Optical properties and laser parameters of Nd3+-doped flouride glasses

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OPTICAL MATERIALS 1 (1992) 217-234 North-Holland

Optical properties and laser parameters of Nd3~-dopedfluoride glasses A. Tesar, J. Campbell, M. Weber, C. Weinzapfel Lawrence Livermore National Laboratory, University of California, Livermore, CA 94550, USA

Y. Lin, H. Meissner and H. Toratani Hoya Optics, inc., Fremont, CA 94538, USA Received 6 March 1992

Optical properties and laser parameters for 27 Nd3~-dopedfluoride glasses are reported. Included are glasses based on zirconium fluoride, hafnium fluoride, and aluminum fluoride and other glasses formed from mixtures of several heavy metal fluorides. Measurements were made of the 4F 41 3/2-. 1i/2 fluorescence spectra and the concentration-dependent 4F 41 fluorescence decays. Judd—Ofelt intensity parameters 4F were derived from absorption spectra and used to calculate the 312—. 1 ,~stimulated emission 2 the lifetimes rangedCross from sections 470 to 650 its. Results for these fluoride are comparedranging with values cross section and theradiative 312 radiative lifetime. showed only a small variation withglasses glass composition, from 2.2 BeF for to 3.4 pm 2-based glasses and for oxide and oxyfluoride laser glasses.

1.

Introduction

The numerous known inorganic glass forming systems [1] provide many possible hosts for laser ions, In the three decades since the initial report of laser action in glass, the spectroscopic properties of lanthanide ions in many different glasses have been in3~ vestigated extensive of Nd in silicate [21 [31,including phosphate [4,5], surveys fluorophosphate [6], and fluoroberyllate [7] glasses. Additional work has been reported for lanthanide ions in beryllium fluoroaluminate [8] and several heavy metal fluoride glasses [9—16].In many cases significant changes in spectroscopic properties, linear and nonlinear refractive indices, and laser parameters are observed when the chemical composition of the glass is altered. These arise from changes in both the glass network forming and network modifying ions. From systematic compositional studies, relationships between composition and properties have been identified which have led to general rules for tailoring optical and laser properties by compositional changes [17—19]. In this work we extend the studies of fluoride based

glasses by investigating a number of new compositions reported in recent years, principally involving a wider compositional range of heavy metal fluoride and aluminum fluoride based glasses. Of the known fluoride glass systems, beryllium floride glasses are generally the most stable and have the largest cornpositional ranges glasses, for glassin formation. Heavy metal fluoride (HMF) contrast, have narrower regions of glass formation [20]. Aluminum fluoride glasses can exhibit higher chemical and mechanical durability than the HMF glasses, but tend to be unstable with respect to devitrification [211. However, small additions of AIF 3 to HMF glasses often increase the glass stability [22]. Processing to control devitrification and hydroxyl impurities is extremely stringent for most HMF and aluminum fluoride glasses. Nevertheless, because of their extended infrared transmission (to 10 i.tm) and smaller probabilities for nonradiative decay of excited rare earths by multiphonon emission [23], fluoride glasses have received increasing interest for applications such as infrared fiber optics, infrared windows, and ultra-lowloss optical fibers [20,24,25]. We have measured the spectroscopic and optical

0925-3467/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

‘-~

217

Volume I, number 3

OPTICAL MATERIALS

properties of Nd3 + in a range of glass-forming fluoride compositions and compared the results with those of various oxide and oxyfluoride glasses. The glass compositions used in this study were chosen from a variety of HMF and fluoraluminates reported in the literature as forming relatively stable glasses. In some instances minor compositional variations were made to make the study more systematic, although we made certain that the compositions still remained within the reported stable glass compositional space. Included are (i) several glasses where zirconium fluoride or hafnium fluoride is the dominant component, (ii) glasses containing large quantities of aluminum fluoride (but still less than 50 mol.%), and (iii) various mixed fluorides containing ZnF 2, PbF2, or CdFT, as major components. Special facilities were developed to handle beryllium and thorium fluorides so that the effects of these additives could be investi3~,the gated. the resultsfor are neodymium specific to Ndare exgeneralAlthough trends observed pected to be applicable to the spectroscopic properties of other trivalent lanthanide ions.

2. Experimental 2.1. Glass melting and sample preparation The majority of the glasses were produced by Hoya Optics, Inc. in the special glove box constructed to handle hazardous materials. The glasses were made from 99.9% pure powdered chemicals which were batched in 10—20 g quantities in a laboratory atmosphere and transported into the nitrogen atmosphere glove box through an antechamber. The antechamber could be evacuated and then backfilled with the glove box gas to avoid introducing outside contamination. The batch materials were poured into 50 ml vitreous carbon crucibles, covered with lids, bottom-loaded into the furnace, and melted at 900°C for two hours. Dry, oxygen-free conditions were maintained inside the glove box by constantly circulating the N 2 gas through a drying column (Vacuum Atmospheres, Inc., Dri-Train Model M040) at a rate ofabout 8 liter/mm. During melting, freon 12 (in a nitrogen carrier gas) was introduced directly into the melt zone of the furnace at a rate of 0.5 1/mm to react with and vent any free oxygen or water. 218

September 1992

This added gas and any melt off gases were vented through a gas lock to a scrubber system. After two hours the crucibles were removed from the furnace and the glasses allowed to cool in the crucibles to reduce thermal shock. A two-hour anneal at a ternperature slightly above the glass transition temperature (—.+ 10°C) followed. Although the optimal melting conditions, casting process, and anneal temperatures are expected to be different for the various compositions included in this study, because of the large number of melts the same melting process was used for all compositions. Due to the hazardous nature of the glasses contaming beryllium and thorium fluorides, grinding and polishing of these samples were performed in a separate isolating glove box using powdered alumina of various grit sizes in a water and glycerol slurry. Polished ~ samples optical spectroscopy had dimensions 5 x 5 X for 2 mm. 2.2. Optical spectroscopy Absorption spectra in the 200 to 1000 nm region were recorded with a resolution of <0.2 nm using a Cary 171 spectrophotometer. A computer-controlled system was used for data collection and analysis [26]. Fluorescence was excited with the 3 50—600 nm output of a filtered xenon arc lamp and recorded with a 0.3-rn McPherson grating monochromator equipped with a PbS detector. Fluorescence decay measurements were made using a xenon flashlamp (-.~3 ~.tsduration), an S-l photomultiplier, and digital signal averaging. All measurements were performed with the samples at room temperature. The experimental uncertainties in the fluorescence linewidths and decay times were typical ±0.2 nm and ±5 j.ts, respectively. -~

2.3. Differential scanning calorimetry The glass transition temperature Tg, the temperature at the onset of crystallization T~,and the ternperature at the crystallization exotherm maximum T~were measured by differential scanning calorimetry (DSC) using a DuPont Model No. 9900 analyzer. Examples of DSC curves are shown in fig. 1. Samples for DSC analysis were small chips (10—20 mg) of glass obtained prior to annealing. These were

Volume 1, number 3

0.11

OPTICAL MATERIALS

265C 0

of 600—200°C.The rate of temperature change was 10°C/mm.

369°C

(a)

September 1992

Tc

-~___~_—

2.4. Other measurements -0.1

Tg

346°C~

Glass densities were determined from the buoyancy upon immersion of deionized water or kerosene. The refractive index at the sodium D line (589

ample 5632 : ZBLAN)

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spectra at 3 p.m. In suggests all cases,the no OH detectable peak was observed. This contentOH is below contaminated with OH, we measured the absorption 1 ppm.

Tc ~8 2

nm) and the reciprocal dispersion (Abbe number) were measured using a Bausch & Lomb Abbe refractometer. Several glasses were analyzed for Nd content using inductively-coupled-plasma mass spectroscopy (ICP/MS). The analyzed values were generally determined to be within ±10% of the nominal concentration. To verify that the glasses were not

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3.1. Glass formation Compositions forming samples sufficient for op-

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Sample 5670: BeAMgCB 400 500 Temperature (°C)

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Fig. 1. Representative DSC curves for fluoride glasses. (a) Topzirconium fluoride glass 5632; (b) aluminum fluoride glass 5678, and; (c) beryllium fluoride 5670 (see table 1 to correlate glass compositions with sample number).

carefully examined under a microscope to ensure that they of surface All DSC curveswere werefreeobtained withcontamination. a heating temperature range of 150—600°Cand a cooling temperature range

minate (8385), an Mn—Ga—lead fluoride, a Cd—Li— othermade sources; these additional included glasses a Ca—Ba—fluoroalualso for several obtained from Al—Pb (CLAP-960l) fluoride All glass, and an Al-alkali earth fluoroberyllate (5670). glasses were doped with a nominal 1 mol% NdF3. Included in the table are values of T~and Tg and references to original investigations of glass formation. Measured densities and refractive indices are given in table 2. The nonlinear refractive index n2 in the long wavelength limit was calculated from nd and p~using the expression [27] K(nd —1 )(n~+2)2 vd[l.52+(nd+2)(nd+l)pd/6nd]’

(1)

where K is an empirically 3 esu. derived constant having a A number of other fluoride compositions were value of 68x10’ melted but did not yield useful samples with the present preparation method; these are listed in an 219

Volume 1, number 3

OPTICAL MATERIALS

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Volume 1, number 3

OPTICAL MATERIALS

September 1992

Table 2 Glass density, Abbe number, and refractive indices at 295 K. Sample

Designation

3)

Density (g/cm

n~

n 2 (l0_i3 esu)

Zirconium fluoride 5640 ABL 5630 ZBLA 5655 ZBT 5601 ZBLAN)I 5656 ZBAT 5672 ZBALBe 5632 ZBLAN)2 5634 ZBAY 5663 ZBTLi

4.63 4.57 4.83 4.42 4.81 4.59 4.35 4.54 4.75

1.5341 1.5129 1.5290 1.5150 1.5217 1.5197 1.5017 1.5167 1.5229

72 75 76 86 75 74 78 71 75

1.02 0.92 0.93 0.73 0.91 0.94 0.82 0.99 0.93

Hafnium fluoride 5636 5638 5657 5658 5659 5660 5661

5.92 5.82 6.05 6.01 6.09 5.77 5.66

1.5242 1.5160 1.5302 1.5195 1.5270 1.5331 1.5341

75 77 76 79 74 73 71

0.93 0.87 0.93 0.85 0.96 1.00 1.04

Aluminum fluoride 8385 ACB 5676 ABC(M)0YL 5678 ABe(M)~Y 5621 AZ(M)OY 5674 ABeCB 5654 AYTB

3.70 3.56 3.66 2.88 3.43 5.01

1.4280 1.4178 1.4257 1.4407 1.4037 1.4946

95 99 86 93 97 87

0.47 0.43 0.55 0.51 0.42 0.68

Other fluorides 5670 5650 9056 9601

3.37 5.36 6.34 5.76

1.3926 1.5104 1.6363 1.5965

94 87 39 62

0.43 0.72 3.40 1.53

HBL HBLA HBT HBAT HBLT HBYLTCs HZBYLTCs

BeAMgCB ZnBAYT PbMnGa CLAP

appendix. These failures may be due to water- or oxide-contaminated batch preparation or, in some cases, to compositions in a region of the phase diagram subject to devitrification. 3.2. Absorption properties Spectroscopic properties of Nd3~are summarized in table 3. In the second column the integrated Nd3~ absorption in the 400—950 nm region relative to that of an Li—Ca aluminosilicate glass (Schott LG-670) is tabulated. Absorption spectra of Nd3 + in a heavy metal fluoride glass and in an A1F 3-based glass are shown in fig. 2; examples of typical silicate and phosphate glass spectra are also shown for comparison. The line strength of electric-dipole transitions be-

tween J states in the Judd—Ofelt treatment is given by [28] 2, (2) S(aJ, bJ’ t=~4,6 Q I I where a and b denote other quantum number specifying the eigenstate, the Q are the Judd—Ofelt intensity parameters, and the doubly-reduced matrix elements of the unit tensor operators U”~are calculated in the intermediate coupling approximation. Nine absorption bands in the 400—1000 nm region were integrated and least-squares fitted to derive a =

set of parameters Q0 (t= 2, 4, 6). The uncertainties in Q0 values given in table 3 are the standard deviations arising from the fitting and do not include Uncertainties in the Nd concentration or those arising 221

Volume 1, number 3

OPTICAL MATERIALS

September 1992

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Volume 1, number 3

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OPTICAL MATERIALS

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ide glasses showed only a small variation with cornposition, ranging from 1049 to 1052 nm. The stimulated emission cross section a at the peak wavelength A1, of the 4F312—~4I11/2 transition was cal

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101111

September 1992

Fig.295 at 2. Comparison K. ofNd

from the baseline subtraction or assumptions inherent in the applications of the Judd.-Ofelt approach. A comparison of the measured and calculated line strengths for two representative fluoride glasses is given in table 4. 3.3. Fluorescence properties The asymmetric 4F 41! 1/12 fluorescence band 3/2—8 representative glass types of Nd3~is shown for three in fig. 3. The band is inhomogeneously broadened and composed of transitions between 12 pairs of Stark levels. The fluorescence band was integrated and divided by the intensity at the peak to yield an

P

2+2 2 _____________

~ 27c L~.Aeff n 61]2} 14~]2+Q (3) x{Q4[U 6[U~ where the doubly reduced matrix elements are [U~41]2=0.l423 and [U161]2=0.4070. The radiative lifetime TR of the 4F 312 state was calculated from ~ = ~ A (4F 4I~-), (4) TR ~ 3/2’ “~

where the summation is over all terminal states and the spontaneous emission probability A for electricdipole transitions is given by .

.

.

=

.

.

.

-

64ir4e2 n n2+2)2 3h ( 2J+ l)A~ 9

(5) The only non-zero matrix elements in eq. (5) are for t=4, 6. The results for a and TR are summarized in table 3. The decay of the fluorescence from the 4F 312 state was generally not a simple exponential even at low Nd concentrations (0.1 mol %) due to site-to-site variations in the glass. As a measure of the nonex-

Table 4 Comparison of measured and calculated line strengths. Final state(s)

4F 2H 4F512, 4S 9/2 712, 31, 2H 4G 112 2F 4G512, 4G712 2K 4G712, 2G912, 2D1312 2K 4P 1112,2D 912, 3/2, 1512 112, 5,2

Wavelength (nm) 867 796 741 678 624 576 521 469 427

Glass 5663

Glass 9056

Smeas

S~01

Sms

Scai

0.95 3.06 3.37 0.59 0.19 4.39 1.77 0.50 0.13

1.02 3.27 3.29 0.22 0.06 4.41 1.49 0.33 0.14

1.03 3.08 2.58 0.21 0.07 3.12 1.24 0.17 0.11

1.07 2.89 2.69 0.19 0.05 3.12 1.36 0.31 0.16

223

Volume 1, number 3

1.0

OPTICAL MATERIALS

______________________________________ Fluorozirconate

wO.8 o C

-

SilICate (LG 670) I

(5640)

U C.)

0.6

-

~0.4

-

0

~

values for the three major compositional series of glasses prepared this study. As expected from other published work, inthe A1F 3 glasses in this study have a higher Tg (and T0) than the ZrF4 and HfF4 compositions. difference between T5 and T0 Moreover is larger forthe theaverage AIF3 versus the HfF4 and ZrF4 glasses in this study:

~8)

.2

~0.2

September 1992

-

Glass type

0 1000

(TS—TO)A.

__________________________________________________________________________

1040

1080 Wavelength (nm)

4F

1120

41

3°-in

Fig. 3. Comparison of the 312—. 1112 fluorescence of Nd fluoride, phosphate, and silicate glasses at 295 K.

ponential character, the first three e-folding times at which the fluorescence had decayed to e e 2 e ~ of its initial value are listed in table 3. The fluorescence decays of several glasses were measured as a function of the Nd concentration. The compositions studied are listed in table 5; most of the samples are from this study but some are from previous work. The effective lifetimes, obtained by averaging over the complete decay at a given Nd concentration, are plotted as a function of the square of the Nd concentration in fig. 4. —

4. Discussion

AIF3

—135°C

ZrF HIF4 4

—105°C —90°C

These data suggest a general decrease in glass stability with composition as A1F3 ~ HfF4 ~ ZrF4. The greater stability observed for the A1F3 glasses agrees with what is often reported in the literature. However, HfF4 and ZrF4 are usually assumed to have similar stabilities because their chemistries (oxidation state and ionic radii) are nearly identical. In general, glass stability is expected to increase as the number of glass components increases (confusion principle) [1,20]. Although the average T0— T0 values above appear to predict the relative behavior for a class of glasses, we found that it does not always hold true for glassto-glass variations. For example, we observed that some fluoride glasses with (T0 T0) = 80°Cwere as easily processed as those with (T0 T0) = 120°C. More complicated formulas involving characteristic temperatures have been invoked, but they are not necessarily more reliable in accounting for the complexities associated with melt viscosity, crystal nucleation and growth, clustering, etc. [29]. —

—

4.1. Glass formation Although the values of T0, T~,and Tg vary for the different glass compositions, the results provide guidance for predicting the ease of glass formation and stability. For example, fig. 5 compares T0 and Tg Table 5

3~fluorescence decay (M denotes mixtures ofdivaleni

Compositions of base glasses used in the concentration quenching studies of Nd and trivalent cationh). Sample number

Designation

Composition (mol.%)

B 101

Fluoroberyllate

49BeF

5621 LG-8 12 5650 5601

AZ(M )0Y Fluorophosphate ZBAYT ZBLAN HBLYCs

2—1 OA1F3— I 4CaF2—27KF 35A1F3—l OMF3—50MF2—5NaF 3OAIF3— 1 OMF3—50MF2—5NaF—5P2O5 3OZnF2—2 1 ThF4—2OBaF2— 1 4A1F3—1 6YF3 56ZrF4—24BaF—6LaF3—4AIF3— 1 ONaF S6HIF4-30BaF2-5LaF,-5YF3-4CsF

-

224

Volume I, number 3

8000,,

OPTICAL MATERIALS

Figure 6 compares the refractive index and Abbe number for the fluoride glasses in this study with those for fluoroberyllate glasses reported elsewhere

l’’’I’’’l’’’l’’’ S

o FlOoroberyllate (BlOl)

•

AZ(M)~V(5621) • HBLVCs • ZnBAYT (5650) V

6000

-

U

~ 400o C.)

-

o

ZBLAN(5601)

-

-

0

•0

z

2000—

-

-

• •

I I I 2(1020/cm3)2 20 40 60 80 [Nd—concentration]

0

100

3~4F

Fig. 4. Effective Nd

312 fluorescence decay times (at 295 K) for several fluoride glasses plotted as a function of Nd concentration. The sample compositions are given in table 5.

_________________

400

_________________

I

(a)

I

I

260

600 (b)

I

September 1992

I

I

[30]. Included in fig. 6 are lines of constant n2 calculated from eq. (1). The nonlinear refractive index is correlated with the linear index and dispersion, therefore fluoroberyllate glasses have the smallest optical nonlinearities [311. Fluorides can be added to silicate and phosphate glasses to reduce the refractive index. In an effort to reduce the nonlinear index of fluorophosphates further, glasses with increasing quantities of A1F3 and other fluoride cornpounds were developed [6]. Eventually it was possible to form glasses in which P205 was eliminated completely [21], such as the fluoroaluminate glasses A1F in table 1. Note that these are invert glasses in that 3 is present in less than 50 mol.%. The HMF glasses, because of the large content of polarizable metal ions, have refractive indices in the same general range of those obtainable with silicate and phosphate glass compositions. However, the dispersion values for HMF glasses are generally less than those for the pure oxide systems.

540

-

I 200

I

-

-

—

I~

I

420 OZr,HtHMF

240

350

-

0• AIF3—glasseS BeF~—glasses

00 0

200 0

I ZrF~

I HtFa

I AIF0

300 0

I ZrF4

I HIF4

I AIF3

-

00

o

“.

1.5

0

0

-

Glass type

Fig. 5. Comparison of measured values for (a) T5 and (b) T~ for three broad classesof fluoride glasses prepared for this study: AIF3, ZrF4 and HfF4 glasses.

“ j

•.

-

•

°‘. 0

-

4.2. Refractive index

S

0

,“~

1.0 ‘

.

-

0

-

Fluoride glasses encompass a wide range of cornpositions. With respect to their refractive indices and dispersion (expressed by the Abbe number ~ they generally fall into the following ranges: __________________________________________________________________________________________

Glass type

Refractive index Abbe number

Fluoroberyllates Fluoroaluminates Heavy metal fluorides

1.3—1.4 1.4—1.5 1.5—1 .6

(ad)

95—110 85—100 70—85

________________________________________________________

1.3

0

~

0

060

0.35

--

-

-‘

toesu~ 0.25

1.2

120

I

100

n~io-

I

80

I

60

40

Abbe number (°~)

Fig. 6. Refractive index (at sodium’D line) versus Abbe number for fluoroaluminate, fluorozirconate, fiuorohafnate, and fluoroberyllate glasses. Also shown are lines of constant nonlinear refractive index n 2.

225

Volume 1, number 3

OPTICAL MATERIALS

4.3. Fluorescence wavelengths and linewidths The peak emission wavelengths and effective bandwidths for the Nd3~fluorescence in various fluoride glasses are shown in figs. 7a and 7b. In general the peak varied between 1.049 to 1.051 mm foremission the fluorozirconates, fluorohafnates, and fluoroaluminates used in this study. These can be compared with BeF 2-based glass [2,30] where there is a measurable shift to shorter wavelengths (fig. 7a). This difference in transition energies is often qualitatively related to the degree of covalent bonding between the cation (Nd) and the associated ligand; the more covalent the bond, the less electron—electron interaction and the lower the transition energy. This reduction in transition energy with increased ______________________

I

I

1051

1.049

-

-

u 1.047

-

---

-~

I

I

I

IHMFI

(HMF)

AIF

1.04C

I

30

I

I

I

3 Glass type

145—60%)

(>90%)

I

I

I

covalency is often referred to in textbooks and articles as the nephelauxetic or “cloud expanding” effect. Using the above arguments, the data in fig. 7 suggest that the BeF (withcomposed a lower A~) nteractZr, lessHf, with 3 + ions than 2those of ieither or Nd However, the electronegativity difference beAl. tween F and Zr, Hf, Al, or Be are nearly identical (table 6). Therefore based on these criteria these glasses would be expected to have about the same degree of ionic (or covalent) character and nearly equivalent peak emission wavelengths. It is interesting compare the emission lineshape 3~in simpletoBeF for Nd 2 versus Si02 glasses (fig. 8). Table 6

Estimated ionic character for various fluoride (based on Pauling electronegativities). Bond Electronegativity Cation field 2) difference (A~,) strength (eZ/a Be—F 2.5 20.8 Al—F 2.5 12.0 Ba—F 3.1 1.1

and oxide bonds

La—F Zr—F Hf—F Th—F Si—O P—O

88 82 84 84 49 39

01

2.9 2.6 2.7 2.7 1.7 1.4

=% ionic

Estimated ionic character % a) 79 79 91

2.3 6.3 6.1 4.4 23.8 43.3

characier= 100[ I —exp( _4

I

)2/4)

I BeF2

-

-

~25-

20

September 1992

(\

-

I

~

-

-

-

.?O.4

-

U

15

I

ZrF4 (HMF)

I

I

I

HfF4 (HMF)

AIF3

BeF2 (45—60%)

I

Glass type 3°4F 41 and, (b) Fig. 7. Comparison of (a) peak emission wavelengths effective emission bandwidths for the Nd 3,2—. 1112 fluorescence and minate, in fluoroberyllate various fluorozirconate, glasses. fluorohafnate, fluoroalu-

226

0.2-

—

BeE2 (v.90%) I 1000

1050

I 1100 1150 Wavelength (nm)

1200

3

Fig. 8. 41 4F Comparison of the fluorescence lineshape for the Nd 312—v 1112 fluorescence in Si02 versus BeF2 glasses.

Volume 1, number 3

OPTICAL MATERIALS

The radii of Si and Be and of F and 0 are similar and both Si and Be are tetrahedrally coordinated. In fact, BeF2 is often thought of as a “model” glass for Si02 [32]. However, apart from valency, a major difference between these glasses is the degree of ionic character in the bond; approximately 80% for Be—F versus 50% for Si—0 (table 6). The larger degree of ionic bonding in the BeF2 system is therefore responsible for the lower electron delocalization (nephelauxetic effect) and hence a higher emission energy as shown in fig. 8. Figure 3 compares the 1/2 emission bands for two representative multicomponent oxide glasses and a fluorozirconate glass. Again the higher transition energy for the HMF glass is related to the greater degree of ionic bonding. Both BeF2 : Nd and Si02 : Nd glasses exhibit line shapes that are similar in character yet different in linewidth. This suggests are that very the similar distributions of 3~ion environments in both Nd although the higher charge on the oxygen leads cases, to significantly greater energy level splitting, The extent of the Stark splitting of the lanthanide J states, and therefore the effective linewidth, is generally smaller in fluoride glasses than in oxide glasses because ofthe smaller field of the monovalent versus divalent ligands (figs. 3 and 8). Ions in the second coordination shell also affect the Stark splitting and effective linewidth. For example, in simple two-component fluoroberyllate glass it has been observed that the smaller the ion field strength [charge/ (ionic radius ) 2] the smaller the linewidth [33]. This was illustrated by the splitting of the 4F 3~ 312 state of Nd in simple BeF 2 glass compared to that in 55BeF2— 45RF (R=Li, Na, K, Rb) glasses where the replacement of small divalent ions with monovalent alkali ions beryllium greatly reduced the larger Stark splitting and inhomogeneous linewidth [33]. However for complex, multicomponent fluoride glasses, the variation in effective bandwidth with field strength is not obvious. For example, fig. 7b cornpares the measured bandwidths for the fluoride glasses in this study with multi- and single-component BeF2 glasses. Based on a simple field strength argument one would expect the bandwidth to increase in the order: Hf< Zr
September 1992

The fact that the field strength argument does not apply well to the glasses in this study is probably related to the fact that both the modifiers and the network formers are being varied and structural factors enter. In previous work on phosphate glasses in which just the modifiers were varied, a clear correlatidn of effective bandwidth with compositionally-averaged modifier field strength was observed [191. However, when both the modifier and network former are both simultaneously varied, no such correlation is obtamed [6,34]. The linewidth ofoptical transitions in phosphate glasses are generally narrower than in silicate glasses although in both cases the lanthanides have oxygen ligands. This illustrates the concomitant importance of local glass structure and cornposition in determining spectroscopic properties. The ranges of variations of L\Ae~ in fig. 7b for the various glass types are This generally small, due10%, forrelall modifier ion changes. is probably to the atively small changes in the molar concentration of the modifiers in the present glasses. The linewidths of the Hf and Zr glasses in which these fluorides are major components are comparable to phosphate glasses, although they are not as large as those observed in many silicate or borate glasses [30]. Table 7 contains comparisons of spectroscopic properties of Nd3~in fluoride and oxide glasses with similar refractive indices. 4.4. Transition probabilities The line strengths of electric-dipole transitions change significantly between fluoride- and oxidebased glass compositions. This is demonstrated in 3~absorption spectra of two fig. 2 where theare Nd fluoride glasses compared with those of phosphate and silicate laser glasses. The relative band intensities change, particularly the peak at —580 nm. The band intensities depend upon different linear combinations of the Judd—Ofelt intensity parameters in table 3. The band at 580 nm is dominated by contributions from Q 2 and this parameter exhibits the largest change with glass composition (tables 3 and 7). Since the Q2 parameter involves the lowest order terms in the expansion of the local electric field, it reflects the asymmetry of values the local 3~site. The small forenQ vironment at the Nd 2 observed for fluoride glasses suggest a more centro227

Volume 1, number 3

OPTICAL MATERIALS

symmetric coordination environment. The general variation in Q2 values with relative integrated absorption for the fluoride glass compositions included in this study are shown in fig. 9. For comparison, we have also included previous data reported for Bebased fluoride glasses [30]. The data shows that BeF2 glasses containing about 45—60 mol % BeF2 have the lowest Q2 values suggesting a nearly symmetric local electric field. Among the glasses in this study, the A1F3 glasses have, on average, somewhat lower Q2 values. Note, however, that the range of values slightly overlays those of the Hfand Zr heavy metal fluoride glasses. 3~absorption in the region 400—950 nm, which Thefrom relative integrated Nd is of interest for flashlamp-pumped laser action, is shown in fig. 10 for various fluoride glasses. In general, integrated absorption is highest for the Zr and Hf glasses. The high BeF 2 content glasses have the lowest absorptions, although those with modest BeF2 concentrations (45— 60%) have integrated absorptions that span the entire range of glass in this study. The values for fluoride glasses are generally smaller than the values of 0.9—1.1 typical of many silicate and phosphate laser glasses [30]. This is a consequence of the smaller 3~line strengths in the fluoride glasses, as reNd in the smaller Judd—Ofelt parameters, and the flected effective absorption linewidths. The total integrated dent on Q absorption is only rather weakly depen2 alone (fig. 11) in spite of the fact that it

a 0~I 0’ ‘C) ‘5

000

r-)’0

0000

‘5

‘5 ‘C) en

0~0 en en en en

-~

‘0,

00

00 ‘0

v-a r- C—i

‘0 —

‘500

r—

v’ —

~

‘~

~

Cl

0. 0

— —

.E

~

~ )0

0:

-

en’5

SC) —

‘0

—

‘0

en

0

C--

0

—‘5

‘0 —

~ ‘0

0’

E

‘0

Ci’

~

——

‘5r-a

c-i

~

0 --

.0

C) c-I

~

I ‘0 <— 1.0 V 0 U

‘0

0 U 0)

‘5 00 coy—

en 0’ en r- r- —

d

—

—

en ‘C)

en en

c-C en c

0

— ~

Sfl

0

en 000

d

— IC)

0

0

r— en C) SC) 0 0

en ‘5

00

c-i en

c

‘5

—

—

~

~0 SC) 0 0

0

narrower

0 I..

0 0

0~

.0

-~0c

r-’5en

en—en

d 0) 0.

‘0, 0 0

4.0

o

0’

00 ‘0

0 0

(~10~ 0’

‘0’~

—

‘0’.’0-~

~

—

~

‘0

~ — — ‘0 ‘5r— 0’ 4,’C)’C)

0) ‘0

0) 0, -a, U C)

0.

0) 0.

UO l-’U

U ol)

~

E

-—

U -

U0.

+

0)

U ‘0

0.0 -.0,

0)

00 ~ CC

Nv.0 0~0, ~ 0.0.0

v U

.0

‘2)

U 0

~2

3.0-

E

0)

00

-

g

1.0--

U -~

— c-a ~

00

0

00 ‘0

o’v~l~

00

ç5

0 C-‘0

0

v-‘0

s-~~

—

~

C) ,~

-

-

.00) ~ ~

I

U

U 0)

I-

0

I

c0

C-- 00

0

Z0

September 1992

0

I!

I

I

ZrF4

I

I

HIF4

A!F3

Glasstype

BeF2

-

BeF2

(45-60%) (v.90%)

Fig. 9. Range of values for the Judd—Ofelt parameter Q2 as a function of fluoride glass type.

228

Volume I, number 3 1.2

OPTICAL MATERIALS

September 1992

4.0

11111

I

I

I

:iJ~:L !o6

:~-~

I

:

~ 0.4

0.2

e0

-

Glass type

-

S

-

I 0.4

I 0.8

Zr,HCHMF ~AlF 0 9O%( 0 BeF0 (45-60%) 0 BeE2 (e I I 1.2 1.6

0

~2.5-

0.7

I 2.0

~.

2.4

-

-

-

-

-

.~-

-

~j

-

-

-

I

ZrF 4

HfF4

A1F3

-

1.0

I

ZrF

I

4

-

~ 0.5

-

-

E

3C- between 450—900 Fig. for nm 10. various Total integrated fluoride glass absorption types (relative of Nd to LG-670 silicate glass).

-

-

1.5

a,

0(pm

0.9

2 2.0 Ui

2)

-

BeF2

BeF2

(45-60%) (>90%) Glass type

Fig. 11. Total integrated absorption relative to LG-670 silicate glass versus Q2 for fluoride glasses in this study and BeF2 glasses reported elsewhere [301.

HfF4 AIF3 Glass type Fig. 12. Range of stimulated emission cross sections for the Zr,

Hf, and Al-based glasses determined in this study.

because of the larger effective linewidths. The optical properties of Nd3~in several other HMF glasses of similar and different compositions from those studied here have been investigated by other researchers [9—16]. Those results are summarized in table 8. Although the compositions are not identical, the values of the Judd—Ofelt parameters and simulated emission cross sections are, in general, in reasonable agreement with those reported here, with the notable exception of the values reported in refs. [121 and [15]. The results in ref. [15] for the 20 mol.% alkali zirconium fluoride glasses are unusual in that they do not exhibit a systematic vanation in the series Li—Na—K as occurs for silicate [3], phosphate [4], and fluoroberyllate glasses [7]. 4.6. Radiative lifetime

is the sole contributing factor to the strong band at 580 nm. 4.5. Cross sections stimulated crossofQ section in eq. (3) is The determined by a emission combination 4, Q6, and &~.eff. Because of the small variations of these with fluoride glass composition, the rangequantities of cross sections for the fluorozirconate and fluorohafnate glasses extend over only about 20% (fig. 12). The cross sections for the fluoroaluminate glasses exhibited an even smaller range of values, about 10%, and tend to be lower than for the other fluoride glasses

4F The radiative lifetime of the 312 state also depends only on the Q4 and Q6 parameters which, as noted above, do not vary much with fluoride glass composition. The correction forathe local field on at the 3~site, however, introduces dependence the Nd refractive index2 (see (3)). Thus,(fig. as shown bylifethe + 2)2eq.dependence 13), the fit to the 1/n ( n times of the BeF 2 and fluoroaluminate glasses are longer than for HMF or oxide glasses largely because of the smaller refractive indices of these glasses. The radiative lifetime calculated from Judd—Ofelt intensity parameters represents an effective average 3~enover the site-to-site variations in the local Nd 229

Volume 1, number 3

OPTICAL MATERIALS

September 1992

Table 8 4F Judd—Ofelt intensity parameters and

41 3~in other fluoride glasses. 3/2—. 11/2 stimulated emission cross sections reported for Nd Q 2) Q 2) Q 2) a (pm2) Ref. 2 (pm 4 (pm 6 (pm

Glass type ZnF 2—CdF2 base ZBAN ZBLA ZBLAN HBLACs PBLA ZBN InF3—BaF2 base ZBAL ZBAN ZBAK

1106

1.40 3.09 1.10 2.10 2.36 1.01 1.95 24—28 7.57 9.42 3.75

I

2.77 3.65 3.80 3.71 4.48 3.73 3.65 8—23 2.54 2.31 5.45

I

C’

0 AlE0 0 BeE2

o •

0

0 500

-. .~

~

-

-

-

/nl000tt 1

-

-

O~’~ 700

(45-60%) C’ BeF2 (o90%I

8 0

~r~’ 0

0 -

—

As

I 300

1.3

1.4

1.5

1.6

Fig. 13. Calculated radiative lifetime versus refractive index for various fluoride glasses. The dashed line shows the expected effeet of the refractive index variation alone (assuming a radiative lifetime of 500 xs at ~D= 1.50).

vironments in a glass. In glasses where the Nd content is low (—~0.1 mol.%) so that nonradiative relaxation by ion—ion interactions is 4Fnegligible, the calculated radiative lifetime for the 3/2 state is in reasonable agreement with the measured average fluorescence lifetime. This is consistent with expectations of high radiative quantum efficiency and negligible nonradiative decay for the heavy metal fluoride glasses [23]. Because of the lower vibrational frequencies of HMF glasses,4F higher 41order processes energy are needed conserve the 3/2—8 15/2 transition in a to purely multiphonon process and hence are less probable than for most oxide glasses. The predicted unit efficiency is also in agreement, within experimental uncertainties, with recent quan230

3.94 3.2 —

2.64 —

2.75 2.90 11—36 10.3 14.3 6.6

[13] [161 [101 [141 1111 1101 [9] [12] [15] [15] [15]

tum efficiency measurements made using an integrating sphere [35]. It is well known that small quantities of OH in the glass structure can significantly reduce the fluorescence lifetimes of lanthanide ions. This may account 3~lifetime (particularly for those glasses melted several years ago as part for some reduction in the Nd of another study and which may have some OH contamination), however this effect was generally small and not investigated in detail.

I OH). Zr ilasses

900

5.77 5.74 5.53 4.62 4.72 6.19 4.17 28—50 13.85 19.42 8.95

The calculated fluorescence branching ratios from the 4F 372 states to the four “Ii states depend upon the ratio of £24 and ~6 [3,36]. The smallest ratio, and hence the smallest branching ratio to ~I /2~ occurs for Zr—Li—Ba—Th glass 5663. For most of the glasses in table 3, Q6 is approximately equal to or slightly larger than £24. The predicted branching ratios to the J=9/2, 11/2, 13/2, and 15/2 states were in the ranges 0.39—0.42, 0.48—0.51, 0.087—0.096, and 0.004—0.005, respectively, for all glasses in this study. For the ZBLAN glass 5670, for example, the branching ratios were 0.41, 0.50, 0.09, and 0.005. 4. 7. Fluorescence quenching At concentrations of ~ 1 mol.%, trivalent lanthanide ions can interact by electric multipolar processes leading to energy migration and concentration 3~this quenching of fluorescence. occurs via pairs of 4F 41In the41case of 41 Nd 312-4 1512: 912--* 15/2 transitions followed by nonradiative decay of the two ions to the ground state. The fluorescence decay times in table 3 exhibit nonexponential behavior and are

Volume 1, number 3

OPTICAL MATERIALS

shorter than the predicted radiative lifetimes because of this additional concentration quenching process. A dipole—dipole quenching process inversely proportional to the sixth power of the ion— ion separation, and thus to the square of the Nd concentration, has been confirmed for several fluoride glasses [37]. effective The measured concentration dependences ofthe fluorescence decay times plotted as a function of the square of the Nd concentration in fig. 4 confirm the presence of this quenching mechanism. This part of the study has recently been published by one of us (Tesar) and the reader is referred to ref. [37] for further details. 4.8. Local structure The spectroscopic properties of Nd3~ line strengths, linewidths, cross sections are affected by the local structure and bonding at the Nd3~site. For most glass forming systems characterized by the continuous random network model (e.g., silicates, phosphates, borates, fluoroberyllates, ...), neodymium ions play the role of network modifiers. The structunes of HMF and other fluoride glasses are not as well established. In some of these glasses neodymium may play the role of a network modifier or, for ZrF 4 glasses, a network former [10]. Many multicomponent, fluoride glasses mayInbetter descnibed by a invert random packing model. eitherbecase, the Nd—F polyhedra are expected to have large fluoride coordination numbers, ~ 7. The local structure and coordination in fluoride glasses has been investigated using X-ray and neutron diffraction [38—42], extended X-ray absorption fine structure (EXAFS) [40,43,44], optical spectroscopy [10,33,45,461 and computer simulations [39,42,47—531.The results indicate that Zr and Hf are coordinated by 7—8 fluorines with the polyhedra being joined at corners or edges [53]. Aluminum is found to be six-fold coordinated [39,51], although distorted A1F 7 polyhedra have been reported for a 40A1F3—20YF3—4OCaF2 glass [42]. Large alkaline earths such as Ba have fluorine coordination numbers of 10—il [51]. The coordination number varies with composition. In a series of BaF2ZrF4 glass simulations, the Zr coordination number (CN) decreased from 8.4 to 7.1 while the Ba CN increased from 8.7 to 10.3 with increasing Ba content. Whereas —

—

September 1992

the average CN for Nd3~in simple BeF2 glass is approximately 7 [42], it increases to approximately 9 for large additions of alkaline ions [43]. These EXAFS findings are in general agreement with predictions based on molecular dynamics simulations of the local coordination [46—48]. In a careful3 +deterin a mination of the fluorine coordination of Dy Dy-Na fluoroberyllate glass [41], a value of 7.3 was found compared to a simulation value of 7.8. (The coordination number will be larger for a larger lanthanide ion such as Nd3 ~ A trivalent lanthanide coordination number of 9.5 has been reported from other heavy metal fluoride glass simulations [53]. As noted by Simmons et al. [52], differences in molecular dynamics simulations can result from differences in the calculation algorithms. Finally, a Mossbauer study [54] of fluorozirconate glass has concluded that whereas Eu3~is tightly bound in well-defined sites, Eu2~is less tightly bound in ill-defined sites having a fluorine CN ranging from 8 to 12 and thus is more like Ba2t There are also site-to-site variations in the local structure and coordination number which give rise to inhomogeneously broadened spectra. These siteto-site variations have been investigated using laserexcited fluorescence line narrowing techniques [55]. Brecher and 3~using Riseberga [45] have interpreted their redistorted nine-fold coordinasults for Eu tion and a point charge model for the local field. Although a general picture of the average coordination and possible site-to-site variations in the local atomic structureat the Nd3 + site emerges from studies such as those cited above, our knowledge and understanding is still too primitive to be useful in predicting or accounting for the compositional dependence of spectroscopic properties in multicomponent glasses.

5. Laser applications In table 7 the optical and spectroscopic properties of several Nd-doped fluoride glasses are compared with corresponding properties for commerciallyavailable oxide and oxyfluoride laser glasses. Included are two silicate glasses (LG-660and LG-670), two phosphate glasses (LHG-8 and LHG-80), and a fluorophosphate (LG-8 12) glass. Since fluoride 231

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glasses span a range of refractive indices, the glasses are grouped by comparable indices of refraction, Both energy storage and energy extraction affect laser operation and efficiency. For broadband optical pumping, the stored energy is dependent on the effective absorption linewidths and line strengths. For xenon flashlamp pumping, the relative absorption of the fluoride glasses is less than that of other glasses in table 7. This is a consequence of the smaller linewidths and smaller Judd—Ofelt parameters. To cornpensate for this, the fluoride glasses can be more

have not been reported to our knowledge, therefore we cannot estimate the magnitude of this effect in fluoride glasses. Hole burning, however, has been shown to have only a small effect on extraction efficiency for silicate, phosphate, and fluorophosphate laser glasses at room temperature [62,631. For high-power lasers where beam quality is a consideration, self-focusing and beam breakup due to intensity-dependent changes in the refractive index are important. These effects are governed by the nonlinear index n2. Fluoroberyllate glasses have the

heavily4F doped because the rate of ion—ion quenching of the 3/2 state, which depends on the £24 and ~ parameters, energy to thewill 4F also be less. Cascade of the absorbed 3/2 state is expected to of occur very rapid multiphonon relaxation for all the byglasses [23,56]. Multiphonon relaxation of the 4F 3/2 state is negligible for fluoride glasses, but appears to reduce the lifetime and radiative efficiency of phosphate glasses by 10% at room temperature [57]. 4F This plus the longer fluorescence lifetimes of the 3/2 state of the fluoride glasses are favorable for flashlamp pumping. For large laser rod or disk components, amplified spontaneous emission (ASE) can limit energy storage [58,59]. The rate of ASE is proportional to the gain coefficient g= oAN, where MT is the inverted population density, times the longest dimension D of the energy storage medium. From both oxide and fluoride glasses, a modest range of s values is possible depending upon the composition. Thus for a given gain, there is a trade-offbetween a, stored energy (SN), and D. The stimulated emission probability also governs the rate of energy extraction. Phosphate glasses, because of the narrow emission bandwidths and large 4F line strengths for the 3/2 state, can provide significantly higher gain and extraction efficiency. All glasses manifest inhomogeneous broadening which can lead to spectral hole burning and reduced energy extraction efficiency under large-signal or saturated gain conditions [60]. Spectral hole burning is dependent on the ratio of the homogeneous to inhomogeneous linewidths. These widths have been measured of a number of different glass types and significant differences are observed [61]. Explicit measurements of the homogeneous of 3~in fluoroaluminate or in variouslinewidths HMF glasses Nd 232

lowest linear and nonlinear refractive indices of all glasses [64], which combined with long 3~ranging lifetimes, stimulated emission for Nd 2 [30],cross andsections satisfactory chemical dufrom 2 to[65], 4 pmmake them very attractive for highrability power laser application. The toxicity and hazards of handling beryllium compounds and the associated high cost has been a deterrent to their use [alexandrite (BeA12O4) and lanthanum beryllate (LaBe2O5) are commercially available materials for the crystalline laser community, however]. The next lowest index glasses are fluoroaluminates and fluorophosphates; these glasses are also attractive for high-power laser applications. The HMF glasses, due to their large content of heavy metals ions, have higher refractive indices in the range ofthose for various oxide glasses and hence offer no advantages with respect to laser beam breakup. In summary, fluoride glasses offer the laser designer a material having low refractive index, medium lasing cross sections, and moderately long fluorescence lifetimes. Their utilization will depend ultimately on the availability of the materials in the sizes and optical quality required and at acceptable cost. For several of the fluoride glass compositions, such as ZBLAN, these do not appear to be significant obstacles.

Acknowledgments We gratefully acknowledge the efforts of G. Greiner in the design and assembly of the glove box and the purge and reactive gas systems used for glass melting. In addition, we thank the following people for samples used in this study: D. generously Blackburn providing and W. Haller (National Bureau of

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Standards), sample 8385; P. Tick and D. Hall (Corning, Inc.), sample 9601; J. Miranday (Université du Maine), sample 9054. This research was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

Appendix The following glass compositions (mol.%) were melted, but they devitrified: • 4OZnF2—2OBaF2—26ThF4—4A1F3—4GaF3— 3YF3—2LaF3—lNdF3 • 3OZnF2—3OBaF2—30InF3—9LaF3—lNdF3 • 3OZnF2—3OBaF2—29InF3— 1 OThF4— 1NdF3 • 3OZrF2—2OBaF2—1OA1F3—9InF3—1OGaF3— 2OThF4—lNdF3 • 28ZnF2— 1 6BaF2—27InF3—28ThF4— 1NdF3 • 28ZnF2—l6BaF2—27YF3—28ThF4—1NdF3 • 45BaF2—44A1F3—1OZrF4—1NdF3 • 45BaF2—44A1F3—1OThF4—lNdF3 • 4OBaF2—39ZnF2—20YF3—lNdF3 • 3OBaF2—2OZnF2—29InF3—lOThF4—1NdF3 • 3OBaF2—22PbF2—29A1F3—8YF3—1OThF4— lNdF3 • 49BeF2—27KF—9A1F3— 1 4LaF3— 1 NdF3 • 4OThF4—39GdF3—2OBaF2—lNdF3 • 38PbF2—15ZnF2—29GaF3—7A1F3—lOThF4— 1NdF3 • 40A1F3— 1OBeF2—34CaF2— 1 5BaF2— 1 NdF3 • 34A1F3—5KF—1OBeF2—l5CaF2—15ZnF2— 1OZrF4—lNdF3 • 32.3A1F3—9.5NaF—1OBeF2—7.5MgF2— l2.2CaF2—1OBaF2—7.5ScF3—1NdF3 • 32.3A1F2—9.5NaF—5BeF2—9.5MgF2—13.2CaF2— 1 OBaF2—9. 5ScF3— 1 NdF3 The following compositions formed glasses, however samples of quality sufficient for optical spectroscopy were not obtained: • 63ZrF4—32BaF2—4GdF3—1NdF3 • 57HfF4—28BaF2—4CsF—3YF3—5LaF3—3NdF3 • 59LiF— 1 OBaF2—2OZrF4— 1 OThF4—1 NdF3 • 33.5A1F333.5PbF21OLiF1.OCdF2l2CdO 1NdF3 I

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