Photoluminescence study of Er-doped zinc–sodium–antimonite glasses

Journal of Alloys and Compounds 611 (2014) 111–116

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Photoluminescence study of Er-doped zinc–sodium–antimonite glasses J. Zavadil a,⇑, Z.G. Ivanova b, P. Kostka c, M. Hamzaoui d, M.T. Soltani d a

Institute of Photonics and Electronics AS CR, Prague, Czech Republic Institute of Solid State Physics Bulgarian Academy of Sciences, Sofia, Bulgaria c Institute of Rock Structure and Mechanics AS CR, Prague, Czech Republic d Laboratoire de Physique Photonique et Nanomatériaux, Universite de Biskra, Algeria b

a r t i c l e

i n f o

Article history: Received 3 April 2014 Received in revised form 13 May 2014 Accepted 14 May 2014 Available online 23 May 2014 Keywords: Antimonite glasses Rare-earth ions Erbium Photoluminescence Stark levels

a b s t r a c t Bulk samples of Er-doped zinc–sodium–antimonite glasses have been investigated by transmission and photoluminescence (PL) spectroscopy. Two series of compositions, (Sb2O3)90 x(Na2O)10(ZnO)x and (Sb2O3)80 x(Na2O)20(ZnO)x, doped with 0.25 mol% Er2O3, have been chosen for this study. Transmission spectra exhibit sharp absorption bands centred at 450, 489, 521, 545, 652, 795, 975 and 1530 nm, which correspond to absorption of Er3+ ions and they are attributed to the optical transitions from the ground state 4I15/2 to the excited states 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2, respectively. The optical gap has been found to vary from 3.09 to 3.15 eV with a tendency to decrease at higher Na2O and/or ZnO contents. Four extrinsic bands due to OH , Si–O, CO2, and (CO3)2 carbonate group vibrations have been identified in the infrared region. Emission spectra are overwhelmed by narrow 4f–4f emission bands. Fine structure of emission bands at 980 and 1530 nm, corresponding to radiative transitions from two lowest excited states of Er3+ ions to the ground state manifold have been investigated at room temperature and at 4 K. A schematic energy diagram of Stark levels splitting for the three lowest manifolds 4I11/2, 4I13/2 and 4 I15/2 has been deduced and the nature of temperature broadening of 4f–4f PL bands has been discussed. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Heavy metal oxide glasses have been the subject of numerous studies due to their interesting physical, thermal and optical properties. They possess high refractive index, optical transmission reaching from visible to middle infrared region [1–4], low phonon energies, high solubility of rare earth ions (RE3+) and non-linear optical properties [5,6] that make them promising materials for optical devices, such as ultrafast optical switches, power limiters and broad band optical amplifiers [7,8]. Among them, antimonite glasses attract an increasing interest because of their interesting properties including stability against devitrification and lower characteristic temperatures making glass processing easier. Infrared transmission is enhanced while refractive index keeps large values [2,3,8–10]. The glass forming ability of antimony oxide Sb2O3 has been predicted by Zachariasen [11] and confirmed in various oxide [12], halide or sulfide [13] systems. This compound participates in the glass network with SbO3 structural units in form of trigonal ⇑ Corresponding author. Address: Institute of Photonics and Electronics AS CR, Chaberska 57, 182 51 Praha 8, Czech Republic. Tel.: +420 266 773 436; fax: +420 284 680 222. E-mail address: [email protected] (J. Zavadil). http://dx.doi.org/10.1016/j.jallcom.2014.05.102 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

pyramids with the oxygen situated at three corners and the lone pair of electrons of antimony at the fourth corner. The presence of this pair could enhance nonlinear optical susceptibility in the antimonite glasses, described by third rank polar tensors [14]. Antimony may also exist in fifth oxidation state, participating in the formation of glass network with SbO4 structural units. Alkali antimonite glasses have been the subject of several studies in the binary Sb2O3–A2O or in the ternary Sb2O3–A2O–MmOn glass systems, where A = Li, Na, K, or Cs and M = Pb or Al [15–17]. In these studies the emphasis has been put on the glass formation and some basic properties such as their high thermal stability, the extended optical window in the infrared and thermal expansion of glasses in question. There are many reports on erbium doped glasses containing Sb2O3 as the second glass former in antimony–borate glasses [18,19], antimony–silicate glasses [20], or antimony–phosphate glasses [21]. In a previous paper, we reported the radiative and spectroscopic properties of Er3+ doped (Sb2O3)70(Na2O)20(ZnO)10 glasses by using the Judd–Offelt analysis [22]. We investigated the effect of Er3+ doping level and examined the potential of these glasses as optical glasses for laser and optical amplifiers. In this work we investigate the influence of host composition, for constant Er2O3 doping concentration, on the transmission and photoluminescence (PL) properties. In addition, the Stark level splitting of the three lowest

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manifolds of Er3+ ions has been deduced from comparison of PL spectra measured at room temperature and at 4 K.

100

4

4

3. Results Prepared glasses exhibit a broad transparency range with short wavelength absorption edge located at around 420 nm, and the long wavelength one placed at about 6.5 lm. The position of the short wavelength absorption edge depends slightly on the glass composition – with increasing Na and/or Zn concentration it shifts towards longer wavelengths. Typical transmission spectra for the 20Na2O series are presented in Fig. 1. The shifts of the absorption edge due to changes in initial glass composition for both investigated series (10Na2O and 20Na2O) are shown in Fig. 2. A modest shift of the absorption edge to longer wavelength could be seen with increasing Na and/or Zn concentration. In view of the fact that the fundamental absorption edge is at about 420 nm, we could not observe transitions to manifolds higher than 4F5/2. Eight absorption

100

4

4

F5/2

H11/2 F7/2 4S

4

4

F9/2

4

Transmittance (%)

Compositions of (Sb2O3)90 x(Na2O)10(ZnO)x, hereinafter referred as 10Na2O series, and of (Sb2O3)80 x(Na2O)20(ZnO)x, below referred as 20Na2O series, where x = 5, 10, 15, 20, were selected for this study. All studied glass samples were doped with 0.25 mol% Er2O3. Bulk glasses were prepared using the conventional melt– quenching method from starting compounds Sb2O3 (Acros, 99%), ZnO (Aldrich, 99%), Na2CO3 (Aldrich, 99.95%), and Er2O3 (Acros, 99.99%), melting was carried out in open quartz glass tubes under air atmosphere. The glass synthesis is described in more detail in [22]. Starting sodium carbonate was thermally decomposed during glass preparation to Na2O and gaseous CO2, which leaves to atmosphere. The amorphous nature of prepared samples was checked by X-ray diffraction (XRD), using a Philips PW3020 diffractometer with Cu Ka radiation. Semiquantitative analysis of prepared samples was performed using Energydispersive X-ray spectroscopy (EDS) detector (EDAX Apollo X) coupled with scanning electron microscope (Quanta 450). Transmission spectra were measured by using Specord 210 Analytic Jena and Nicolet 6700 FTIR spectrometers in the VIS and IR regions, respectively. Photoluminescence spectra were measured at 4 and 300 K by using an optical He closed cycle cryostat. The 1 m focal length monochromator coupled with GaAs photomultiplier and/or a cooled high purity Ge detection system enables sensitive and high resolution measurement in the spectral range of 400–1800 nm by using the lock-in technique and computer controlled data collection. Typical spectral resolution in reported experiments was in the range of 0.04–0.08 nm and 30 measurements were typically collected for each wavelength of the spectrum. The Ar ion laser (514.5 nm line) was used for excitation. Transmission and photoluminescence spectra were obtained by measuring polished samples whose thickness was 1.19 ± 0.06 mm for 10Na2O series, and 1.18 ± 0.02 mm for samples from the 20Na2O series.

I11/2

I9/2

F9/2

3/2

20Na2O series 70Sb2O3 20Na2O 10ZnO 65Sb2O3 20Na2O 15ZnO

40

10Na2O series 85Sb2O3 10Na2O 5ZnO 80Sb2O3 10Na2O 10ZnO

20

0 400

500

600

700

800

900

1000

Wavelength (nm) Fig. 2. Transmission spectra of two samples from 10Na2O and from 20Na2O series each, are shown. A small shift of the absorption edge due to changing glass composition could be seen.

bands corresponding to transitions from the ground state 4I15/2 to excited manifolds of Er3+ ions have been found, within the range of transparency of studied glasses. Seven bands falling into the range of 400–1100 nm could be seen in Figs. 1 and 2. The absorption band corresponding to transition 4I15/2 ? 4I13/2 at 1530 nm can be seen in Fig. 3, where typical infrared transmission spectra are shown. Infrared transmission spectra of studied 10Na2O and 20Na2O series are found to be similar. Four noticeable absorption bands, besides Er3+ related transition 4I15/2 ? 4I13/2 at 1530 nm, could be seen in Fig. 3. Two strong absorption bands at 3 lm (3260 cm 1) and 5.5 lm (1800 cm 1), together with two minor ones at 2.3 lm (4348 cm 1) and 4.2 lm (2380 cm 1) are found. The broad bands at 3 lm and 5.5 lm correspond to OH groups and Si–O bonds, respectively [23]. The weak band at about 4.2 lm is due to fluctuations in concentration of atmospheric CO2 during measurements [23]. A very weak band at 2.3 lm marked by arrow in Fig. 3, is probably due to Si–OH vibrations [24] with possible contribution from (CO3)2 carbonate groups [25]. Contamination by Si comes from the crucible used for preparation of the glass. We have also estimated the values of optical gap (Eg) from calculated absorption coefficients. Absorption coefficient was

4

I9/2

I11/2

80

3/2

(80-x)Sb2O3 20Na2O xZnO:Er2O3 4

75Sb2O3 20Na2O 5ZnO

60

(80-x)Sb2O3 20Na2O xZnO:Er2O3 40

75Sb2O3 20Na2O 5ZnO 70Sb2O3 20Na2O 10ZnO 65Sb2O3 20Na2O 15ZnO 60Sb2O3 20Na2O 20ZnO

Transmittance (%)

I13/2

20

4

4

4

60

80

Transmittance (%)

F5/2

80

2. Experimental

H11/2 F7/2 4S

4

60

CO3

CO2

60Sb2O3 20Na2O 20ZnO

40

OH band 20

Si-O

0

0 400

500

600

700

800

900

1000

Wavelength (nm) Fig. 1. Transmission spectra of (Sb2O3)80 x(Na2O)20(ZnO)x glasses doped with 0.25 mol% of Er2O3 are shown for x = 5, 10, 15 and 20. The assignment of curves to samples is indicated in the inset. Narrow absorption bands due to 4f–4f uptransitions from the ground state of Er3+ ions are also indicated.

2

4

6

8

10

11

12

Wavelength (micrometers) Fig. 3. Typical infrared spectra for two samples from 20Na2O series are shown. Absorption band due to 4I15/2 ? 4I13/2 transition in Er3+ ions is shown together with four extrinsic bands. The assignment of impurities to respective absorption bands is indicated.

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PL intensity (arb. u.)

60Sb2O3 20Na2O 20ZnO:Er2O3

8

T=4 K 6

4

0 500

(b)

(b) 70Sb2O3 20Na2O 10ZnO:Er2O3

(a)

40 30

600

4

S3/2- I13/2

F9/2- I15/2 700

800

900

Wavelength (nm) Fig. 5. Low temperature PL spectrum of Er doped (Sb2O3)60(Na2O)20(ZnO)20 glass, measured in the range 500–900 nm, is shown. Three narrow bands due to 4f–4f radiative transitions within Er3+ ions are indicated in the figure.

8 4

6

4

I13/2- I15/2

60Sb2O3 20Na2O 20ZnO:Er2O3

T=4 K 4

4

I11/2- I15/2

4

4

4

2 S 3/2- I13/2 4

4

S 3/2- I11/2

0 800

1000

1200

1400

1600

Wavelength (nm) Fig. 6. Low temperature PL spectrum of Er doped (Sb2O3)60(Na2O)20(ZnO)20 glass, measured in the range 800–1700 nm, is shown. Four narrow bands due to 4f–4f radiative transitions within Er3+ ions are indicated in the figure.

10

(a) 80Sb2O3 10Na2O 10ZnO:Er2O3

4

4

4

PL intensity (arb. u.)

-1

Absorption coefficient (cm )

50

S3/2- I15/2

2

70 60

4

4

10

PL intensity (arb. u.)

evaluated from measured transmittance by using approximate relation a(k) = d 1  ln (T(k) 1), where T(k) is measured transmittance as a function of wavelength k, and d is the sample thickness. The approximate formula for a(k) given above neglects the manifold reflections but here it is used only for the estimation of trends in changes of optical band gap as a function of host composition. Absorption coefficients for typical samples from 10Na2O and 20Na2O series are plotted as a function of energy in Fig. 4. Tauc’s method [26] could not be used for the evaluation of the optical band gap since the studied bulk glasses were too thick for the evaluation of absorption coefficient in the region of a(k) P 104 cm 1. Thus, we estimated Eg values from the intersection of the extrapolated straight part of the a(k) dependence with the abscissa (energy axis). It follows from inspection of obtained results that the gap (approximately 3.1 eV) decreases with increasing Na and/or Zn concentration. Low-temperature PL (LTPL) spectra of studied glasses have been studied in more detail. A broad band luminescence of the host glass, usually centred at about the mid-gap energy [9,13,27–29], has not been observed on studied samples even at low temperature. PL spectra are overwhelmed by 4f–4f inner shell transitions in Er3+ ions, both at room temperature and at 4 K. Typical LTPL spectra in visible and IR range are shown in Figs. 5 and 6. All emission bands due to 4f–4f transitions that could be excited by 514.5 nm line have been found. They are located at 550, 670, 850, 980, 1230 and 1530 nm, respectively. Emission bands at 980 and 1530 nm, corresponding to radiative transitions from the two lowest excited manifolds (4I11/2 and 4I13/2) to the ground state manifold 4I15/2, have been examined in more detail. Both these PL bands bear the same ‘‘finger print’’ of the Stark levels splitting of the ground state 4I15/2. PL spectra of these two emission bands have been measured at 4 K and at 300 K in order to deduce the Stark levels energy diagram, corresponding to the three manifolds involved, from the observed fine structure. The two investigated series of samples exhibit qualitatively similar temperature dependence and fine structure due to Stark level splitting. The spectra differ only by relative strengths of individual transitions contributing to the observed fine structure. In view of the fact that the Stark level splitting manifested as a fine structure of PL bands is composition independent, we will further describe only spectra of 20Na2O series. PL bands at 980 (4I11/2 ? 4I15/2 transition) and at 1530 nm (4I13/2 ? 4I15/2 transition), measured at 4 (curves (a), (b)) and at 300 K (curves (c) and

8

(80-x)Sb2O3 20Na2O xZnO:Er2O3 4

T=300 K

T=4 K

4

I11/2- I15/2

(a) x=5

(c) x=5

(b) x=20

(d) x=20 (b)

6

(a) 4

(d)

2

20

H11/2

4

I11/2

10

4

2 4

F9/2 4S 3/2

0 940

0 1,0

1,5

2,0

(c)

F7/2

2,5

3,0

3,5

960

980

1000

1020

1040

Wavelength (nm)

Energy (eV) Fig. 4. Absorption coefficients for two samples, one from each composition series, with equal ZnO content (x = 10) are shown. The assignment of curves to samples is indicated in the inset. A small shift of absorption edge towards lower energy with increasing Na2O content is seen.

Fig. 7. A fine structure of the emission band at 980 nm due to 4I11/2 ? 4I15/2 transitions of Er3+ ions is demonstrated for two samples as measured at 4 and 300 K. The curves (a) and (c) correspond to (Sb2O3)75(Na2O)20(ZnO)5 host and curves (b) and (d) correspond to (Sb2O3)60(Na2O)20(ZnO)20 host. Spectra measured at 4 K are labeled (a) and (b) while spectra measured at 300 K are labeled (c) and (d).

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(80-x)Sb2O3 20Na2O xZnO:Er2O3

PL intensity (arb. u.)

8 4

T=300 K (c) x=5 6

4

I13/2- I15/2 (a)

(d) x=20

T=4 K (a) x=5 (b) x=20

(b) 4

(c) 2

(d) 0 1440

1480

1520

1560

1600

1640

Wavelength (nm) Fig. 8. A fine structure of the emission band at 1530 nm due to 4I13/2 ? 4I15/2 transitions of Er3+ ions is demonstrated for two samples as measured at 4 and 300 K. The curves (a) and (c) correspond to (Sb2O3)75(Na2O)20(ZnO)5 host and curves (b), (d) correspond to (Sb2O3)60(Na2O)20(ZnO)20 host. Spectra measured at 4 K are labeled (a) and (b) while spectra measured at 300 K are labeled (c) and (d).

(d)), are shown in Figs. 7 and 8, respectively. For better clarity of figures, we are plotting only spectra for minimum and maximum ZnO content, i.e. 5 and 20 mol% ZnO. The curves (a) and (c) correspond to the (Sb2O3)75(Na2O)20(ZnO)5 host, and curves (b), (d) correspond to the (Sb2O3)60(Na2O)20(ZnO)20 host. It is clearly seen from Figs. 7 and 8 that lowering of temperature leads to considerable narrowing of 4f–4f related PL bands. The fine structure due to Stark levels splitting is better resolved at 4 K. The

4

4

I13/2- I15/2 60Sb2O3 20Na2O 20ZnO

Intensity (arb.u)

T=300 K

(b)

T=4 K

Measured Fitted

(a) 1450

1500

1550

1600

1650

Wavelength (nm) Fig. 9. Er3+ related PL band at 1530 nm for the (Sb2O3)60(Na2O)20(ZnO)20 glassy host measured at 4 and 300 K is shown in parts (a) and (b), respectively. The deconvolution into Gaussians that better elucidates fine structure of the band is demonstrated.

resolution is also slightly improved with increasing zinc concentration and lower content of sodium in the glass. It should be noted that broadening of emission bands takes place primarily in the high energy part of PL spectra, and is caused by thermal excitation of electrons within 4I11/2 and 4I13/2 manifolds. The de-convolution of measured PL bands into Gaussians enables the conversion of the fine structure of PL bands into schematic Stark levels energy diagram for three investigated Er3+ manifolds. The de-convolution of the PL band at 1530 nm for (Sb2O3)60(Na2O)20(ZnO)20 host, measured at 4 and 300 K, is shown in Fig. 9. The de-convoluted spectra in Fig. 9 correspond to curves (b) and (d) presented in Fig. 8. A schematic energy diagram of Stark levels with indicated electronic transitions, for the three lowest Er3+ manifolds in studied glasses, is shown in Fig. 10. Energies of all eight sublevels [30] of the ground state manifold 4I15/2 have been specified, and participation of thermally excited energy levels in upper manifolds 4I13/2 and 4I11/2 indicated. Radiative transitions from 4I13/2 and 4I11/2 manifolds are marked as F21, F31, F41 and G21, G31, G41, G51, respectively. Labels i and j in transitions Fij and Gij stand for the i-th and j-th energy level of the upper and lower manifolds, respectively. The energies assigned to respective Stark levels are indicated on the right-hand-side of Fig. 10. It is demonstrated that only sublevels (in manifolds 4I13/2 and 4I11/2) within the range of electronic thermal energy 208 cm 1 participate in observed radiative transitions.

4. Discussion The short wavelength absorption edge of studied glasses is located at about 420 nm and exhibits little dependence on glass composition. A modest shift to longer wavelength has been found with increasing Na and/or Zn concentration, represented by the shift of the optical gap. All relevant absorption bands of Er3+ ions that fall into the investigated spectral range have been found. IR transmission spectra exhibit absorption bands due to impurities. Two strong absorption bands at about 3 lm (3260 cm 1), and 5.5 lm (1800 cm 1) and two weak ones at 2.3 lm (4348 cm 1) and 4.2 lm (2380 cm 1) have been found in the spectral range of 2–6 lm. Strong and relatively broad bands at 3 lm and 5.5 lm are interpreted as due to the presence of OH- groups and Si–O bonds, respectively [23]. The presence of Si, manifested by Si–O related absorption band, was verified by semi-quantitative analysis of prepared samples using the EDS analysis. It was found that although the ratio of metal elements constituting the glass components does not change significantly, contamination of the glass melt by silicon (SiO2) from crucible material occurs. The determined concentration of Si reaches up to 7 cat% of Si in investigated glasses. A weak band at 4.2 lm is related to fluctuations of atmospheric CO2 concentration in optical path in spectrometer, and very weak band at 2.3 lm is probably due to Si–OH vibrations [24] and residual (CO3)2 carbonate groups [25]. All these bands are extrinsic, i.e. their intensity may be reduced or they can be completely removed by achieving and maintaining higher purity during glass synthesis [31]. Impurities present in the glass may influence its properties, among others also lifetimes and quantum efficiencies of 4f–4f radiative transitions of RE ions [32], but we do not expect their significant effect on the described characteristics (absorption/ emission bands positions and intensities). The long wavelength absorption edge is situated at about 6.5 lm for all studied glass compositions and its position is almost independent of the composition. It follows from LTPL spectroscopy that Sb2O3–Na2O–ZnO glass systems does not exhibit a detectable broad band luminescence

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I11/2

10400

10383 (+164) 10344 (+129)

10300

10304 (+83) 10272 (+51) 10221 (0)

10200

4

6800

I13/2

6744 (+222)

-1 Energy (cm )

6700 6651 (+129)

6600 6562 (+40) 6532 (0)

F18

6500

F17 G18

F15 F14

F41

F13

400 F11

F21

G41

G15

F31

F12

G51

G17 G31

G13 G11

G21

4

I15/2

290-305

300

240-245 180-200

200

115-119

100

66-72 47-52 23-35 0

0

Fig. 10. A schematic energy diagram showing the Stark components of 4I15/2, 4I13/2 and 4I11/2 manifolds is presented together with indicated radiative transitions responsible for the observed fine structure. The energies assigned to respective Stark levels are indicated on the right-hand-side.

mediated by deep energy states in the band-gap of the host. PL spectra are dominated by narrow radiative transitions within 4f shell of embedded Er3+ ions that could be detected at both room and low temperature. All emission bands due to 4f-4f transitions (found at 550, 670, 850, 980, 1230 and 1530 nm) that could be excited by 514.5 nm line, have been found. It means that corresponding transitions are supported by the investigated glass series in Sb2O3–Na2O–ZnO system. Emission bands at 980 and 1530 nm that correspond to radiative transitions from the two lowest excited manifolds (4I11/2 and 4 I13/2) to the ground state manifold 4I15/2 have been investigated at 4 K and at room temperature in order to deduce energies of corresponding Stark levels. The observed broadening of PL bands due to elevated temperature (when going from 4 to 300 K) takes place primarily in the high energy part of PL spectra, and is caused by thermal excitation of electrons within 4I11/2 and 4I13/2 manifolds. The broadening at room temperature correlates well with electronic thermal energy that could be estimated as kT  208 cm 1, where k is the Boltzman constant. In view of the fact that Er3+ ions are characterized by an odd number of 4f electrons, the number of Stark levels in a given manifold is always the same (8 for 4I15/2 manifold), irrespective of structural conditions and the symmetry of the host [30]. Thus the fine structure of 980 and 1530 nm emission bands measured at low temperature enables to identify all 8 Stark levels characterizing the 4I15/2 manifold of Er3+ ions. Measurements at room temperature further enable to identify

contributions due to radiative transitions from thermally excited Stark levels of higher manifolds (4I11/2 and 4I13/2). A schematic energy diagram for Stark levels splitting of the three lowest Er3+ manifolds in studied glasses has been deduced from measured fine structure of PL bands at 980 and 1530 nm. Besides a quantitative estimation of energy level splitting, the diagram also reveals the nature of the broadening of 4f–4f PL bands at elevated temperature. As stated above, the electronic thermal energy is estimated as 208 cm 1 for the room temperature. Thus only three or four thermally excited sublevels of 4I13/2 and 4I11/2 manifolds, respectively, participate in the room temperature PL emission.

5. Conclusions Antimonite glasses in two series of compositions (Sb2O3)90 x (Na2O)10(ZnO)x, and (Sb2O3)80 x(Na2O)20(ZnO)x, where x = 5, 10, 15, 20 doped with 0.25 mol% Er2O3 have been prepared and characterized by EDS, transmission and PL spectroscopy. Glasses exhibit relatively broad transmission range. The VIS and IR absorption edges have been found at 420 nm and 6.5 m, respectively. All relevant absorption bands of Er3+ ions in whole transmission range have been observed. The transmission in infrared region is impaired by four extrinsic bands due to OH , Si–O, CO2, and (CO3)2 carbonate group vibrations.

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It has been shown by using LTPL spectroscopy that studied Sb2O3–Na2O–ZnO glass system does not exhibit a detectable broad band luminescence of the host mediated by deep energy states in the band-gap. PL spectra are dominated by narrow radiative 4f–4f transitions within Er3+ ions that have been observed both at room and low temperature. It has been shown that studied host glass supports all Er3+ related radiative transitions, located at 550, 670, 850, 980, 1230 and 1530 nm, that fall into investigated spectral range. The attention has been concentrated on the fine structure of two PL bands at 980 and 1530 nm that represent transitions from the two lowest excited states 4I11/2 and 4I13/2 to the ground state manifold 4I15/2. A detailed measurement of these PL bands at 4 K and 300 K, followed by the de-convolution of spectra into Gaussians, enabled to deduce the schematic energy diagram of Stark levels splitting for the three lowest manifolds of Er3+ ions. Such a procedure enables to assign energies to Stark levels of individual manifolds and reveals the nature of temperature related broadening of PL bands when temperature is elevated from 4 to 300 K. Acknowledgment The work was supported by the Czech Science Foundation, Project No. P106/12/2384. References [1] M. Poulain, Ann. Chim. Sci. Mater. 28 (2003) 87. [2] M. Nalin, M. Poulain, M. Poulain, J.L. Ribeiro Sidney, M. Messaddeq, J. NonCryst. Solids 284 (2001) 110. [3] I.V. Kityk, J. Phys. Chem. B 107 (2003) 10083. [4] J.C. Sabadel, P. Armand, D. Cachau-Herreillat, P. Baldeck, O. Doclot, A. Ibanez, E. Philippot, Solid State Chem. 132 (1997) 411. [5] W.H. Dumbaugh, Phys. Chem. Glasses 19 (1978) 121.

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