Spectroscopic properties of Er3+ and Yb3+ doped phosphate–borate glasses

Journal of Molecular Structure 1010 (2012) 85–90

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Spectroscopic properties of Er3+ and Yb3+ doped phosphate–borate glasses N. Sdiri a,⇑, H. Elhouichet a,b,⇑, C. Barthou c, M. Ferid a a

Laboratoire de physico-chimie des matériaux minéraux et leurs applications, Centre National de Recherches en Sciences des Matériaux, B.P. 95, Hammam-Lif 2050, Tunisia Département de Physique, Faculté des Sciences de Tunis, Campus ElManar 2092, Tunisia c Institut des Nanosciences de Paris, Université P. et M. Curie, Centre Nationale de la Recherche Scientifique, UMR-7588, 4 place Jussieu, Boîte Courrier 840, 75252 Paris Cedex 05, France b

a r t i c l e

i n f o

Article history: Received 11 October 2011 Received in revised form 21 November 2011 Accepted 22 November 2011 Available online 1 December 2011 Keywords: Er3+-doped and Er3+ Yb3+-codoped in phosphate–borate glass Judd–Ofelt analysis Photoluminescence Luminescence lifetime

a b s t r a c t In this work, Judd–Ofelt analysis is applied to an extensive series of Er3+-doped and Er3+, Yb3+-codoped in phosphate–borate glass in order to evaluate their potential as both glass laser systems and amplifier materials. A spectroscopic investigation is presented. The phenomenological Judd–Ofelt parameters X2, X4 and X6 are determined for both rare-earth ions together with their quality factors and compared to the equivalent parameters for other host glasses. The absorption cross section for the 4 I13=2 ! 4 I15=2 transition is determined. Photoluminescence (PL) and its decay behaviour studies were carried out for the transition 4 I13=2 ! 4 I15=2 . Published by Elsevier B.V.

1. Introduction Rare earths (REs) doped glasses have attracted much attention due to their potential technological applications. In the case of Er3+ ions the optically excited luminescence originating from the dipole-forbidden 4 I13=2 ! 4 I15=2 transition has a wavelength of 1.54 lm that matches one of the minimum loss windows of commercial silica-based optics fibres. In the construction of integrated light amplifiers it is desirable to obtain the maximum gain within small component dimensions. Borate glass is a particularly suitable optical material because of its high transparency, low melting point, high thermal stability, different coordination numbers, and good solubility of rare-earth ions [1,2]. Phosphate glasses exhibit very important physical properties such as low melting temperature, high thermal expansion coefficient, low glass transition temperature Tg, low softening temperature, low viscosity and high ultraviolet (UV) transmission [3,4]. Phosphates glasses with various compositions are of exceptional importance due to their interesting linear and nonlinear optical properties [5–7]. Another important feature of these glasses is their ability to incorporate large amounts of transition metal without reduction of glass forming ability. The incorporation of transition metals in phosphate glasses is known to improve several phys-

⇑ Corresponding authors. Address: Faculté des sciences de Monastir, Monastir 5000, Tunisia. Tel.: +216 97694850 (N. Sdiri). E-mail address: [email protected] (N. Sdiri). 0022-2860/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.molstruc.2011.11.036

ico-chemical properties like chemical resistance against atmospheric moisture [8]. When doped with Yb3+, phosphate glasses it could be used for ultrahigh power laser systems, presenting superior energy storage capability, a gain bandwidth able to support amplification of femtosecond pulses. It also presents a flat absorption spectrum around 940 nm that makes it suitable for diode pumping [9]. However, the relatively poor chemical durability of the phosphate glasses often limits their usefulness. Recent studies show that the chemical durability can be much improved by properly adding small amounts of one or more oxides, such as Al2O3, B2O3, PbO, SnO, ZnO and Fe2O3, to the phosphate glasses [10]. On other hand phosphate glasses are regarded as better hosts for Er3+ ions compared to silicate glasses, because of their higher phonon energy, more solubility of RE3+ ions and smaller upconversion coefficient of the 4 I13=2 level [11]. In addition the optical pump excites the Er3+ ions indirectly, via energy transfer from Yb3+ ions. In contrast to Er3+, the Yb3+ ion offers a broad absorption band from 800 to 1100 nm, with a particularly high peak absorption cross section [12,13]. The purpose of this paper is to introduce the graded a new phosphate–borate glasses by including Er and Er–Yb in different amounts. Many potential host materials for rare earth ions have been developed. However, there are few reports describing the relationship between the spectroscopic properties and the spectroscopic factor (X4/X6). In this work, the effects of spectroscopic factor modifiers on spectroscopic properties of Er3+-doped and Er3+, Yb3+-codoped with a small amounts, in a new phosphate–borate

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glass were studied. On the other hand, in this paper we have shown stretched exponential (SE) decays in our glasses.

980 nm light from a diode laser, detected by an InGaAs detector and photomultiplier tube.

2. Experimental procedure

3. Results and discussion

Phosphate–borate glasses were prepared according to the following compositions: 85P2O5–10B2O3–(5  y)Na2O–yEr2O3 (y = 0.05, 0.1, 0.15 mol%) for doped samples (PBNE1, PBNE2, PBNE3 samples, respectively), 85P2O5 + 10B2O3 + (4.9  z)Na2O + 0.1Er2O3 + zYb2O3 (z = 0.05, 0.1 mol%) for co-doped samples (PBNEY1 and PBNEY2 samples, respectively). For each batch, about 4 g raw materials were fully mixed and then melted at 1000 °C over 1 h in high-temperature furnace. After heated, the melt was cast at home temperature into a preheated (at 300 °C) graphite mould to from glass. The quenched samples were annealed at around 650 °C in muffle furnace for 30 min. The obtained glasses were cut into pellet (of thickness 2.5 mm and diameter 2 cm), and then optically polished for spectra measurements. Glass samples with two faces polished were used in the measurements of the refractive index, transmittance, absorption, fluorescence emission and lifetime at room temperature. The refractive index was determined by means of the Brewster angle measurement [14]. The densities (qg) of samples were measured at room temperature using Archimedes principle with acetone as buoyant liquid. A cylinder shape glass sample was weighed in air (Wair) using electronic balancer (±0.0001 g) manufactured by Melter Toledo, Wac the glass sample weight in buoyant and qac the acetone density of the buoyant. A glass disc was weighed in air (Wair) and immersed in acetone an reweighed (Wac). The acetone density using here is 0.789 g cm3. The relative density is given by following the relation [14]:

3.1. Structural analysis

qg ¼ qac

W air W ac

ð1Þ

The refractive index is measured using the Brewster’s angle method with He–Ne laser at 25 °C. The concentration of the rare-earth ions is an important parameter, which affects the laser gain of the host material. The number density N of the laser-active ions i.e. the number of ions per cubic centimetre can be evaluated using the relation [15]: 3

N ðions=cm Þ ¼

yqg NA

ð2Þ

M

where qg is the density of the glass, NA is the Avogadro’s number; y is the mole fraction of rare earth oxide and M is the average molecular weight of the glass. The basic characteristic physical properties measured of the different glasses used are summarised in Table 1. Powder XRD pattern was obtained on D5000 Siemens diffractometer with a copper tube. The optical absorption of Er3+ in the borate–phosphate glass was obtained in the wavelength range of 200–2000 nm, by using a Varian 5000 UV/Vis/NIR Spectrophotometer. The infrared photoluminescence spectra around 1530 nm were measured by Zolix SBP500 spectrometer under the excitation of

Fig. 1 shows XRD patterns (PBNE2). This figure show a generally amorphous background, but with a peak appearing at h = 28.047°, which mean that the small crystalline exists in it. This feature can be explained by the fact that B2O3, P2O5 cannot form a glass by themselves but can form a glass as a mixture with network formers to certain content. The strongest Bragg peaks characterise the formation of crystalline boric acid (H3BO3) [16,17]. The average grain size of the crystallite is determined to be 8 nm from the full width at half maximum of the most intense peak making use the Scherer’s equation, D = 0.9k/b cos h [18], where k is the wavelength of X-ray radiation (1.541 Å), b is the FWHM in radians of the XRD peak and h is the angle diffraction. The feature has been also observed by Xusheng et al. [19] in fluorosilicate glass using Eu3+ probe. 3.2. Absorption spectra and Judd–Ofelt analysis 3.2.1. Absorption spectra The optical absorption of the polished samples was obtained in the wavelength range of 200–2000 nm, at room temperature by using a Varian 5000 UV/Vis/NIR Spectrophotometer. The absorption coefficient (a(k)) was calculated from the relation [20]:

1 d

  I I0

aðkÞ ¼ Ln

ð3Þ

  where d represents the thickness of the glass samples and Ln II0 corresponds to absorbance. Optical absorption of Er3+-doped and Er3+, Yb3+-codoped in phosphate–borate glass in the wavelength range 200–1600 nm is shown in Fig. 2. Five emission bands locating at 379, 505, 630, 982 and 1530 nm they are assigned to the 4 I15=2 ! 4 G11=2 , 4 I13=2 ! 4 G11=2 , 4 G11=2 ! 4 D3=2 , 2 F7=2 ! 2 F5=2 , 4 I15=2 ! 4 I13=2 transition, respectively. Spectra of serie1 and serie2 samples are similar in shape with small differences in absorbance. However, for samples (PBNY1, and PBNY2), the absorption corresponding to the 2 F7=2 ! 2 F5=2 transition is relatively intense.

Table 1 Physical properties of Er3+-doped and Er3+, Yb3+-codoped in phosphate–borate glass. Glass sample

PBNE1

PBNE2

PBNE3

PBNEY1

PBNEY2

Refractive index (n) Density (q) (g/cm3)

1.52 3.60 145.03

1.52 3.61 145.22

1.52 3.62 145.35

1.52 3.62 145.42

1.52 3.62 145.62

0.74

1.49

2.24

2.99

3.74

Molecular weight (M) (g) N (1019 ions/cm3)

Fig. 1. XRD patterns of the glass ceramics (PBNE2). Inset: A zoom around 2h = 28°.

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sr ¼ P

PBNE1;PBNE2;PBNE3;PBNEY1;PBNEY2 (1)

10

(2)

4 4

(3)

-1

α (cm )

(4) (5)

5

(1)

(4)

(2) (3)

I15/2 → G11/2 G11/2 → 4 D3/2 4

200

400

600

At ðJ i ! J f Þ bðJ i ! J f Þ ¼ P Jf At ðJ i ! J f Þ

(5)

800 1000 1200 1400 1600 1800 2000

λ (nm) Fig. 2. Room temperature absorption spectra Er3+-doped and Er3+, Yb3+-codoped in phosphate–borate glass samples (PBNE1, PBNE2, PBNE3, PBNEY1, and PBNY2).

3.2.2. Judd–Ofelt analysis Judd–Ofelt theory has been substantially depicted in literature [21]. Some important formulas are briefly presented here. The theoretical transition strength from level i to f can be expressed as the following:

" #Z 3hcð2J þ 1Þ 9n Sexp ¼ aðkÞdk 8p3 e2 kN ðn2 þ 2Þ2 band X Scalc ðJ i ! J f Þ ¼ Xt jhðSi ; Li ÞJi kU ðtÞ kðSf ; Lf ÞJf ij2

ð4Þ ð5Þ

t¼2;4;6 2

Smd ¼

h jhðSi ; Li ÞJ i k~ L þ 2~ SkðSf ; Lf ÞJ f ij2 16p2 m2 c2

ð6Þ

where Scalc and Sexp are the calculated and experimental line strength of an electric-dipole transition respectively, Smd is the magnetic dipole line strengths, k is barycentre wavelength at a given absorption band, c, n, h, m, and N are light speed in vacuum, refractive index, Planck’s constant, the masse of the electron, and number of rare-earth ions per unit volume, respectively. The k~ L þ 2~ Sk is the magnetic dipole operator, which is independent of host material and can be calculated through formula [22]. The terms hðSi ; Li ÞJi kU ðtÞ kðSf ; Lf ÞJf i are doubly reduced matrix elements of the unit-tensor operator U(t), independent of the host materials, and the values tabulated in [23] are adopted in our calculation. The theoretical transition strength, given by (4), can be fitted to the one from experiment, given by (5), using least-squares fitting, allowing the extraction of the phenomenological JO intensity parameters Xt (t = 2, 4, 6). The root mean-square deviation (rms) of the fit drms is determined by:

"Pl

drms ¼

 Scalcði;jÞ Þ2 ðq  pÞ

i¼1 ðSexpði;jÞ

ð9Þ

! Jf Þ

where the summation is carried out over all lower-lying level f compared with level i. The fluorescence branching ratio b is given by:

I11/2→ 4 G11/2 2 F7/2 → 2 F5/2 4 I15/2 → 4 I13/2

4

0 0

1 Jf At ðJ i

#

ð7Þ

where q is the number of transitions and p is the number of parameters determined. The calculated average value of drms is 0.074  1020 cm2. Using the obtained intensity parameters, the total spontaneous transition probability between the Ji and Jf levels At, and excited radiative lifetime sr can be calculated by: " # 64p4 At ðJ i ! Jf Þ ¼ 3hð2J þ 1Þk3 ! 2 nðn2 þ 2Þ X 2 ðtÞ 3 Xt jhðSi ; Li ÞJ i kU kðSf ; Lf ÞJ f ij þ n Smd  9 t¼2;4;6

ð8Þ

ð10Þ

The tensor elements are independent of the host and can be easily calculated from the tables of Nielson and Koster [23–25]. Table 2 shows the reduced matrix elements of transitions for Er3+, Yb3+ in this work; All values of Judd–Ofelt analysis for our samples in this work are tabulated in Tables 3–5. In Table 3 we have listed the values of average wavelength, R integrated absorption coefficients ( band aðkÞdk ¼ C), and experimental (Sexp) and calculated (Scalc) absorption line strengths for absorptions of Er3+-doped in PBNE1, PBNE2, PBNE3 and Er3+, Yb3+-codoped in PBNEY1, PBNEY2 samples. In Table 4 we find the Judd–Ofelt coefficients Xq (q = 2, 4, 6), while Table 5 reports the spontaneous emission probabilities (Aed, Amd), fluorescence branching ratio (b) and the radiative lifetimes sr, of various excited states of Er3+-doped in PBNE1, PBNE2, PBNE3 and Er3+, Yb3+-codoped in PBNEY1, PBNEY2 samples. Analysis of Table 4 shows that the X2 values are higher for the PBNE3, PBNEY1, PBNEY2 samples, this feature reflect the asymmetry of the local environment at the Er3+ ion site and exhibits the dependence on the covalency between rare earth ions and ligands anions [26]. However the values of X2 relatively smaller for the other samples (PBNE1, PBNE2) indicates in fact the presence of ionic banding between rare earth ions and the surrounding hosts, it reflects the relatively large symmetry of the local environment at Er3+, this result can to justify the peak appearing at XRD pattern in Fig. 1. More over, the X4 values are gained by lowering the covalency of r chemical bond between RE ion and ligand anions [27]. In addition, the vibronic-dependent parameter (X6) exhibits a large relatively values that explain a high rigidity of the metal– ligand bond donation from the ligands (such as PO4 tetrahedra) [28,29]. The values of the spectroscopic factor, equal to the ratio X4/X6 < 1 for all samples, the result reveal that our samples have a high rigidity of the metal–ligand bond, in addition the ration is almost constant with increasing the Er3+-doped and Er3+, Yb3+codoped. This feature is due to the invariance of the matrix host. Table 5 reveals that the branching ratios b, increase for 4 G11=2 ! 4 I13=2 transition but it decrease for 4 G11=2 ! 4 I15=2 transition with increasing the Er3+, Yb3+-codoped. However the ratios, decrease 4 G11=2 ! 4 I13=2 transition and it increase for 4 G11=2 ! 4 I15=2 transition with increasing Er3+ doped. Added to, the radiative lifetimes of 4 I13=2 level decrease with increasing the Er3+ doped and Er3+, Yb3+-codoped. This feature reflects the increasing of phonon energy.

Table 2 Values of reduced matrix elements for the absorption transitions of Er3+and Yb3+ in our sample glasses at room temperature. Transition

k (nm)

hU(2)i2

hU(4)i2

hU(6)i2

4

I15=2 ! 4 G11=2

379

0.9222

0.5300

0.1165

4

I13=2 ! 4 G11=2

505

0.1012

0.2666

0.2580

4

G11=2 ! 4 D3=2

630

0.0000

0.0054

0.0154

2

F7=2 ! 2 F5=2

982

0.0278

0.0003

0.3937

4

I15=2 ! 4 I13=2

1530

0.0195

0.1173

1.4328

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N. Sdiri et al. / Journal of Molecular Structure 1010 (2012) 85–90

Table 3 Values of the average wavelengths, integrated absorption coefficients, and experimental and calculated lines strengths of Er3+-doped in PBNE1, PBNE2, PBNE3, and Er3+, Yb3+codoped in PBNEY1, PBNEY2 samples. Transition k (nm)

C (nm cm1)

PBNE1

Sexp (1020 cm2) Scalc (1020 cm2)

C (nm cm1)

PBNE2

Sexp (1020 cm2) Scalc (1020 cm2)

C (nm cm1)

PBNE3

Sexp (1020 cm2) Scalc (1020 cm2) Transition k (nm)

C (nm cm1)

PBNEY1

Sexp (1020 cm2) Scalc (1020 cm2)

C (nm cm1)

PBNEY2

Sexp (1020 cm2) Scalc (1020 cm2)

4 I15=2 ! 4 G11=2 379

4

I13=2 ! 4 G11=2 505

4 G11=2 ! 4 D3=2 630

0.873 2.976 2.974

0.903 1.959 1.958

0.053 0.078 0.086

8.905 7.339 7.339

1.925 3.258 3.235

1.112 1.171 1.196

0.577 0.421 0.703

22.657 9.273 9.254

6.845 7.523 7.522

4.783 3.426 3.424

0.181 0.088 0.135

41.663 11.343 11.341

4 I15=2 ! 4 G11=2 379

4 I13=2 ! 4 G11=2 505

4 G11=2 ! 4 D3=2 630

6.188 5.219 5.207

4.955 2.659 2.735

0.245 0.089 0.115

7.390 2.339 2.666

23.675 9.855 9.751

8.289 5.589 5.604

6.268 2.689 2.614

0.299 0.087 0.104

10.786 2.729 2.413

53.291 8.689 8.786

Table 4 Intensity parameters Xn (in units of 1020 cm2) for samples PBNE1, PBNE2, PBNE3, PBNEY1, PBNEY2. Sample

X2

X4

X6

rms (1020cm2)

X4/X6

PBNE1 PBNE2 PBNE3 PBNEY1 PBNEY2

1.440 1.283 5.143 3.270 3.786

2.022 2.501 3.584 2.688 2.698

4.936 6.237 7.552 6.541 5.860

0.005 0.004 0.002 0.249 0.358

0.410 0.400 0.474 0.410 0.460

3.3. ð4 I13=2 ! 4 I15=2 Þ Luminescence lifetime and quantum efficiency As already mentioned in the introduction, this transition is of the utmost importance as far as the Er-doped and Er, Yb-codoped glass is used as active material to achieve optical amplification. The large bandwidth of this transition is in fact desirable for tuneable lasers to have a wide wavelength range over which they deliver relatively constant power. Similarly, optical amplifiers are more useful if they provide gain that is relatively independent of signal wavelength. This relaxes the wavelength tolerances on transmitters in a single-channel system, and increases the number of optical channels that can be multiplexed without gain compensation techniques in wavelength division multiplexed systems. 4

2 F7=2 ! 2 F5=2 982

4 I15=2 ! 4 I13=2 1530

Table 5 Average wavelengths, calculated radiative rates for electric dipole transitions, Aed, and magnetic dipole transitions, Amd, branching ratios b, and radiative lifetimes for samples PBNE1, PBNE2, PBNE3, PBNEY1, PBNEY2. k (nm) Transition in PBNE1 I13=2 ! 4 I15=2

4 4

D3=2 ! 4 G11=2

4

G11=2 ! 4 I13=2

! 4 I15=2

4

4 I15=2 ! 4 I13=2 1530

Transition in PBNE2 I13=2 ! 4 I15=2

4 4

D3=2 ! 4 G11=2

4

G11=2 ! 4 I13=2

! 4 I15=2

Transition in PBNE3 I13=2 ! 4 I15=2

4

D3=2 ! 4 G11=2

4

G11=2 ! 4 I13=2

! 4 I15=2 Transition in PBNEY1 I13=2 ! 4 I15=2

Amd (s1)

b

sr (ms)

1530

338.46

52.41

1.000

2.558

630

180.81

0

1.000

5.530

39

0.219

0.335

0

0.780

0.007

505

2939.03

379

10562.33

1530

427.66

630

129.81

0

505

2537.00

39

379 7928.84

4

Aed (s1)

0

52.41

0.754

1530

524.51

630

203.99

52.41 0

1

2.083

1

7.703

0.245

0.095

0.095

1

1.733

1

4.902

505

51399.37

39

0.658

0.019

379

26700.41

0

0.341

0.037

1530

455.72

2

F7=2 ! 2 F5=2

982

477.24

4

D3=2 ! 4 G11=2

630

4

G11=2 ! 4 I13=2

505

3989.22

39

0.176

0.248

379

18523.12

0

0.821

0.053

4

! 4 I15=2

68.772

52.41

1

1.968

0

1

2.095

0

1

14.540

3.3.1. ð I13=2 ! I15=2 Þ luminescence The emission spectrum in the infrared ranges for PBNE1, PBNE2, PBNE2, PBNEY1, and PBNEY2 glasses excited at 980 nm is shown in Fig. 3. The broad infrared fluorescence spectrum covering the 1470– 1625 range arises from the 4 I13=2 ! 4 I15=2 transition for the PL peak around 1530 nm. Fig. 3 shows, that intensities luminescence increased with Erdoped in phosphate–borate glasses. On the other hand the PL intensity in phosphate–borate Er, Yb-codoped is more important than glasses Er-doped.

g¼

3.3.2. Lifetime and quantum efficiency From the spontaneous emission probability At (Eq. (8)) the radiative lifetime sr ¼ A1 for transition can derived. A comparison t with the measured lifetime smeas yields the radiative quantum efficiency:

The measured lifetime can be determined experimentally by measuring the luminescence decay for the respective transition. To characterise the infrared emission band, the fluorescence lifetime 4 I13=2 level measured. To do so, the pump light exciting the fluorescence is abruptly switched off and the fluorescence

Transition in PBNEY2 4 I13=2 ! 4 I15=2

1530

401.82

2

F7=2 ! 2 F5=2

982

556.81

4

D3=2 ! 4 G11=2

630

4

G11=2 ! 4 I13=2

505

4034.23

39

0.170

0.245

379

19836.31

0

0.829

0.050

! 4 I15=2

smeas srad

67.226

52.41

1

2.201

0

1

1.795

0

1

14.875

ð11Þ

89

7 6 5

PB7

35

8

6

PB6

4 PB5 0,04

4

3

PBNE1 PBNE2 PBNE3

3

8

Intensity (×10 normalized)

3

Intensity (×10 normalized)

9

Intensity (×10 normalized)

N. Sdiri et al. / Journal of Molecular Structure 1010 (2012) 85–90

0,08

0,12

0,16

3+

Er (mol%)

3 2 1 0

PBNYE1 PBNYE2

30 25 20 15 10 5 0

1400

1450

1500

1550

1600

1650

1700

1400

1450

1500

λ (nm)

1550

1600

1650

1700

λ (nm)

Þ

ð12Þ

where I(t) is the emission intensity after the pulsed excitation, s the lifetime of the excited state, I(0) a constant, and p that has a value between 0 and 1 is a phenomenological parameter of disorder [32]. The results calculated radiative lifetimes srad, experimentally determined lifetimes smeas, Quantum efficiencies g, and the phenomenological parameter of disorder p are summarised in Table 6. Table 6 shows that disorder and the quantum efficiencies increases with Er doped or Er/Yb co-doped. In Table 7, the quantum efficiency g in different host materials are compared. On other hand, the initial increase luminescence in PBNEY2 sample is due to the populating of 4 I13=2 level from the higher energy levels directly excited, that result an effective energy transfer process between Yb3+ and Er3+ ions during smeas1 = 1.091 ms. In fact the excitation was at 980 nm, assuring the absorption by Yb3+, the de-excitation is through tow steps: first, a phonon assisted decay to the 4 I13=2 level, and after that a photon emission with k = 1530 nm with luminescence lifetime smeas2 = 1.475 ms. This feature of energy transfer between Yb3+ and Er3+ ions is represented in the energy diagram (Fig. 5) [34]. In addition, the nanocrystallites that have been observed in our material are embedded in the glass matrices homogeneously, the result is against the fact of phonon energy of borate, that have decreased the quantum efficiency. Hence the glass ceramics retained

-1

1xe

-2

1xe

-3

1xe

-4

1xe

-5

0

2

4

6

8

time (ms) time (ms) -2

0

2

4

6

8

10

0

1xe

-1

1xe

12

14

16

PBNEY1

1xe

τmeas=1.142ms τmeas1

1xe

-2

1xe

1xe

-3

1xe

τmeas2

1xe

-4

-1

-3

-5

-7

1xe-9

1xe

PBNY2

-5

1xe

1xe-11

τmeas1=1.091ms;τmeas2=1.475ms

0

5

10

15

20

Ln of luminescence Intensity (a.u)

IðtÞ ¼ Ið0Þeð

p t smea

PBNE1 PBNE2 PBNE3 tmea=0.614ms;tmea=0.78ms;tmea=0.884ms

0

1xe

1xe

Ln of luminescence Intensity (a.u)

intensity recorded as function of time. The fluorescence intensity is proportional to the spontaneous decay rate. This, in turn, is proportional to the population of the upper level if there is no interaction between the dopant ions. Therefore, in the simplest case, the fluot rescence decay is a single exponential decay IðtÞ ¼ Ið0Þesmea , where smea is the lifetime of the energy level. For the measurement, the sample is excited with laser light at 980 nm, which populates the 4 I11=2 level, not the 4 I13=2 level. The excited Er3+ ions relax into the 4 I13=2 level. The energy gap between these two levels is smaller than the gap between the 4 I13=2 level and the ground state 4 I15=2 . The lifetime of the 4 I11=2 level (10 ls, [30]) is therefore much shorter than the lifetime of the 4 I13=2 level and does not distort the result of the measurements. Typical measured decay curves are shown in Fig. 4 that presents the time decay measurements of the transition from Er-4 I13=2 level to 4 I15=2 for the Er doped (PBNE1, PBNE2, and PBNE3) and Er/Yb codoped (PBNEY1, PBNEY2) glasses. The numerical results presented in Fig. 4 verify the expected stretched exponential relaxation. We recall that a stretched exponential decay is of the form [31]

Ln of luminescence Intensity (a.u)

Fig. 3. PL spectra relatives to 4 I13=2 ! 4 I15=2 transition, under 980 nm excitation wavelength, for PBNE1, PBNE2, PBNE3, PBNEY1, and PBNEY2 glasses. Inset shows the dependence of PL intensity on Er3+ concentrations in Er-doped phosphate–borate glasses.

25

time (ms) Fig. 4. Luminescence decay curves for the 4 I13=2 ! 4 I15=2 transition on PBNE1, PBNE2, PBNE3, PBNEY1, and PBNEY2 glasses samples under 980 nm excitation. The line is the best fit of a stretched exponential decay function. The luminescence lifetime (smea) was determined by the best fitting.

Table 6 Parameter of disorder p, and quantum efficiencies for the 4 I13=2 ! 4 I15=2 transition of Er3+ in PBNE1, PBNE2, PBNE3, PBNEY1, and PBNEY2 glasses. Samples

PBNE1

PBNE2

PBNE3

PBNEY1

PBNEY2

Parameter of disorder, p Quantum efficiencies, g

0.606 0.240

0.656 0.37

0.687 0.510

0.632 0.580

0.646 0.670

90

N. Sdiri et al. / Journal of Molecular Structure 1010 (2012) 85–90

Table 7 Comparison of the optical parameters in different host materials at room temperature on 1530 nm.

smeas

Host PBNE3 PBNYE2 Er3+:Ca(PO3) Pb–In–(PO4)

X4/X6

Ref.

(ms)

Quantum efficiency g (%)

0.884 1.474 3.900 2.800

51.00 67.00 38.61 44.00

0.47 0.46 1.69 2.56

Present work Present work [31] [33]

E (cm-1) 2

1000

ET

F5/2

4

I9/2

4

I11/2

References [1] [2] [3] [4] [5]

[6] [7] [8] [9] [10] [11]

4

980nm

2

0

1530nm 4

F7/2 Yb3+

I13/2

I15/2

Er3+

Fig. 5. A schematic representation of energy transfer (ET) from Yb3+ Er3+ (? radiative transition, non-radiative transition) [34].

[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

an excellent transparency, a good luminescence behaviours [35–37], and a high rigidity.

[24]

4. Conclusion

[25] [26] [27] [28]

The spectroscopic properties of Er-doped and Er/Yb-codoped phosphate–borate glasses with different ions concentrations were thoroughly investigated at room temperature with the aim of getting further insights on the energy levels. The absorption spectra and Judd–Ofelt analyses evidence the incorporation of rare earth ions phosphate–borate glasses. In addition, the 4 I13=2 -fluorescence lifetime was carefully measured. The short distances between Er3+ and Yb3+ ions favour the inter-ionic interactions, and thus results in an efficient energy luminescence of Er3+. The energy transfer efficiency increases with Er3+ content, and reaches a maximum of 67% in the 0.1 mol% Er3+/0.1 mol% Yb3+ co-doped glass ceramic. The results obtained provide useful guidelines for choice of Er and Yb concentration as well as for modelling and optimising the performance of lasers and optical amplifiers based Er-doped and Er/Yb-codoped borate–phosphate glasses.

[29] [30] [31] [32] [33] [34]

[35] [36] [37]

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