosphate glasses, since the energy levels of Er3+ ions are too many and the fluorescence emitted by Er3+ masks the Raman spectra. Zhang et al. studied the structure of Yb3+ doped fluorophosphate glasses with 20 mol% Al(PO3)3 and 80 mol% alkaline earth fluoride content through Raman

spectra, and found that the main role of Yb3+ ions is the network former [19], so in these glasses the main role of Er3+ can be taken as the network former. In Fig. 1 it can be found that the glasses with 7–9 mol% Al(PO3)3 content contain more O–P–O and P–O–P bonds than the glasses with other compositions. Their networks are much stronger [20]. Hence more Er3+ ions were incorporated in the networks which contain more phosphorus. Compared with FP3, FP5 and FP11, the environment of Er3+ ions in FP7 or FP9 is more inclined to that of the phosphate glasses, so their fluorescence widths decrease. For the samples with different content of NaF, FWHM decreases with the increase of NaF content. With the introduction of NaF the ratio of fluorine to phosphorus decreases, and the environment of Er3+ inclines to that of the phosphate glasses, so the fluorescence width decreases. McCumber theory is usually used to calculate the stimulated-emission cross section semi ðlÞ of Er3+: 4I13/2 -4I15/2 transition: semi ðlÞ ¼ sabs ðlÞ expð  hnÞ=KT,

(2)

where K is the Boltzmann constant, T is the temperature of samples, e is the net free energy required to excited one Er3+ from the 4I15/2 state to 4I13/2 state at temperature T. e was determined using the procedure provided in Ref. [21]. sabs ðlÞ is the absorption cross section at give wavelength, and can be calculated by formula (3) sabs ðlÞ ¼

Fig. 4. Upconversion spectra of samples with various content of Al(PO3)3 under 970 nm excitation.

2:303 ODðlÞ, Nl

(3)

in formula (3), N is the concentration of Er3+ ions; l is optical length; ODðlÞ is optical density. The stimulatedemission cross sections at peak wavelength 1528 nm of all samples are listed in Table 2. With the increment of Al(PO3)3 content, stimulated-emission cross sections

Table 2 The measured lifetime tmea, the stimulated-emission cross section semi, the full width at half maximum FWHM, semi  tmea and semi  FWHM of Er3+ 4I13/2-4I15/2 Glasses

tmea (ms)

semi (  1021 cm2)

FWHM (nm)

semi  tmea

semi  FWHM

FP3 FP5 FP7 FP9 FP11 FP5Na4 FP5Na12 FP5Na20 FP5Pb2 FP5Pb6 FP5Pb14 Telluritea Germanateb Silicatec

6.88 6.94 7.92 8.49 6.82 6.96 7.72 8.47 7.57 6.72 7.53

8.40 8.50 8.56 8.90 8.93 8.82 7.93 7.18 8.53 8.76 9.02 7.50 5.68 5.50

63 62 59 59 67 62 61 57 61 70 62 65 53 40

57.8 59.0 67.8 75.6 60.9 61.4 61.2 60.8 64.5 58.8 67.9

529.5 527.2 504.9 525.0 598.4 547.0 483.8 409.4 520.0 612.9 559.2 487.5 301.0 220.0

a

Ref. [25]. Ref. [26]. c Ref. [25]. b

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increase, since they increase with the increment of refractive index n (semi / ðn2 þ 2Þ2 =n) [22,23]. 4. Discussion Multiphonon relaxation is an important impact factor to the upconversion properties of glasses. When referring to the multiphonon relaxation, generally researchers focused on the influence of maximum phonon energy. However, the maximum phonon energy remains constant for the glasses prepared by us. As a matter of fact, multiphonon relaxation rate can be expressed by formula (1) [24]:   expð_op =kTÞ p aDE Wp ¼ C e , (4) expð_op =kTÞ  1 in formula (4), _op is the maximum phonon energy; p ¼ DE=_op , DE is the energy gag between the upper and lower energy level; a ¼  ln =ðÞ=_op , e accounts for the exact nature of the ion–phonon coupling and is insensitive as the log dependence in the formula; k is the Boltzmann constant, T is the temperature of samples; op is the multiphonon relaxation rate. The parameter C, generally considered as constant, is positive relative to the phonon density of states of the glass. C1rðoÞ5=3 , where rðoÞ is the phonon density of states. It can be found from formula (4) that with the increase of phonon density of states, the multiphonon relaxation rate rises. For these glasses, the variation tendency of phonon density of states with the increment of Al(PO3)3 content can be indicated by the intensity variation of peak e in Raman spectra. In Figs. 1 and 2, in order to compare the intensity of peak e, all the graphs are normalized according to the intensity of peak b, since the peak b is ascribed to [AlF4] vibration and is insensitive to the phosphate content. It can be found from the inset in Fig. 1 that the phonon density of states increases with the increase in phosphate content. In Fig. 2 phonon density of states of FP5Na20 is higher than that of FP5Na12, and phonon density of states of FP5Pb14 is higher than that of FP5Pb6. As a result, the dependence of the upconversion-luminescence intensity on the composition is contrary to that of the phonon density of states on the composition. It is important to mention that the upconversion luminescence of FP3 is intense enough to be seen by the naked eye at excitation power as low as 100 mW. Generally fluorophosphate glasses are not thought to be the suitable matrix for upconversion luminescence because of their high phonon energy. However, for the fluorophosphate glasses prepared by us, the maximum phonon density is comparative low, and intense upconversion luminescence can still be obtained. Till now the most popular matrix for upconversion luminescence is tellurite or fluorozirconate glasses such as ZBLAN (ZrF4–BaF2–LaF3–AlF3–NaF). However, the phonon energy of tellurite glasses is not low enough for effective upconvision. Fluorozirconate glasses must be melted under a reactive atmosphere condition in order to

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avoid metallic zirconium deposition on surface of the glass, but the reactive atmospheric conditioned manufacturing processes are very complex and expensive. Further more, the raw materials of tellurite and fluorozirconate glasses are expensive. Here we present a fluorophosphate glass with low content of phosphate as a matrix candidate, the upcoversion luminescence is intense, and no expensive raw material or special atmospheric conditioned preparation is required. Multiphonon relaxation does no harm the infrared emission around 1530 nm, since the energy gap is 6545 cm1, and is six times of the phonon energy. For these series fluorophosphate glasses under 970 nm excitation, high phonon density is more preferable because it contributes to the relaxation of 4I11/2-4I13/2 and holds back the upconversion. In Table 2, semi  tmea and semi  FWHM are used to valuate the gain properties and bandwidth properties respectively. For the samples with various content of phosphate, gain properties of FP7 and FP9 are better. Though the introduction of NaF increases the stability against crystallization, it decreases the bandwidth properties. The introduction of PbF2 contributes to the gain properties. On the whole, it is difficult to obtain a material with the highest stability, the best gain properties and bandwidth properties. There should be a compromise between these properties according to the demand. For the sample of FP5Pb6 and FP11, the bandwidth properties and the stimulated-emission cross sections are better than tellurite glasses [25], germanate [26] and silicate glasses [25]. 5. Conclusions Er3+/Yb3+ codoped fluorophosphate glasses were prepared and their thermal stability and spectroscopic properties were studied. Refractive index increases with the increment of Al(PO3)3 or PbF2 content, and decreases with the increment of NaF content. Through the introduction of NaF or PbF2 with proper content, the stability against crystallization is increased. The variation of Al(PO3)3 or NaF content in the composition affects not the maximum phonon energy but the phonon density. The introduction of PbF2 decreases the phonon energy slightly. Intense green and red upconversion luminescence was observed for the fluorophosphate glass with low phosphate content. A glass matrix for upconversion luminescence requiring neither expensive raw material nor special atmospheric conditioned preparation is provided. Infrared luminescence around 1530 nm was studies. Fluorophosphate glasses with the bandwidth properties and the stimulated-emission cross sections better than tellurite glasses, germanate and silicate glasses are obtained. Through the introduction of NaF, the bandwidth properties are decreased. Through the introduction of PbF2 the gain properties are increased. This work might provide helpful references for the development of upcoversion luminescence and fiber amplification.

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