Crystal growth and spectral characterizations of Ho3+-doped Li3Ba2La3(MoO4)8 crystal

JOURNAL OF RARE EARTHS, Vol. 35, No. 4, Apr. 2017, P. 368

Crystal growth and spectral characterizations of Ho3+-doped Li3Ba2La3(MoO4)8 crystal SONG Mingjun (宋明君)1,*, ZHANG Nana (张娜娜)1, MENG Qingguo (孟庆国)1, WANG Lintong (王林同)1, LI Xiuzhi (李修芝)2, WANG Guofu (王国富)2 (1. School of Chemistry and Chemical Engineering, Weifang University, Weifang 261061, China; 2. Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou 350002, China) Received 2 July 2016; revised 22 September 2016

Abstract: A large Ho3+:Li3Ba2La3(MoO4)8 crystal with high optical quality and well-developed appearance was grown by the flux method. The main spectral properties of the crystal, including the absorption spectra, fluorescence spectra and fluorescence decay curves were recorded at room temperature. The Judd-Ofelt (J-O) theory was applied to calculate the oscillator strength parameters Ωt (t=2, 4, 6), spontaneous emission probabilities, fluorescence branching ratios, and radiative lifetimes of Ho3+ ions undergoing transitions from ground state 5I8 to the excited states. The stimulated emission cross-section for the 5I7→5I8 transition was estimated to be 1.32×10–20 cm2 at 2045 nm by Fuchtbauer-Ladenburg (F-L) equation and the quantum efficiency of the 5I7 level was calculated to be 89%. Keywords: crystal growth; optical properties; Judd-Ofelt theory; rare earths

Solid-state lasers operating in the eye-safe spectral region around 2 μm have a large variety of applications in fields of medicine, remote sensing, gas detection, etc. Among the rare earth ions, Ho3+ and Tm3+ are the most frequently studied active ions to realize laser operation in this spectral region. The 5I7→5I8 transition of Ho3+ ions gives rise to a strong emission band around 2.05 μm, which is characterized by high gain cross-sections and long-operating laser wavelength. However, Ho3+ ions have no absorption to match the emission of commercially available laser diodes, so it has to be co-doped with a second ion, e.g. Tm3+, Yb3+, Nd3+, which serves as sensitizer to improve the pump efficiency[1–4]. By contrast, the 3F4→3H6 transition of Tm3+ ions has been paid more attention for the development of 2.0 μm lasers in recent years, since Tm3+ ions possess a strong absorption band around 800 nm and can be efficiently pumped by the high power and well-developed AlGaAs lasers. Moreover, under the 800 nm radiation, the 3F4 upper laser level could be effectively populated through a cross-relaxation process (3H4+3H6→3F4+3F4), which means that a pump quantum efficiency near two can be expected. During the last decade, a new series of triple molybdate compounds, with the general formula Li3Ba2RE3(MoO4)8 (RE=La-Lu), have aroused extensive concern as promising solid state laser materials[5–10]. The main advantages of these crystals include moderate growth condition, broad absorption and emission bands, as well as high

quantum efficiency. Previously, we reported some physical and spectral properties of Nd3+, Er3+ and Dy3+doped Li3Ba2Re3(MoO4)8 crystals, which confirmed them to be good candidates for solid-state lasers[5–7]. Meanwhile, Cascales et al. demonstrated the efficient laser operations around 1.0 and 2.0 μm in Yb3+:Li3Ba2Gd3(MoO4)8[8] and Tm3+:Li3Ba2Lu3(MoO4)8[9] crystals, respectively. It is worth mentioning that with a Ti:laser as the pump source, a maximum output power of 515 mW around 2.0 μm, with a slope efficiency up to 71%, was obtained in Tm3+:Li3Ba2Lu3(MoO4)8 crystals, which is the best laser performance obtained in Tm-doped disordered crystal so far. However, as far as we know, the detailed spectroscopic properties of Ho3+ ions in these compounds have never been reported. So, in this paper a systematic investigation on the spectral properties of Ho3+:Li3Ba2La3(MoO4)8 crystal was presented, and the main spectroscopic parameters relevant to laser applications were calculated to access its potential laser performance around 2.0 μm.

1 Experimental In our previous research, it was found that Li3Ba2La3(MoO4)8 crystal melt incongruently and cannot be grown by the convenient Czochralski method[5]. As a result, the crystal was grown by the top seeded solution growth method (TSSG) using Li2MoO4 as flux. The ini-

Foundation item: Project supported by the Natural Science Foundation of Shandong Provincce (ZR2014JL029, BS2015CL012, ZR2015BM005) and the Project of Shandong Province Higher Educational Science and Technology Program (J14LA52, J15LA09) * Corresponding author: SONG Mingjun (E-mail: [email protected]; Tel.: +86-536-8785283) DOI: 10.1016/S1002-0721(17)60921-9

SONG Mingjun et al., Crystal growth and spectral characterizations of Ho3+-doped Li3Ba2La3(MoO4)8 crystal

tial Ho3+ concentration in the raw materials was 4 at.% and the molar ratio of Li3Ba2La3(MoO4)8 to Li2MoO4 is 1/5. A single crystal bar with [010] orientation was used as seed. Other details about crystal growth are similar to those of Er3+:Li3Ba2La3(MoO4)8 and Dy3+:Li3Ba2La3(MoO4)8 crystals[5,7]. The as-grown Ho3+:Li3Ba2La3(MoO4)8 crystal with dimensions of 50 mm×20 mm×10 mm was obtained, as shown in Fig. 1(a). We can see that the grown crystal possesses regular appearance and well- developed faces, and no cracks or inclusions is found within the crystal. However, the grown crystal is slightly opaque as the volatilized MoO3 corrods the surface of the crystal during the growth process. As the Li3Ba2La3(MoO4)8 crystal belongs to the monoclinic system, whose indicatrix principal axes are not totally consistent with the crystallographic axes, we must first determine the indicatrix principal axes for spectral measurement. In the monoclinic system, one of the indicatrix principal axes is parallel to the crystallographic b axis and the other two principal axes are in the same plane as crystallographic a and c axes but rotated. Detailed orientation process can be found in Ref. [7]. Since the precise values of the refractive index for the three principal axes were not measured, the three indicatrix principal axes are herein temporarily named as a', b' and c', respectively. As shown in Fig. 1(b), the principal axis collinear with the crystallographic b axis is named as b'; a' is clockwise rotated with respect to crystallographic a axis by 25º, while c' is clockwise rotated with respect to crystallographic c axis by ~26º, as the sample is viewed from the minus direction of b crystallographic axis. The accurate concentration of Ho3+ ions in the sample, which is an important parameter in the spectral analysis, was measured to be 2.43 at.% by the inductively coupled plasma atomic emission spectrometry (ICP-AES) method. Thus, the corresponding concentration and the segregation coefficient of Ho3+ ions in the grown crystal were 1.12×1020 cm2 and 0.61, respectively. The polarized absorption spectra were recorded using a spectrophotometer (Lambda-900, Perkin-Elmer) with a wavelength range

369

from 300 to 2200 nm. The polarized fluorescence spectra and the fluorescence decay curves were measured using an Edinburgh Instruments FLS920 spectrophotometer. All the measurements were carried out at room temperature.

2 Results and discussion 2.1 Absorption spectra and Judd-Ofelt analysis Fig. 2 shows the polarized absorption spectra of Ho3+:Li3Ba2La3(MoO4)8 crystals a measured at room temperature. For all polarizations, seven evident absorption bands around 360, 418, 455, 537, 643, 1160 and 1955 nm can be observed, which can be assigned to the intrinsic transitions of Ho3+ ions from the ground state 5I8 to the excited multiplets of 5G5+3H6, 3G5, 5G6+5F1, 5S2+5F4, 5 F5, 5I6 and 5I7, respectively. One can see that the shapes of the absorption spectra for the three polarizations show little difference, but the absorption intensities depend strongly on the polarizations and absorption bands for E//b′ are in most cases much more intense than those for the other two polarizations. Since it was put forward by Judd and Ofelt in 1962[11,12], the Judd-Ofelt (J-O) theory has been widely used in the analysis of spectroscopic properties of rare earth ions in crystals and glasses. According to the J-O theory, the experimental oscillator strength fexp of a transition from the ground multiplet 5I8 to an upper J′ multiplet can be obtained from the corresponding absorption band by the following equation: mc

2

∫σ

(λ )dλ (1) πe λ abs where m and e are the mass and charge of an election, respectively, and c is the velocity of light, q is the polarization of the absorption spectra, c is the velocity of light, λ abs is the mean wavelength of the absorption bands and ∫ σ aq (λ )dλ is the integrated absorption cross f exp, q =

2

2

q

a

sections. The total oscillator line strength includes both the electric-dipole (ED) and the magnetic-dipole (MD)

Fig. 1 Grown crystal of Ho3+:Li3Ba2La3(MoO4)8 (a) and relative position between the optical indicatrix and the crystallographic axes (b)

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where the reduced matrix elements of unit tensor operators U(t) ( t=2, 4, 6) can be found in Ref. [14]. ED

By a least-fitting between f cal ,q calculated by Eq. (5) ED

and f exp,q calculated using Eqs. (2) and (3), the oscillator

Fig. 2 Polarized absorption spectra of Ho3+:Li3Ba2La3(MoO4)8 crystals

transitions. Thus, it should be expressed as: f exp, q = f exp, q + f q ED

MD

ED

(2) MD

where f exp,q and f q

are the oscillator line strengths of

the ED and MD transitions, respectively. In most cases, the MD transition is much smaller than the corresponding ED transition and thus can be ignored. But for the 5 I8→5I7 transition of Ho3+ ions, the MD transition also provides a significant contribution to the results and should be calculated by the following formula: nh

fq ( J → J ′) = MD

6mc(2 J + 1)λ

(3)

4 f [α LS ]J L + 2S 4 f [α ′L′S ′]J ′ n

n

2

where h is Plank constant, n is the refractive index of the host, L and S are the quantum numbers of the total orbit and total spin angular momentum, respectively, α represents other quantum numbers that may be necessary to define the level uniquely[11], and L + 2 S is the reduced matrix element of unit tensor operators, which was reported by Carnall et al.[13]. The refractive indices n can be calculated according to Sellmeier equation: n = A+ 2

Bλ

2

− Dλ

(4)

2

λ −C where A, B, C, and D are Sellmeier parameters and were reported in Ref. [10].

The f

2

ED cal,q

2

are calculated using the following formula: 8π mc

( n + 2)

3h(2 J + 1)λ

9n

2

fcal,q ( J → J ′) = ED

∑Ω

t = 2,4,6

2

RmsΔf =

N

∑( f

(5) (t )

4 f [α ′L′S ′] n

2

ED exp, q

− f cal, q ) /( N − 3) MD

2

(6)

1

where N is the number of absorption bands used in the Table 1 Intensity parameters of Ho3+:Li3Ba2La3(MoO4)8 (Ho3+:LBLM) and other crystals Crystals

Ω2/

Ω4/

Ω6/

(10–20 cm2) (10–20 cm2) (10–20 cm2)

Ref.

Ho3+:YAG

0.04

2.67

1.89

[15]

Ho3+:Lu3Ga5O12

0.34

2.07

1.38

[16]

Ho3+:LuLiF4

2.24

2.31

1.44

[17]

Ho :NaLa(MoO4)2

16.19

4.21

1.00

[18]

Tm3+/Ho3+:LiGd(MoO4)2

18.90

5.99

1.41

[1]

Yb3+/Ho3+:NaGd(MoO4)2

17.43

5.41

1.52

[2]

Ho :LBLM (E//a′)

17.63

3.09

1.00

Ho3+:LBLM (E//b′)

20.57

7.74

1.32

Ho3+:LBLM (E//c′)

14.02

4.09

1.05

Ho3+:LBLM (eff)

17.40

4.97

1.12

3+

3+

×

4 f [α LS ] U n

t ,q

2

strength parameters Ωt (t=2, 4, 6) for three polarizations were obtained and are listed in Table 1, in which the effective oscillator strength parameter Ωeff is defined as Ωeff=(Ωa'+Ωb'+Ωc')/3. It can be seen from Table 1 that the oscillator strength parameters of Ho3+:Li3Ba2La3(MoO4)8 crystals are similar to those of other molybdate crystals such as Ho3+:NaLa(MoO4)2, Tm3+/Ho3+:LiGd(MoO4)2 and Yb3+/Ho3+:NaGd(MoO4)2. However, the values of Ω2 of Ho3+:Li3Ba2La3(MoO4)8 crystals are much larger than those of Ho3+:YAG, Ho3+:Lu3Ga5O12, and Ho3+:LuLiF4 crystals. Such phenomenon can be observed also in other Ho3+-doped molybdate crystals presented in Table 1. It is suggested that the Ω2 parameter of Ho3+ ion critically depends on the intensity of the hypersensitive transition of 5I8→5G6[19]. Generally, the stronger the transition of 5 I8→5G6 is, the larger the value of Ω2 will be. This can be deduced from the reduced matrix elements involved in the J-O calculation, i.e., the ||U(2)||2 for this transition is 1.52, whereas those for other transitions are generally below 0.22[14]. So, in the present case, the intensive absorption band around 455 nm is mainly responsible for the large value of Ω2 in the title crystal. Besides, the spectroscopic quality factor Ω4/Ω6, an important characteristic in predicting the stimulated emission in laser active medium, is calculated to be 4.44, which is higher than those of other crystals listed in Table 1, implying that the Ho3+:Li3Ba2La3(MoO4)8 crystal is a promising material for efficient laser action. To justify the results obtained, the root-mean-square deviation between the experimental and calculated oscillator line strengths is introduced as:

This work

SONG Mingjun et al., Crystal growth and spectral characterizations of Ho3+-doped Li3Ba2La3(MoO4)8 crystal ED

ED

above calculation process. The results of f cal,q , f exp,q and Rms∆f , as well as the relative error Rms error, are all displayed in Table 2. The spontaneous emission probability of the ED and MD transitions from J-multiplet to a lower J′-multiplet can be calculated by the following equation: 8π e n 2

ED(MD)

A

( J → J ′) =

2

Table 3 Spontaneous emission probabilities, fluorescence branching ratios and radiative lifetimes of Ho3+:Li3Ba2La3(MoO4)8 crystals Transitions 5

F4→5F5 5

I4

2

2

mc λ em

f

( J → J ′)

ED(MD)

(7)

70.8+7.8

91.5+8.0

61.1+8.0

0.7

50.0

34.7

0.3

427.9

264.4

2.6

783.5

8.1

5

755

1018.8

2504.0

1310.1

13.7

5

545

6458.9

12208

7666.3

74.6

3330

1.1

2.7

1.5

0.1

1942

60.6

96.2

66.1

2.0

5

1385

52.4

76.4

55.8

1.6

5

1012

249.4

417.5

227.6

7.9

5

755

1162.6

1540.9

1206.7

34.6

5

I8

549

1834.9

2373.1

1871.9

53.8

F5→5I4

4658

0.1

0.3

0.2

0

2282

15.9

24.7

16.4

0.3

I5 I6 I7

5

5

I5

5

1430

190.4

328.5

207.4

3.4

5

974

1033.6

2053.1

1184.2

19.9

4581.6

76.4

I6 I7

5

I8

660

3720.9

8032.6

I6 → 5I7

2934

32.1+22.5

49.8+22.8

I8

1180

263.1

381.6

I7 → 5I8

1980

116.4+45.3

175.9+45.6

5

5 5

/s

3173

1435.5

I4

2.2 Fluorescence spectra and emission cross sections

A +A

β/%

628.4

5

J′

/s

MD –1

222.3

S2→ F5

can be obtained. The values of AED, AMD, β and τr of some typical transitions are listed in Table 3.

A +A

ED

1012

5

J′

/s

E//c′

MD –1

1364

5

cence branching ratio β = A( J → J ′) / ∑ A( J → J ′)

A +A

ED

5

I8

A( J ′ → J ) of multiplet J and the fluores-

E//b′

MD –1

31.5

I7

q

nm

E//a′ ED

1887

I6

For the biaxial crystal, the polarization averaged spontaneous emission probability is defined as A( J → J ′) = ∑ Aq ( J → J ′) / 3 . Then the radiative lifetime

λem/

5

I5

where λ em is the mean wavelength of emission bands. Then the total spontaneous emission probability should be ED MD (8) Aq ( J → J ′) = Aq ( J → J ′) + Aq ( J → J ′)

τr = 1/ ∑

371

33.9+22.9 16.7 270.7

83.3

121.8+45.1 100

τr/ μs

85

266

140

2729 5454

Fig. 3 shows the polarized fluorescence spectra of Ho3+:Li3Ba2La3(MoO4)8 crystals in the wavelength range of 500–1450 nm under excitation of 455 nm, i.e., the Ho3+ ions were excited to the 5G6+5F1 multiplets. According to Fig. 3, the emission spectra in the visible wavelength region consist of three emission bands around 545, 660 and 755 nm, which belong to the transition of 5S2+5F4→5I8, 5F5→5I8, and 5S2+5F4→5I7, respectively. For the emission spectra in the near-infrared region, two eminent emission bands around 1000 and 1180 nm can be observed. The emission band around 1000 nm is actually composed of two resonant transitions, namely the transition of 5F5→5I8 with a peak at 984 nm and the Table 2 Polarized experimental and calculated oscillator strengths of Ho3+:Li3Ba2La3(MoO4)8 crystals Transitions λabs 5

I8→ 5

I7

5

nm

E//a′

E//b′

E//c′

fexp

fcal

fexp

fcal

fexp

fcal

(10–6)

(10–6)

(10–6)

(10–6)

(10–6)

(10–6)

1955 2.03

1.54(ED) 0.58(MD)

2.32(ED)

2.54

0.59(MD)

1.97

1.59(ED) 0.58(MD)

I6

1160 1.04

1.07

1.38

1.54

1.07

1.09

F5

643 4.14

4.12

9.20

8.31

4.67

4.70

5

S2+ F4

537 3.93

3.85

9.63

7.71

6.13

4.80

G6+5F1

455 90.60

90.68

116.82

116.79

74.54

74.46

5 5 5

3

G5

418 5.15

5.53

12.06

13.92

6.67

7.14

G5+3H6

360 19.72

18.88

25.56

25.54

15.40

15.97

5

Fig. 3 Polarized fluorescence spectra of Ho3+:Li3Ba2La3 (MoO4)8 crystals in the wavelength range of 500–1450 nm

Rms∆f

0.59×10–6

1.41×10–6

0.80×10–6

Rms error

1.7%

3.1%

2.7%

5

S2+5F4→5I6 with a peak at 1021 nm, while the emission band around 1180 nm can be assigned to the transition of 5 I6→5I8. Besides, there is a weak band around 1380 nm, which can be ascribed to the transition of 5S2+5F4→5I5. Due to the restriction of the test condition, only the unpolarized fluorescence spectra around 2 μm were recorded under the excitation of 455 nm. The 5I7→5I8 transition of Ho3+ presents a broad emission band extending

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from 1850 to 2150 nm, with FWHM of 65 nm, as shown in Fig. 4. The emission cross-section of this transition can be calculated from the fluorescence spectra using the Fuchtbauer-Ladenburg (F-L) equation[20]:

σ em (λ ) =

λ

5

8πn cτ r 2

I (λ )

∫ λ I (λ )dλ

(9)

where I(λ) is the fluorescence intensity at wavelength λ, and τr is the radiative lifetime calculated by J-O theory. The calculated emission cross-section of the transition 4 I13/2→4I15/2 is shown in Fig. 4, in which the unpolarized absorption cross-section is also presented for comparison. The peak emission cross-section is 1.32×10–20 cm2 at 2045 nm, which is comparable to those of Ho3+:YAG (1.59×10–20 cm2 at 2090 nm)[21] and Ho3+: YLF (1.55×10–20 cm2 for π polarization at 2050 nm)[22], in which efficient lasers around 2 μm were achieved[23]. To estimate the possible output wavelength during laser operation, the gain cross section of the title crystal was introduced as (10) σ g = βσ em (λ ) − (1 − β )σ abs (λ ) where σem and σabs are the emission and absorption crosssections around 2 μm respectively, and β is the ratio of excited Ho3+ ions to the total populations. The unpolarized gain cross-sections with several β values are shown in Fig. 5. As we can see from Fig. 5, to achieve laser operations a minimum value β=0.3 is needed. When the inversion ratio reaches 0.5, the positive gains in a wide range from 1974 to 2150 nm are observed, with a peak of 0.37×10–20 cm2 at 2045 nm. The fluorescence decay curve of the 5I7 multiplet was recorded by monitoring the emission of the 5I7→5I8 transition at 2050 nm under excitation of 455 nm, as shown in Fig. 6. The linear relationship exhibits a single exponential behavior of the fluorescence decay and the fluorescence life time τf=–1/k is calculated to be 7.33 ms, which is much larger than the radiative life time τr listed in Table 3. Such a large discrepancy is mainly caused by the re-absorption effect, which commonly observed in Ho3+-doped crystals[18]. To eliminate the re-absorption

Fig. 5 Unpolarized gain cross-sections of Ho3+:Li3Ba2La3(MoO4)8 crystal with different β values

Fig. 6 Fluorescence decay curves of Ho3+:Li3Ba2La3(MoO4)8 crystal for the 5I7 state

effect, the lifetime of the diluted powder sample was also measured in this work. The detailed processes are as follows: First, a piece of Ho3+:Li3Ba2La3(MoO4)8 crystal was ground into fine powder by ball milling. Then, the Ho3+:Li3Ba2La3(MoO4)8 powder was diluted to a lower concentration of 0.25 at.% (1.1×1019 cm2) with the pure Li3Ba2La3(MoO4)8 crystal powder. Finally, the diluted powder was immersed into ethylene glycol (EG), which was used as refractive index matching fluid to reduce the internal reflection within the particles, and filled into a quartz tube for measurement. The fluorescence lifetime of powder sample was measured to be 4.86 ms and thus the quantum efficiency was estimated to be 89%.

3 Conclusions

Fig. 4 Unpolarized absorption and emission cross-sections for the 5I7→5I8 transition of the Ho3+:Li3Ba2La3(MoO4)8

Ho3+:Li3Ba2La3(MoO4)8 crystals were obtained by the conventional top seeded solution growth (TSSG) method from a Li2MoO4 flux. The polarized absorption spectra, fluorescence spectra and fluorescence decay curves of the crystal were investigated. The polarized absorption spectra were analyzed in the framework of J-O theory and the main spectroscopic parameters were calculated. The effective oscillator strength parameters were deter-

SONG Mingjun et al., Crystal growth and spectral characterizations of Ho3+-doped Li3Ba2La3(MoO4)8 crystal

mined to be Ω2=17.40×10–20 cm2, Ω4=4.97×10–20 cm2, and Ω6=1.12×10–20 cm2. The fluorescence lifetime of the 5 I7 level was determined to be 4.86 ms, and the stimulated emission cross section for the 5I7→5I8 transition was estimated to be 1.32×10–20 cm2 at 2045 nm.

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