Yb3+-codoped oxyfluoride glasses

Yb3+-codoped oxyfluoride glasses

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 232–238 Contents lists available at ScienceDirect Spectrochimica Acta...

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 232–238

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Optical transitions of Ho3+ in oxyfluoride glasses and upconversion luminescence of Ho3+/Yb3+-codoped oxyfluoride glasses Li Feng a,⇑, Yinsu Wu b a b

Institute of Materials Science and Engineering, Shijiazhuang University of Economics, Shijiazhuang 050031, PR China College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050016, PR China

h i g h l i g h t s 3+

g r a p h i c a l a b s t r a c t 3+

3+

 Ho -singly doped and Ho /Yb -

codoped SiO2–BaF2–ZnF2 glasses were prepared.  Judd–Ofelt analysis indicated the glass structure changed with change of glass composition.  The intensity of upconversion luminescence decreased with increasing ZnF2 content.

a r t i c l e

i n f o

Article history: Received 9 May 2014 Received in revised form 28 July 2014 Accepted 27 January 2015 Available online 4 February 2015 Keywords: Upconversion Optical transitions Oxyfluoride glass Judd–Ofelt

a b s t r a c t Optical properties of Ho3+-doped SiO2–BaF2–ZnF2 glasses have been investigated on the basis of the Judd– Ofelt theory. Judd–Ofelt intensity parameters, radiative transition probabilities, fluorescence branching ratios and radiative lifetimes have been calculated for different glass compositions. Upconversion emissions were observed in Ho3+/Yb3+-codoped SiO2–BaF2–ZnF2 glasses under 980 nm excitation. The effects of composition, concentration of the doping ions, and excitation pump power on the upconversion emissions were also systematically studied. Ó 2015 Elsevier B.V. All rights reserved.

Introduction Upconversion luminescence of rare-earth doped various glasses has been extensively investigated [1–10], due to the potential applications of these materials in areas like visible lasers, optical

⇑ Corresponding author. Tel.: +86 311 87208281. E-mail address: [email protected] (L. Feng). http://dx.doi.org/10.1016/j.saa.2015.01.066 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

amplifiers, optical data storage, color display, biomedical diagnostics and sensors, etc. [11–16]. Among the rare-earth ions, Ho3+ ion is one of the most important active ions due to its convenient energy level structure exploitable in upconversion processes. For example, the Ho3+ has a relatively long-lived 5I7 level which can act as a good population reservoir for possible upconversion processes. On the other hand, Yb3+ ion has only two spin–orbit states (2F7/2 ground state and 2F5/2 excited state), which guarantee a large cross

L. Feng, Y. Wu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 232–238

section around 980 nm [17]. Therefore, Yb3+ ion can act as sensitizers that can be pumped efficiently by laser diodes at 980 nm, thus allowing for an efficient energy transfer from Yb3+ to Ho3+ ions. The upconversion efficiency of rare-earth ions is greatly enhanced through energy transfer, which has been demonstrated in Ho3+/ Yb3+-codoped glasses [12,18]. The host materials play an important role in obtaining highly efficient upconversion luminescence. Among various glass hosts, the oxyfluoride glasses are one of the most attractive hosts because they contain the advantages of oxide glasses and fluoride glasses, and they have not only higher chemical and mechanical stability but also lower phonon energy. Recently, we have prepared Ho3+singly doped and Ho3+/Yb3+-codoped SiO2–BaF2–ZnF2 oxyfluoride glasses. In this article, radiative properties of Ho3+-singly doped glasses were analyzed on the basis of the Judd–Ofelt theory. The dependence of upconversion emission intensity on composition, Yb3+ and Ho3+ concentration, and pump power was investigated. Experimental The oxyfluoride glasses used in this work were prepared with the following composition in mol%: 50SiO2–(50  x)BaF2–xZnF2(SBZx), where x = 10, 20, 30. The amount of Ho3+ dopant was varied from 0.002 to 1 mol%, and Yb3+ was varied from 0 to 22 mol%. The starting materials are reagent grade SiO2, BaF2 and ZnF2, and high purity Ho2O3 and Yb2O3. Accurately weighted 10 g batches of raw materials were fully mixed and then melted at 1250 °C for 1 h in a corundum crucible. The melts were poured on a preheated stainless steel plate and then they were annealed to the room temperature. The obtained glasses were cut and polished for measuring their optical properties. Absorption spectra of the Ho3+-singly doped glasses were recorded in the 4000–32,000 cm1 region on a Cary 5000 UV– Vis–NIR spectrophotometer equipped with double out-of-plane Littrow monochromator. Density and refractive index of the sample were measured using Archimedes’ principle (using distilled water as an immersion liquid) and on an Abbe refractometer (using monobromonaphthalene as adhesive coating). The values of density and refractive index for Ho3+-singly doped SBZ10, SBZ20 and SBZ30 glasses are 4.1538 g/cm3 and 1.6049, 4.0624 g/cm3 and 1.6078, and 4.8978 g/cm3 and 1.6391, respectively. Based on density, the concentration of Ho3+ for the three glasses were calculated to be 0.3677 mol/1000 cm3 (equivalent to 2.214  1020 ions/cm3), 0.3842 mol/1000 cm3 (equivalent to 2.313  1020 ions/cm3) and 0.4972 mol/1000 cm3 (equivalent to 2.993  1020 ions/cm3), respectively. Upconversion emission spectra were recorded with a SPEX Fluorolog-3 spectrofluorometer equipped with a R928P photomultiplier tube. The excitation light for upconversion luminescence was a 980 nm laser diode with a maximum power of 1 W.

the electric dipole contributions. The Judd–Ofelt theory also supposes that only the magnetic dipole transitions with selection rules DS = DL = 0, DJ = 0, ±1 can be considered to be non-negligible. The calculated oscillator strengths Pcal can be expressed by

Pcal ¼

" # 2 8p2 mcm nðn2 þ 2Þ 3 S þ n S ed md 3hð2J þ 1Þe2 n2 9

X

D 

 

 

E 2 

Xk  ðS; LÞJU ðkÞ ðS0 ; L0 ÞJ0 

ð3Þ

2 e2  ðS; LÞJ kL þ 2SkðS0 ; L0 ÞJ 0  2 2 4m c

ð4Þ

Sed ¼ e2

k¼2;4;6

Smd ¼

E D   where Xk are the Judd–Ofelt intensity parameters, and U ðkÞ  the reduced matrix elements, which are taken from Carnall et al. [22]. The Judd–Ofelt intensity parameters Xk can be derived from a least square fitting of experimental and theoretical electric dipole oscillator strengths. Using the Judd–Ofelt intensity parameters, the total radiative transition probability can be calculated from the following relation:

  A ðS; LÞJ; ðS0 ; L0 ÞJ 0 ¼ Aed þ Amd ¼

" # 2 64p2 m3 nðn2 þ 2Þ Sed þ n3 Smd 3hð2J þ 1Þ 9

Z

eðmÞdm

The fluorescence branching ratio b and the radiative lifetime

   A ðS; LÞJ; ðS0 ; L0 ÞJ 0 0 0 0   b ðS; LÞJ; ðS ; L ÞJ ¼ P 0 0 0 S0 ;L0 ;J 0 A ðS; LÞJ; ðS ; L ÞJ

srad ¼ P

S0 ;L0 ;J 0 A



1  ðS; LÞJ; ðS0 ; L0 ÞJ 0

ð6Þ

ð7Þ

Results and discussion Absorption spectra and the Judd–Ofelt analysis Absorption spectra of 1 mol% Ho3+-singly doped SBZ10, SBZ20 and SBZ30 glasses in the spectral range from 4000 to

ð1Þ

where e(m) = 2.303 OD(m)/l is the measured absorption coefficient at a given frequency m, OD(m) the optical density, l the thickness of sample, m and e are the electron mass and charge, respectively, c is the speed of light and N the number of absorbing ions in unit volume. According to the Judd–Ofelt theory [20,21], the experimental oscillator strength contains the electric dipole and the magnetic dipole contributions. So, magnetic dipole contributions have to be subtracted from the experimental oscillator strengths to obtain

ð5Þ

srad can be obtained by the expressions

From the absorption spectrum, the experimental oscillator strengths Pexp of the electronic transitions can be calculated using the expression [19]

mc pe2 N

ð2Þ

where n is the refractive index; Sed and Smd are the calculated electric dipole and magnetic dipole line strengths, respectively, which can be given by

Theoretical background

Pexp ¼

233

Fig. 1. Absorption spectra of SBZ10, SBZ20 and SBZ30 glasses.

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L. Feng, Y. Wu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 232–238

Table 1 Experimental and calculated oscillator strengths of SBZ10, SBZ20 and SBZ30 glasses. Transition

SBZ10

SBZ20

SBZ30

Pexp

Pcal

Pexp

Pcal

Pexp

Pcal

5

1.24

1.43

0.61 2.26 3.42 0.94 18.21 2.14 0.56 4.35 0.25 4.20 1.85 1.55 7.60

1.46 (Ped) 0.47 (Pmd) 1.07 2.77 3.72 1.16 18.67 2.55 0.54 4.08

1.28

?5I6 ?5F5 ?5F4, 5S2 ?5F3 ?5F2, 3K8, 5G6, 5F1 ?5G5 ?5G4, 3K7 ?3H5, 3H6, 5G2 rms 106 X2 1020 X4 1020 X6 1020 P Xk 1020

1.28 (Ped) 0.47 (Pmd) 0.95 2.41 3.25 1.02 18.28 2.19 0.48 3.90

1.30 (Ped) 0.48 (Pmd) 0.96 2.49 3.33 1.03 19.34 2.34 0.49 4.12

I8 ? 5I7

0.74 2.62 3.82 1.25 18.61 2.53 0.63 4.49 0.24 4.08 2.15 1.76 7.99

0.65 2.44 3.44 0.99 19.32 2.30 0.57 4.39 0.18 4.34 1.93 1.52 7.79

Table 2 Values of radiative transition probabilities A, fluorescence branching ratios b and radiative lifetimes srad of SBZ10, SBZ20 and SBZ30 glasses. Initial state

Final state

SBZ10

SBZ20

SBZ30

A

b

srad

A

b

srad

A

b

srad

5

5

65.97

1.000

15,158

75.10

1.000

13,316

69.87

1.000

14,312

5

5

I7 5 I8

16.98 160.36

0.096 0.904

5639

19.25 182.61

0.095 0.905

4954

18.06 169.42

0.096 0.904

5334

5

I6 I7 5 I8

5.78 80.48 57.21

0.040 0.561 0.399

6970

6.51 91.88 65.52

0.040 0.561 0.399

6101

6.20 84.76 60.54

0.041 0.559 0.400

6601

5

I5 I6 I7 5 I8

6.76 85.08 385.68 1555.09

0.003 0.042 0.190 0.765

492

7.65 97.05 441.44 1795.81

0.003 0.041 0.189 0.767

427

7.20 90.87 418.12 1682.99

0.003 0.041 0.190 0.766

455

5

F5 I5 5 I6 5 I7 5 I8

0.43 31.81 135.74 840.82 1107.01

0 0.015 0.064 0.398 0.523

473

0.50 36.39 155.66 960.36 1264.39

0 0.015 0.065 0.397 0.523

414

0.48 33.56 144.19 883.45 1163.13

0 0.015 0.065 0.397 0.523

449

5

S2 F5 5 I5 5 I6 5 I7 5 I8

0 8.57 117.68 221.39 305.09 3003.39

0 0.002 0.032 0.061 0.083 0.822

274

0 8.66 135.07 256.43 355.66 3453.74

0 0.002 0.032 0.061 0.084 0.821

238

0 9.49 125.68 242.16 338.54 3214.01

0 0.002 0.032 0.062 0.086 0.818

254

5

F4 S2 F5 5 I5 5 I6 5 I7 5 I8

1.72 0.10 21.21 135.12 325.07 1151.53 1804.61

0.001 0 0.006 0.039 0.094 0.335 0.525

291

1.78 0.10 23.11 157.72 374.16 1332.69 2061.17

0 0 0.006 0.040 0.095 0.337 0.522

253

1.89 0.11 23.21 150.45 348.73 1253.52 1896.10

0.001 0 0.006 0.041 0.095 0.341 0.516

272

5

0.10 1.75 0.04 1.34 9.48 53.35 8.05 243.72 112.99 876.37 4401.20 3269.77

0 0 0 0 0.001 0.006 0.001 0.027 0.013 0.098 0.490 0.364

111

0.11 1.95 0.05 1.57 9.68 53.76 9.40 244.37 129.53 928.51 4363.68 3822.39

0 0 0 0 0.001 0.006 0.001 0.026 0.014 0.097 0.456 0.399

105

0.10 1.91 0.04 1.50 10.49 58.63 8.99 268.48 123.23 967.28 4857.50 3654.85

0 0 0 0 0.001 0.006 0.001 0.027 0.013 0.097 0.488 0.367

100

I7 I6

5

I5

I8

5

5

F5

5 5

5

S2

5

5

F4

5

5

F3

5 5

5

G5

F1 G6 K8 5 F2 5 F3 5 F4 5 S2 5 F5 5 I5 5 I6 5 I7 5 I8 5 3

32,000 cm1 are shown in Fig. 1. The thicknesses of three samples are 2.80, 2.90 and 3.16 mm, respectively. The profiles and positions of the absorption bands are similar for the three glasses. The absorption bands are attributed to 4f–4f transitions of Ho3+ from

the ground state 5I8 to the excited states 5I7, 9I6, 5I5, 5F5, (5F4, 5S2), 5 F3, (5F2, 3K8), (5G6, 5F1), 5G5, (5G4, 3K7) and (3H5, 3H6, 5G2). From the absorption spectra it can be observed that the absorption edge in the UV region shifts towards longer wavelength as the amount

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L. Feng, Y. Wu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 232–238 Table 3 P Values of sk, sk and P of various Ho3+ doped hosts. System

Intensity parameter(109 cm)

Oscillator strength(106)

3+

1 LaF3:Ho [28] 2 HoP5O14 [26] 3 HClO4–DClO4 [22] 4 HoCl3–CH3OH [29] 5 HoCl36H2O–CH3OH [29] 6 [HoW10O35]7 [30] 7 HoCl3–C2H5OH [29] 8 HoCl36H2O–C2H5OH [29] 9 YAlO3:Ho3+ [28] 10 HoCl3–n–C3H7OH [29] 11 HoCl36H2O–n–C3H7OH [29] 12 Ho(PW11O39)2 [30] 13 Ho(PW11O39) [30] 14 15BaO–85TeO2 [31] 15 20Na2O–80TeO2 [31] 16 35ZnO–65TeO2 [31] 17 Ca3Sc2Ge3O12 [32] 18 ZBLA [33] 19 ZBLA [33] 20 SANZ10 (this work) 21 SANZ20 (this work) 22 SANZ30 (this work)

P1

P2

s2

s4

s6

Rsk

Rs⁄k

4.35 7.44 6.00 10.49 11.58 12.83 14.25 15.24 7.45 16.88 17.60 14.16 17.48 35.77 41.27 39.01 7.80 10.46 – 18.21 18.61 19.32

1.48 2.67 3.24 3.58 3.82 – 4.64 4.64 2.52 5.28 5.54 – – – – – 2.37 – 3.37 4.35 4.49 4.39

1.86 2.34 0.47 ± 0.18 3.689 ± 0.303 3.847 ± 0.303 4.55 ± 0.17 6.179 ± 0.148 6.192 ± 0.404 3.91 7.173 ± 0.192 7.104 ± 0.534 4.63 ± 0.14 6.39 ± 0.06 14.332 ± 0.647 16.822 ± 0.534 14.997 ± 0.676 0.19 3.45 3.47 6.76 6.59 7.18

2.21 2.21 4.05 ± 0.21 2.872 ± 0.448 3.570 ± 0.435 3.02 ± 0.25 2.531 ± 0.229 2.894 ± 0.606 5.12 3.285 ± 0.274 3.449 ± 0.794 3.80 ± 0.20 3.86 ± 0.09 6.164 ± 1.183 6.842 ± 0.794 7.887 ± 1.234 5.67 3.15 3.17 2.98 3.47 3.19

1.41 2.47 3.96 ± 0.21 2.424 ± 0.303 2.754 ± 0.303 3.26 ± 0.18 2.167 ± 0.148 2.234 ± 0.404 3.29 2.437 ± 0.192 2.601 ± 0.534 5.20 ± 0.14 4.34 ± 0.07 4.570 ± 0.430 3.442 ± 0.497 4.667 ± 0.450 1.21 2.62 2.64 2.50 2.84 2.51

5.48 7.02 8.48 ± 0.60 8.985 ± 1.054 10.171 ± 1.041 10.83 ± 0.60 10.877 ± 0.525 11.32 ± 1.414 12.32 12.895 ± 0.658 13.154 ± 1.862 13.63 ± 0.48 14.59 ± 0.22 25.066 ± 2.26 27.107 ± 1.825 27.551 ± 2.36 7.07 9.22 9.28 12.24 12.90 12.88

5.25 7.66 7.94 9.72 10.35 11.20 12.18 12.48 7.48 13.78 14.32 12.00 14.02 25.10 28.43 27.06 7.40 9.77 9.14 13.02 13.32 13.41

Fig. 2. Oscillator strength P as a function of intensity parameters original data and ‘‘j, d’’ for present data.

P

sk . ‘‘h, s’’ for

of ZnF2 is increased. The behavior indicates structural changes might have occurred. Experimental and theoretical oscillator strengths and Judd– Ofelt intensity parameters are calculated using Eqs. (1) and (2), and the results are given in Table 1. In addition, the 5I8 ? 5I7 transition contains a substantial magnetic dipole component, and the value of magnetic dipole oscillator strength for this transition is also shown in Table 1. The accuracy of the fit is given by the root mean square deviations (rms) which are also presented in Table 1. All of the rms deviations for three glasses are very small which indicate a good fitting between experimental and calculated oscillator strengths. The oscillator strengths of all the transitions except 5 I8 ? 5F2, 3K8, 5G6, 5F1 transition are slightly higher in SBZ20 glass which indicate that non-symmetric component of the electric field acting on Ho3+ is slightly higher in SBZ20 glass. The magnetic dipole contribution of the 5I8 ? 5I7 transition is constant and independent of the ligand structure and host composition.

Fig. 3. Upconversion spectra of 0.06 mol% Ho3+ and 6 mol% Yb3+ codoped SBZ10, SBZ20 and SBZ30 glasses under 980 nm excitation.

The Judd–Ofelt intensity parameters, particularly X2, are closely related to the glass composition. X2 is sensitive to asymmetry of the rare earth sites and covalency between rare earth ions and ligand ions. X2 will increase with increase in asymmetry and covalency [23–25]. From Table 1, it can be observed the value of X2 is smaller for SBZ20 glass which is ascribed to lower asymmetry and/ or covalency between rare earth ions and ligand ions in SBZ20 glass. On the other hand, the values of X4 and X6 are larger for P SBZ20 glass. The sum of intensity parameter Xk is also larger for SBZ20 glass. In addition, the orders of intensity parameter Xk (X2 > X4 > X6) for all the three glasses are similar. Radiative transition probabilities, fluorescence branching ratios and radiative lifetimes were calculated using Eqs. (5)–(7), respectively. The values were listed in Table 2. It is observed that the SBZ20 glass possesses slightly larger the radiative transition prob-

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Fig. 4. Effect of Yb3+ concentration on the upconversion luminescence of Ho3+/Yb3+-codoped SBZ10, SBZ20 and SBZ30 glasses: (a) SBZ10; (b) SBZ20; (c) SBZ30.

Fig. 5. Effect of Ho3+ concentration on the upconversion luminescence of Ho3+/Yb3+-codoped SBZ10, SBZ20 and SBZ30 glasses: (a) SBZ10; (b) SBZ20; (c) SBZ30.

abilities and shorter the radiative lifetimes for all the excited states except 5G5 level. For all the samples, it is found that the lifetime of 5 I7 level is the longest. From the Table 2, it is also observed that the magnitudes of branching ratios are high for 5I7 ? 5I8, 5I6 ? 5I8,

5

F5 ? 5I8 and 5F4 ? 5I8 transitions which suggest these transitions are most suitable for laser action. Su reported that oscillator strength P and intensity parameter P sk has a linear relation [26,27]

L. Feng, Y. Wu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 232–238

237

P

In the present case, sk for SBZ10, SBZ20 and SBZ30 glasses were calculated to be 13.02  109, 13.32  109 and 13.41  P 109 cm respectively obtained using Eqs. (10)–(12). sk for SBZ10, SBZ20 and SBZ30 glasses were calculated to be 12.24  109, 12.90  109 and 12.88  109 cm respectively P  according to Eq. (9), which are well consistent with sk . It is also obviously shown in Fig. 2 that the present data are almost on straight lines of P1 and P2.

Upconversion luminescence

Fig. 6. Dependence of upconversion emission intensity of 0.06 mol% Ho3+ and 6 mol% Yb3+ codoped SBZ10 glass on excitation power under 980 nm excitation.

Fig. 7. Simplified energy level diagram of Ho3+ and Yb3+ ion and possible upconversion luminescence mechanisms of Ho3+/Yb3+-codoped glasses. Solid straight lines with upward and down arrows indicate pumping and upconversion transitions, and radiation transitions, respectively; dot lines and wavy arrows denote energy transfer and nonradiative relaxation, respectively.

P¼a

X

where

P

sk þ b

ð8Þ

sk = s2 + s4 + s6, sk can be obtained by expression

Xk ¼ 9:0  1012 

9n ðn2

þ 2Þ

2

sk

ð9Þ

For Ho3+ ion, the linear relations are given below by fitting different data (Table 3, Fig. 2).

P1 ð5 I8 ! 5 G6 Þ ¼ 1650:6 P2 ð5 I8 ! 3 H6 Þ ¼ 401:4 X

sk ¼

P

sð1Þ k þ 2

P

X

X

skð2Þ

6 sð1Þ k  5:66  10

skð2Þ  0:30  106

ð10Þ ð11Þ

ð12Þ

Fig. 3 shows the upconversion emission spectra of 0.06 mol% Ho3+ – 6 mol% Yb3+-codoped SBZ10, SBZ20 and SBZ30 glasses excited by 980 nm laser at room temperature. For all the glasses, the shapes and properties of spectra are similar, and the red emission is much stronger than the green one. The emission bands centered at 547 and 660 nm correspond to 5F4, 5S2 ? 5I8 and 5F5 ? 5I8 transitions, respectively. The orders of the intensity of upconversion luminescence for three glasses are SBZ10 > SBZ30 > SBZ20, which may be attributed to the decreased radiative lifetimes of intermediate levels (see Table 2). The decrease of radiative lifetimes reduces the probabilities of intermediate levels further excited. In addition, the intensity of upconversion luminescence is also affected by phonon energy and/or electron–phonon coupling strength [34–36]. An increase in both phonon energy and electron–phonon coupling strength raises nonradiative relaxation rate, and thus reduces the intensity of upconversion emission. Fixed the concentration of 0.06 mol% Ho3+, the dependence of green and red emission intensity on Yb3+ concentration of Ho3+/ Yb3+-codoped SBZ10, SBZ20 and SBZ30 glasses is described in Fig. 4. The intensity of both green and red emissions increases with increasing Yb3+ concentration and then decreases at higher concentration. The results indicate the important role of the Yb3+ ions in the energy transfer process. Both the energy transfer from Yb3+ to Ho3+ and energy back-transfer from Ho3+ to Yb3+ may exist in Ho3+/Yb3+-codoped system under 980 nm excitation. The rate of energy transfer from Yb3+ to Ho3+ is dominant at the lower Yb3+ concentration, which is responsible for the increasing emission intensity. However, at the higher Yb3+ concentration, the rate of energy back-transfer from Ho3+ to Yb3+: 2F7/2(Yb3+) + 5F4, 5 S2(Ho3+) ? 2F5/2(Yb3+) + 5I6(Ho3+) becomes so large that the upconversion luminescence intensity decreases. Fixed the concentration of 6 mol% Yb3+, the dependence of green and red emission intensity on Ho3+ concentration of Ho3+/ Yb3+-codoped SBZ10, SBZ20 and SBZ30 glasses is presented in Fig. 5. With increasing Ho3+ content, the intensity of both green and red emission increases and then decreases at higher concentration. At the lower Ho3+ concentration, increasing Ho3+ concentration benefits the populations on the excited levels of Ho3+ and thus the emission intensity increases. At the higher Ho3+ concentration, the cross-relaxations of among different levels of Ho3+ become intense and thus the emission intensity decreases. In frequency upconversion processes, the upconversion emission intensity IUP will be proportion to the nth power of infrared excitation intensity IIR, that is: IUP / (IIR)n, where n is the number of IR photons required to excite the emitting state. A plot of log IUP versus log IIR yields a straight line with slope n. Such plots for 5F4, 5 S2 ? 5I8 and 5F5 ? 5I8 transitions are shown in Fig. 6, and the values of n obtained are 1.94 and 1.81, respectively. The results indicate that a two-photon absorption process is responsible for two emission bands. The possible upconversion mechanism including energy transfer is depicted in Fig. 7. Firstly, the Yb3+ is excited from the 2F7/2 level to 2 F5/2 level under 980 nm pumping, and then transfers its energy to Ho3+. Thus Ho3+ is raised from the 5I8 ground state to 9I6 excited state.

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L. Feng, Y. Wu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 232–238

This is a phonon-assisted energy transfer process because of energy mismatch between the 2F5/2 level of Yb3+ and 5I6 level of Ho3+. Secondly, the Ho3+ in 5I6 level is promoted to 5F4 or 5S2 level by the same energy transfer from Yb3+. In addition, the 5F4 and 5S2 level of Ho3+ can be also populated through excited state absorption. Finally, the green emission around 547 nm associated with 5F4, 5 S2 ? 5I8 transition take place. The red emission centered at 660 nm is originated from the 5F5 ? 5I8 transition. Population of the 5F5 level is possibly accomplished by two different channels. One channel is that Ho3+ in 5F4 or 5S2 level relaxes nonradiatively to 5F5 level. The other channel is closely related to the 5I7 level populated by nonradiative relaxation from the 5I6 excited state. The Ho3+ ion in 5I7 level can be excited to 5F5 level by two processes mentioned above: excited state absorption and energy transfer from Yb3+. In the case of our sample pumped at 980 nm, the intensity of red emission is much stronger than that of green emission. This result possibly indicates the quantities of Ho3+ ions in 5I6 level relaxing nonradiatively to the lower 5I7 level are much greater than those excited to the upper 5F4 and 5S2 level. In other words, it possibly results from the longer lifetime of 5I7 level. Therefore, the latter channel can be considered to be dominant. Conclusions Optical properties of Ho3+-singly doped and Ho3+/Yb3+-codoped SBZ10, SBZ20 and SBZ30 glasses were investigated. Judd–Ofelt intensity parameters and some radiative properties such as radiative transition probabilities, fluorescence branching ratios and radiative lifetimes were obtained on the basis of the Judd–Ofelt theory. The value of X2 is smaller while the values of X4, X6 and P Xk are larger for SBZ20 glass. The sum of intensity parameter obtained using the Judd–Ofelt theory is in good agreement with one calculated using the linear relations reported by Su. The SBZ20 glass possesses slightly larger the radiative transition probabilities and shorter the radiative lifetimes for all the excited states except 5G5 level. The upconversion emission bands centered at 547 and 660 nm of Ho3+/Yb3+-codoped glasses were observed under 980 nm excitation, and the orders of the intensity of upconversion luminescence are SBZ10 > SBZ30 > SBZ20. The influence of Yb3+ concentration on all the emissions is obvious, which indicates the important role of the Yb3+ ions in the energy transfer process. Power-dependent studies reveal that both green and red emissions result from two-photon processes. Acknowledgements This work was supported by the Nature Science Foundation of Hebei Province (E2012403011), Foundation for The Excellent

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