Optical transition properties of rare-earth ionsin nonfluoride-halide glasses

Masanori Shojiyaa, Kohei Kadono', Masahide Takahashic, Ryoji Kanno', and Yoji Kawamotoa

aDjvjsjon of Science of Materials, Graduate School of Science and Technology, Kobe University,Nada, Kobe 657-0013, Japan

bOsaka National Research Institute, Ikeda, Osaka 563-8577, Japan

CVenture Business Laboratory, Kobe University, Nada, Kobe 657-0013, Japan

ABSTRACTOptical transition properties of Er3 ions in ZnC12-KC1-BaCI2 glass were studied and were discussed on the comparison

with those in the other glass systems, i.e., ZrF4-based and Ga2S3-based glasses. Judd-Ofelt analysis was performed

using eight absorption bands of Er3 in the ZnC12-based glass. Among the Judd-Ofelt intensity parameters, the 2 was

larger than that of the ZrF4-based glass. This is probably due to the covalency in the bonds of the rare..earth and ligand

ions in comparison with those of the ZrF4-based glass. Decay curves of the emission from the 4F512, 47a4S312 and

4F912 levels were measured. From the lifetime data and the radiative transition probabilities calculated using the Judd

Ofelt intensity parameters, multiphonon relaxation rates are estimated for the four excited levels. The multiphononrelaxation rates of the 4F512, 4F712, and 4F9 were much smaller than those in the ZrF4-based and Ga2S3-based glasses.

This is the consequence of the extremely low-phonon-energy property of the ZnCl2-based glass. The multiphononrelaxation rates were inversely proportional to the exponential of the energy gap between the emission and the nextlower levels (a so-called "energy-gap low") as well as the other glass systems.Keywords: Nonoxide glass, Halide glass, Phonon energy, Rare-earth ion, Multiphonon relaxation

1. INTRODUCTION

Since the discovery of the glass formation in the system based on ZrF41, the science and technology of fluoride glasses

have been much advanced.2 Particularly, the optical properties of the glasses, e.g., infrared transmission andspectroscopy of rare-earth ions doped in the fluoride glasses have received much attention. Researches and developmentsfor application to optical devices have been intensively performed. The characteristics of the fluoride glasses areattributed to the low-phonon-energy property of the host in addition to the intrinsic superiority of glass materials foroptical devices - transparent in wide spectrum range, optically isotropic and homogeneous, producible, and so on. Suchinvestigations of the fluoride glasses have continued to stimulate the research interest of the other kind of nonoxide

glass systems, e.g., nonfluoride-halide glasses (chloride, bromide, and iodide glasses) and chalcogenide glasses.Moreover, after the success of the erbium-doped fiber amplifiers at 1 .5 tm for optical telecommunication, the effort todevelop the optical amplifiers at 1.3-Rm window has much more spurred the investigation of the nonoxide glasses ashost materials of rare-earth ions.3

We have carried out studies on the nonfluoride-halide glass systems - the glass formation, structural analyses, andphysical and chemical properties.4 Since these glass systems have lower phonon energies than the glasses based onfluorides, the infrared transmission is superior to the fluoride glasses, and more importantly, we can expect to observeemission bands which is hard to detect even in fluoride glasses because of the large multiphonon relaxation rate. Theseemission bands will have potential application for new fluorescence materials, optical amplifiers, and lasers.

1K.K. (Corresponding author): E-mail address: kadono@onri.go.jp; Tel: 81-727-51-9642; Fax:81-727-5 1-9627

SPIE Vol. 3280 • 0277-786X1981$10.00 23

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In spite of the promising applications, however, there are only a few investigations for fundamental spectroscopyof rare-earth ions in the nonfluoride-halide glasses.57 In this paper, we report the optical transition properties of Er3ions in ZnC12-based glass as an example of rare-earth ions in glasses based on nonfluoride-halide systems.

2. EXPERIMENTAL PROCEDURE

Reagent grade anhydrous chlorides were used for preparation of glasses. Zinc chloride was purified by passing hydrogenchloride gas through the melt at 450 °C and then distilling under vacuum. Other chlorides were dried at elevatedtemperature under vacuum. Batches of the dehydrated starting materials having desired compositions were melted inquartz crucibles at 500 to 600 °C. Small amounts of ammonium chloride was add to the batches for chlorination ofthe trace of hydrate and oxide impurities. After the chlorination reaction finished and the residual ammonium chloridewere completely drove out, the melts were quickly cooled into brass molds at room temperature. The obtained glasseswere transparent and slightly pink. All preparation processes were carried out in a glove box filled with dry argon.

The refractive index was measured by the immersion method using an appropriate liquid mixtures. The densitywas determined by the flotation method. The preparation methods for the fluoride and sulfide glasses were describedelsewhere.8'9 The compositions of the glasses used in the present experiments are shown in Table I.

Absorption spectra were obtained with a HITACHI U-3500 spectrometer in the range from 200 to 1600 nm.Decay curves of the emissions from 4F512(+4F3), 4F712, 4S312(÷2H1112), and 4F9 levels were measured for the glasses

containing 0.5 cation mol% of Er3 at room temperature. Excitation to these levels was performed by the use of anoptical parametric oscillator (Spectra-Physics, Quanta-Ray MOPO-730) pumped with a Q-switched Nd:YAG laser(Spectra-Physics, Quanta-Ray GCR23O-1O). The decay curves were detected by a photomultiplier and stored in a digitaloscilloscope.

Table I Compositions, densities, and refractive indices of glasses and number densities of Er3 ion in the glasses

Composition x (mol%)Number density

Density Refractiveof Er3 ion

index(g/cm3)(1020 ions/cm3)

For the glasses of the composition of x=3.0 mol%

5OZnCI23OKC120BaC12+xErCl3 0.5, 3.0 2.871 3.70 1.615

57ZrF438BaF2.(5x)LaF3xErF3 0.5, 3.0 4.734 4.96 1.525

600aS31210GeS2•(30-x)La5312xEr5312 0.3, 3.0 3.921 5.01 2.440

3. RESULTS

Figure 1 shows the absorption spectra of the ZnCl2-based, ZrF4-based, and Ga253-based glasses containing 3 mol% of

Er3. The line strengths of the absorption bands were obtained from the spectra. The Judd-Ofelt analysis was performedusing the eight absorption bands for the ZnCl2-based glass. The detailed description of the Judd-Ofelt analysis is shown

elsewhere.6'9'10'11 We used eight absorption bands in order to determine the Judd-Ofelt intensity parameters for theZnCl2-based glasses. The obtained parameters are shown in Table II together with those of the ZrF4-based and Ga253-

based9 glasses. Using these parameters, we can calculate a given spontaneous emission probability between the levelsofJ and J', A(J'M, which are shown in Table III.

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NE0

CN

C0rID

rIDCt.2

CI-0C

C

1600200 400 600 800 1000 1200 1400

Wavelength (nm)

Figure 1. Absorption spectra of the Er3 doped ZnCl2-based, ZrF4-based, and Ga2S3-based glasses.

Table II Judd-Ofelt parameters of Er3 in nonoxide glasses

Host glass(10-20 cm2)

ZnCI2-based 4.73 0.65 0.21

ZrF4-based 3.10 1.52 0.99

Ga2S3baseda 6.58 2.11 1.00

aReferen 9.

The lifetimes of the emissions from the excited levels, 4F512(+4F312), 4F712, 4S312(+2H1112), and 4F912 were

obtained as average lifetimes, tav, defined by the following equation,

t = ftI(t)dt/ fI(t)dt . (1)

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I9I2

4F912

4s3,2-.

Transition

i3/2 15/2I11/2— I15/2

I13/2

I15/2

I13/2

'15/2

I13/2

I11/2

I9/2

I15/2

I13/2

'11/2

I15/2

I15/2

I15/2

I15/2

I15/2

I13I2

11/2

I9/2

Aed (s1)

25.6

49.6

4.4

49.4

8.0

5630

691

219

177

353

827

358

19.8

17.5

Branching ratio

1.00

0.92

0.08

0.22

0.53

0.23

0.01

0.01

Table III Calculated spontaneous emission probabilities of Er3 in

ZnC12-based glass

Amd (s1)

42.9

11.9

2.5

438 0.89

30.7 0.06

21.8 0.04

3.0 0.01

200 0.65

85.9 0.28

7.9 0.02

15.2 0.05

4F-i12

4F312

2H912

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The observed lifetime, t, is written as

1/r=W+WR (2)where WR and WNR je radiative and nonradiative transition probabilities, respectively. The main parts attributed to thenonradiative transition probability are the multiphonon relaxation process and the cross relaxation process between ions.Since the concentration of Er3 (0.5 mol%) is small, the cross relaxation process is negligible and the differencebetween the total transition probability, lIT, and the radiative transition probability becomes the multiphononrelaxation rate. Table IV shows the luminescence properties of the 4F512(+4F312), 4F712, 4S312(+2H1112), and 4F912

levels. For comparison, the luminescence properties of the 4S312(+2H1112), and 4F912 levels of Er3 in the ZrF4-based

and Ga253-based9 glasses are also shown in Table V. The radiative quantum efficiency, , in these tables is calculated

by the following equation;i=1OOxtxW . (3)

Table IV Luminescence characteristics of excited levels of Er3 ion in ZnCI2-based glass

Level 4F912 4S312 (H1112) 4F712 4F512 (4F3)

Lifetime (tm) 1200 700 5.3 40

Transition probability (s-i) 830 1430 1.8x105 2.5x104

Calculated spontaneous emission

probability (s )860 960 690 215

Multiphonon relaxation rate (s-i) 340 470 1.9x105 2.5x104

Quantum efficiency (%) 72 67 0.4 0,9

Energy gapa (cm1) 2788 3058 1310 1643

aEnergy gap between the level and the next lower-lying level.

Table V Luminescence characteristics of excited levels of Er3 ion in ZrF4-based and Ga2S3baseda glasses

Glass host ZrF4-based glass Ga2S3-based glass

Level 4F912 3/2 (H1112) 4F912 3/2 (H1112)

Lifetime (rim) 140 490 89.6 33.9

Transition probability (s-i) 7140 2040 1.1x104 3.0x104

Calculated spontaneous emission

probability (s1)1170 1530 8004 9188

Multiphonon relaxation rate (s-i) 5970 510 3157 20310C

Quantum efficiency (%) 16 75 71 31

Energy gaps (y1) - 2832 3131 2776 3084

aReference 9.bEnergy gap between the level and the next lower-lying level.CThe nonradiative relaxation from the 453 level of Er3 in the Ga2S3-based glass includes

the other process except for the multiphonon relaxation (see Ref. 9).

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4. DISCUSSION

The absorption spectrum of Er3 in the ZnC!2-based glass in Figure 1 shows that the bands of 380 nm and 525 nm are

clearly larger than those of the ZrF4-based glass. These bands are assigned to the transitions of 4I1512—4G1112 and

4I1512—'2H11, respectively, which are so-called hypersensitive transitions. The reduced matrix elements u(2) of these

transition are large and, therefore, the Judd-Ofelt � 2 parameter becomes large in the case that hypersensitive absorptions

and emissions are strongly observed. In fact, the 2 parameter for the ZnC12-based glass is obviously larger than thatof the ZrF4-based glass as shown in Table II and also those of the other fluoride glass systems.12 The �2 2parameter is

sensitive to local structure and bonding nature around rare-earth ions. It is believed that the Q2 increases with thecovalency of the bond between the rare-earth and ligand ions. Therefore, the large 2 parameter for the ZnC12-based

glass is probably due to the covalency in the bonds compared with those of the fluoride glasses. The characteristics aremore pronounced for the Ga2S3-based glass, in which the bonds between rare-earths and ligands are expected to be much

more covalent.

In Tables IV and V, the multiphonon relaxation rates of the 4F912 of Er3 in the ZnCl2-based glass is evidently

smaller than those of the ZrF4-based and Ga2S3-based glasses. On the other hand, the multiphonon relaxation rate of

the 4S3 is comparable with that of the ZrF4-based glass. The energy gap from the 4F912 level to the next lower level

is about 2800 cnv1 while that for the 4S312 is about 3100 cm1. Therefore, it is expected that the multiphononrelaxation of the excited levels for the energy gap less than about 3000 cm1 is pronouncedly suppressed in the ZnC12

based glass. This is more clearly observed for the 4F712 and 4F5 levels, the energy gap of which are 1310 and 1640

cur1, respectively. While the multiphonon relaxation rates of these levels are estimated to be 1067 1 in the ZrF4based glass, those of the ZnCl2-based glass are iO s1 which are smaller by one to two order. Therefore, the radiative

quantum efficiencies of these levels are still not negligible (0.4 - 0.5 %), and the emissions from these levels areobserved.

The theory of multiphonon relaxation for rare-earth ions describes the relaxation rate WMPR as inverselyproportional to the exponential of the energy gap1315,

p,/MPR[fl(7) 1 . (4)Here, C and a are host-dependent parameters, ziE is the energy gap, p is the number of phonons required to bridge thatgap, and n(T) is the Bose-Einstein distribution function. The parameters, C, a, and p are insensitive to the rare-earthions and energy levels. In Figure 2, the multiphonon relaxation rates of Er3 in the ZnCl2-based glass are plotted

against the energy gap together with those in other glass systems.118 The multiphonon relaxation rates of someexcited levels of NcP in ZnC12-based glass with the same composition as the present study are also plotted.7 The

dependence of the multiphonon relaxation rates for the ZnC12-based glass obviously follows the exponential law as well

as the other glass systems.

5. CONCLUSIONS

Judd-Ofelt analysis and emission lifetime measurements were performed of Er3 ions in the ZnC12-based glass. The 2

parameter sensitive to the local structure and bonding nature around rare-earth ions was large compared with that of theZrF4-based glass. This may be due to the relative covalency in the bonds between the erbium and ligand ions. The

multiphonon relaxation rates of the 4F512, 4F712, and 4F912 levels estimated from the calculated radiative transition

probabilities and the measured lifetimes were much smaller than those of the ZrF4-based and Ga2S3-based glasses

because of the low-phonon-energy property of the ZnC12-based glass. The dependence of the multiphonon relaxationrates for the ZnCI2-based glass followed the exponential law as well as the other glass systems.

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00

1

0 1000 2000 3000 4000 5000Energy gap to next-lower level (cm-i)

Figure 2. Multiphonon relaxation rates of rare-earth ions in various glasses as a function of the energy

gap to the next-lower level, open circle (Er) and squar (Nd); ZnCl2-based glass, closed circle (Er); Ga2S3-

based glass.

ACKNOWLEDGMENT

One of the authors (M.S.) acknowledges the support of the Research Fellowships of the Japan Society for thePromotion of Science for Young Scientists.

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