Spectroscopic investigation of Er3+ doped BaF2 ...

Optical Materials 82 (2018) 104–109

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Spectroscopic investigation of Er3+ doped BaF2 single crystal a,c

A. Bitam , S. Khiari J.P. Jouarte

b,c,∗

c

c

c

c

T d

, M. Diaf , H. Boubekri , E. Boulma , C. Bensalem , L. Guerbous ,

a

University of Tamanrasset, Algeria University of El-Tarf, El-Tarf, 36000, Algeria Laser Physics, Optical Spectroscopy and Optoelectronics Laboratory, Badji Mokhtar Annaba University, POB 12, 23000, Annaba, Algeria d Laser Department, Nuclear Technique Division, Nuclear Research Center of Algiers, 02 Bd Frantz Fanon, 16000, Algiers, Algeria e ECATHERM/GRESPI, Reims Champagne-Ardenne University, UFR SCIENCES, Moulin de la Housse, B.P. 1039, 51687, Reims Cedex 2, France b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Spectroscopic analysis Spectral parameters Judd-Ofelt theory Laser emission

We report on the spectroscopic investigation of Er3+ doped BaF2 single crystals. High-quality crystals have been gown by the Bridgman-Stockbarger pulling technique. The room temperature absorption, excitation, emission and fluorescence decay spectra of the luminescent Er3+ ions inserted in BaF2 fluoride single crystals have been investigated. Using the Judd-Ofelt (JO) theory, the intensity parameters of Er3+ ions have been calculated to be Ω2 = 0.949 × 10−20 cm2, Ω4 = 0.975 × 10−20 cm2 and Ω6 = 1.258 × 10−20 cm2. These parameters were then used to calculate the radiative transition probabilities (AJJ′), branching ratios (βJJ′) and radiative lifetimes (τrad) of the main laser emitting levels of Er3+ ions. The obtained spectroscopic properties are compared to those of Er3+ transitions in other fluoride and oxide hosts. The excitation spectrum in the UV–Visible spectral range is very close to the absorption spectrum indicating that all observed absorption levels can excite the Erbium green emission. The emission spectrum is mainly dominated by the green emission alongside a weak red emission. For the main transitions, there is a good agreement between the emission spectrum and the spontaneous emission probabilities given by the JO analysis. Using the Fuchtbauer–Ladenburg method, the emission cross-section of the three main visible emission were determined in addition to other important laser parameters such radiative quantum efficiency and optical gain.

1. Introduction Nowadays, crystalline lasers play a special role in modern laser physics and nonlinear optics. The amplifying medium is generally an inorganic material doped with luminescent elements. Such medium is used as single crystals, glasses, glass ceramics or optical fibers. The most materials studied are doped with rare earth ions (RE) which constitute the optically active centers by considering the 4f–4f transitions. Among all solid-state laser materials, the fluorides have the advantageous to be transparent in a large electromagnetic domain and they have low maximum phonon frequency leading to a large number of potential emitting levels by limiting the non radiative emissions since they gave rise to laser emissions at different wavelengths of the optical spectrum. In this context, MF2 difluoride crystals (M = Alkaline earth element, Cd or Pb) with fluorite structure are of great interest as material for photonics, in general, and specially in solid state laser. BaF2 is an alkali earth fluoride and a great deal of attention is attributed to its optical properties [1–3]. These optical properties are related to the structural

∗

Corresponding author. University of El-Tarf, El-Tarf, 36000, Algeria. E-mail addresses: [email protected] (S. Khiari), [email protected] (M. Diaf).

https://doi.org/10.1016/j.optmat.2018.05.034 Received 3 March 2018; Received in revised form 24 April 2018; Accepted 11 May 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.

and electronic properties of BaF2 such as a very large band gap. Much work has been devoted to the search of new laser host materials. Barium fluoride BaF2 doped with rare earth ions TR3+ occupies a wide range of applications such as lasers [4], optical communications [5]. In the research, BaF2 doped with RE3+ is known as an excellent scintillator [6] for the detection of X-rays or high energy particles. A such fluoride material have a relatively low phonon energy and a large optical transparency over a wide wavelength range from ultraviolet (UV) to far infrared (FIR) from 0.2 to 14.0 μm and is used to make optical components, such as windows for infrared spectroscopy. On another hand, rare earth ions like Er3+ are considered as luminescence centers in crystals as well as in glasses [7–10]. A great interest is paid to erbium due to its emission band at 1.53 μm for optical transmission (erbiumdoped fiber amplifier), eye-safe light detecting and in the visible region (blue-green) for data storage. The emission of erbium Er3+ in the visible range can be obtained by up-conversion process. The laser performance of Er3+ has been observed in glass fibers [11] and crystals [12]. Indeed, fluorides offer an advantage more than the oxides because


A. Bitam et al.

the lifetime of the electronic states is longer in these systems, this comes from the low energy phonon of this hosts leading to a small nonradiative transition. In this paper, we report the spectral properties of Er3+ ions doped BaF2 crystal. Absorption, excitation, emission and fluorescence decay spectra are recorded at room temperature. The Judd-Ofelt (JO) analysis was applied using the absorption spectra recorded at room temperature in order to deduce the emission transition probabilities, branching ratios and radiative lifetimes.

2. Physical properties of BaF2 crystals Fig. 1. A polished sample of Er3+: BaF2 crystal.

The barium difluoride (BaF2) crystallizes in the fluorite structure and has the Fm3m space group with four unit formula per unit cell [13]. Its lattice constant is 6.20 Å. It has a melting point temperature of 1360–1480 °C [14]. The transmission range of BaF2 is 0.2–14 μm, due to its large bandwidth of 11 eV [15]. During the synthesis of BaF2: Er3+crystal, the Er3+ ion substitutes the Ba2+ divalent ion, short-range charging compensation will then be possible. For the site tetragonal (C4v), charge compensation is provided by a fluorine ion F− in interstitial position in the center of the nearest neighbor cube to ensure the electrical neutrality of the crystal. The phonon energy is 319 cm−1 [16]. Such value remains low compared with oxides, which leads to high radiative transitions in these materials. The optical and physical properties of BaF2 make it versatile optical material. The whole of these properties and others are summarized in Table 1.

3. Crystal growth and X-ray ray characterization Fig. 2. X-Ray diffraction pattern of barium fluoride sample BaF2: Er3+(2% mol.).

Er3+: BaF2 is grown by the use of Bridgman-Stockbarger pulling technique. The crucible that contains the powder mixture is placed in a vacuum furnace in fluorine atmosphere. The BaF2 initial powder is purified several times using repeated growth of single crystals. The erbium ion is introduced in the form of erbium trifluoride (ErF3) after the purification step. The pulled crystals have a diameter of more than 8 mm and they are 20–30 mm in length. Fig. 1 shows a part of the pulled cylindrical crystal and a sample obtained from a cut slice. Since it has a cubic structure, we have cut the samples in a direction perpendicular to the cylinder axis. The samples were checked in polarized light and have been found to be free of macles and crackles. They could easily be cut into laser bulk single crystals with high optical quality. The crystal structure was checked by X-ray diffraction (XRD) pattern (Fig. 2) using a Philips X'Pert Pro diffractometer. The XRD diffractogram of Fig. 2 shows a set of diffraction lines. The diffraction peaks are ascribed to the cubic structure according to the file (JCPDS 04–0452). The fitted value of lattice parameter is 6.23 Å which is in agreement with those obtained in the literature [13].

4. Judd–Ofelt (JO) formalism The absorption lines of trivalent rare-earth ions (RE) in a crystalline host are due to intra-configurational f–f transitions. JO model [18,19] has been employed to describe the absorption and photoluminescence properties of RE in many ion-host combinations [20]. Fitting procedure of the experimental absorption oscillator strengths to the theoretical ones allows evaluating the phenomenological JO parameters Ω2 , Ω4 , Ω6 , which can be used to calculate other important parameters, as radiative transition probabilities, radiative lifetimes and branching ratios. In the framework of the Judd–Ofelt theory, we need the average wavelength of each absorption band which is often given by the following formula:

∫ λDO (λ ) dλ λ =

J →J′

∫ DO (λ ) dλ

(1)

J →J′

The measured line strengths SJJmes ′ of the chosen bands are determined using the following expression:

Table 1 Physical characteristics of BaF2 crystal. Properties

Ref.

9n ⎛ ⎞ ⎛ 3hcε0 ⎞ 1 (2J + 1) SJJmes ′ = 2 2 2 2 ⎝ (n + 2) ⎠ ⎝ 2π e ⎠ λ ⎜

Crystal structure Space group Lattice parameter Number of unit formula Symmetry site Melting temperature Refractive index Energy gap Phonon energy Thermal conductivity Thermal expansion coefficient Density

Cubic Fm3m 6.2 Å 4 C4v 1386 °C 1.475 11 eV 319 cm−1 11.72 W m-1 K-1 18.1 10−6/K 4.9 g/cm3

[13] [13] [13] [13] [14] [15] [13] [17] [16] [16] [15] [17]

⎟

∫ σabs (λ) dλ

(2)

Where J and J′ represent the total angular momentum quantum numbers of the initial and final levels, respectively, n is the refractive index of the material, h is Planck's constant, c is the vacuum light celerity, ε0 is the vacuum permittivity, e is the electron charge, λ is the average wavelength of the J→J′ absorption transition. The magnetic dipolar line strength is: 2 → → ⎛ h ⎞ 4f N α [L, S ] J L + 2 S 4f N α′ [L′, S′] J ′ SJJMD ′ = ⎝ 4πmc ⎠

105

2

(3)


A. Bitam et al.

The theoretical line strength of electric dipole transition is given by the following equation:

(SJJED′ )

cal

=

∑

Ωt

U (t )

2

(4)

t = 2,4,6

2

are Where Ω2, Ω4 and Ω6 are the JO intensity parameters, and the squared reduced matrix elements of rank t (t = 2,4,6) between the two multiplets characterized by the quantum number (S, L, J) and (S′, L′, J′). The values of these matrix elements do not depend on the nature of the ligands but depend only on angular momentum of the Er3+ states. The matrix elements U(t) used in the present work were tabulated by Kaminski [21] for the Er3+ ions. The J-O parameters are influenced by the host and particularly on the transition probabilities. The measured electric dipole transition strength is given by:

U (t )

2 ⎛ 9n ⎞⎟. S MD SJJEDmes = SJJmes ′ ′ − ⎜ 2 JJ ′ ( n + 2)2 ⎠ ⎝

(5)

The JO parameters Ωt (t = 2, 4, 6) are determined by fitting the measured line strength to the calculated ones. To evaluate the accuracy of the Judd–Ofelt parameters and the theoretical line strength, the root mean-square (rms) deviation between measured and calculated line strengths of the transitions is given by the equation: q

∑

δ=

((SJJEDcal )i − (SJJEDmes )i ) ′ ′

i=1

2

q−p

(6)

where p = 3 and q is the number of considered transitions fitting procedure of the experimental absorption line strengths to the theoretical ones allows evaluating the phenomenological JO parameters Ω2 , Ω4 , Ω6 , which can be used to calculate other important spectroscopic parameters such as radiative transition probabilities (AJJ′), branching ratios (βJJ′ and radiative lifetimes (τJJ′). The spontaneous emission probability is identified and written as.

AJJ ′ = AJJED′ + AJJMD ′ =

2 2 16π 3e 2 ⎛ n (n + 2) SJJED′ + n3SJJMD ⎞ ′ 3 9 3h (2J + 1) ε0 λ ⎝ ⎠ ⎜

⎟

(7) Fig. 3. Absorption spectra of BaF2: Er3+(2% mol.).

The radiative lifetime of the excited manifold can be calculated as:

1 = τJrad

∑ AJJ ′ J′

Table 2 Measured and calculated line strengths of different bands obtained from the absorption spectrum of Er3+ in BaF2.

(8)

Thus all possible transitions from a given excited manifold 2S+1LJ′ to all possible lower lying 2S+1LJ ones are considered. The fraction of photons emitted from an excited manifold 2S+1LJ′ to a single lower one 2S+1 LJ is the luminescence branching ratio βJJ ′ and is expressed as:

βJJ ′ =

AJJ ′ ∑J ′ AJJ ′

Transition 4I15/2 →

λ (nm)

SJJEDmeas (10−20cm2) ′

SJJEDcal (10−20cm2) ′

4

1518.1 975.3 801.9 653.1 543.4 520.2 487.0

2.039 0.523 0.244 1.141 0.188 1.181 0.725

1.934 0.527 0.161 1.129 0.287 1.183 0.935

I13/2 I11/2 4 I9/2 4 F9/2 4 S3/2 2 H11/2 4 F7/2 4

(9)

5. Absorption spectra For spectroscopic measurements, the crystal is cleaved and polished in order to obtain parallel face samples with 1.8 mm thickness. The absorption spectra from 400 to 1700 nm were recorded on a Cary 500 spectrophotometer at room temperature. As BaF2 is an isotropic crystal, the absorption spectra were obtained without using the birefringent polarizer of the used spectrophotometer. The spectra are displayed in Fig. 3. These spectra, ranging from 400 to 1700 nm consists of seven Er3+ absorption bands located around 1518.1, 975.3, 801.9, 653.1, 543.4, 520.2 and 487.0 nm. The observed bands are attributed respectively to the multiplets 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4 I13/2. The bands involving levels 4F7/2, 2H11/2, and 4I13/2 are intense compared to other bands. They are used in the fitting procedure. Table 2 lists the measured (Smeas) and calculated (Smeas) transition strengths of different bands obtained from the absorption spectrum of Er3+ in BaF2. A least-square fitting of Smeas to Scal provides the three JO

Table 3 Judd-Ofelt parameters and spectroscopic quality factors of Er3+ ions in different hosts. Material

Ω2

Ω4

Ω6

References

BaF2 CdF2 CaF2 LiGdF4 Lu2SiO5 (Gd0.7Y0.3)2SiO5

0.949 1.284 1.39 0.905 4.451 5.52

0.975 0.413 1.34 2.47 1.614 1.38

1.258 1.272 2.24 4.92 1.158 0.91

This paper [22] [23] [24] [25] [26]

parameters and are obtained equal to: Ω2 = 0.949, Ω4 = 0.975 and Ω6 = 1.258 (in 10−20 cm2 units). These parameters are in accordance with those calculated for other fluoride hosts (Table 3) [22–24]. 106


A. Bitam et al.

However, the value of Ω2 is influenced by the valency of the ligand ions and increases with oxides compounds [25,26]. On the other hand, Ω4 and Ω6 parameters depend on the rigidity of the host matrix in which the rare earth ions are incorporated. A series of calculation were carried out while varying the number of transitions used in the fitting procedure. The quality of the fit is then estimated by calculating the standard deviation rms between the experimental and calculated line strengths of the transition. The found value of rms is δ = 0.134 × 10−20 cm2. This low value of rms means a good agreement between measured and calculated line strengths of the transitions. The JO parameters can now applied to evaluate radiative transition probabilities and the branching ratios from the upper J manifolds, 4I13/ 4 4 4 4 2 4 2, I11/2, I9/2, F9/2, S3/2, H11/2, F7/2 to all lower -lying J′ manifolds. These parameters can be determined by Eq. (6) and (8) to evaluate radiative transition probabilities and the branching ratios from the upper J manifolds, 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2 and 4F7/2 to all lower -lying J′ manifolds. Besides of this, the use of equation (7) provides the value of the radiative lifetimes for all seven upper states. The values of the radiative transition probabilities, the branching ratios and the radiative lifetime are given in Table 4.

Fig. 4. Excitation spectrum of Er3+ in BaF2 at room temperature.

of BaF2: Er3+ (2% mol.) single crystal recorded between 200 and 500 nm. We monitored the green emission at 530 nm which corresponds to 4S3/2 + 2H11/2 → 4I15/2 transition. Many bands are observed and could be assigned to transitions from the 4I15/2 ground level to 4F7/ 4 4 2 4 2 2 4 3+ 2 . It 2, F5/2 + F3/2, H9/2, G11/2 + G9/2 + K15/2+ G7/2, D7/2 of Er seen that the excitation spectrum and the absorption spectrum are alike within the range from 200 to 500 nm. These several bands of Er3+ can excite the green emission. It is remarkable that the excitation transition at 376 nm is intense. The emission spectrum shown in Fig. 5 of Er3+ ions doped BaF2 recorded within the range from 400 to 700 nm, is obtained under excitation wavelength of 376 nm. It indicates that the dominating emission is at 551 nm (4S3/2 → 4I15/2). Also, two weak fluorescence lines at 440 nm and at 650 nm are observed due to 4F5/2 → 4I15/2 and to 4F9/ 4 2 → I15/2 transitions, respectively. The signal in green region is several times more intense that in the red and blue regions. Fig. 6 depicts the room temperature time-resolved emission spectra of BaF2: Er3+ recorded between 400 and 700 nm under excitation wavelength of 376 nm. After, the decay curves of the green (2H11/ 4 4 4 4 2 + S3/2 → I15/2) and the red ( F9/2 → I15/2) are recorded (Fig. 7). The decay of luminescence at 652 nm (4F9/2 → 4I15/2 transition) is fitted by the exponential dependence to give (τexp = 0.95 ms). One can see from Fig. 7 (b) that the decay of luminescence is nonexponential. It has been fitted by a double exponential function which leads to an effective lifetime of 0.56 ms. When we compare all the results of the fluorescence dynamics with those already published in the literature (Table 5), we

6. Excitation, emission and fluorescence decay measurements The photoluminescence measurements were performed at room temperature using a Perkin Elmer LS 50 B spectrometer that operates a Xe lamp as the excitation source. The excitation wavelength was 376 nm. This device works in a wide spectral range, extending from 200 to 900 nm. We show in Fig. 4 the room temperature excitation spectrum Table 4 Electric and magnetic dipole emission probabilities, lifetimes and branching ratios in BaF2 doped with Er3+ ions. Average wavelength λ (nm)

AED (s−1)

AMD (s−1)

β

τrad (ms)

I13/2 → 4I15/2

1518.1

74.90

30.72

1

9.47

I11/2 → 4I15/2 4 I13/2

975.3 2727.9

89.77 12.27

0 6.88

0.824 0.176

9.18

I9/2 → 4I15/2 4 I13/2 4 I11/2

801.9 1699.8 4510.6

59.227 34.94 0.573

0 0 0.962

0.619 0.365 0.016

F9/2 → 4I15/2 4 I13/2 4 I11/2 4 I9/2

653.1 1146.1 1976.4 3517.9

768.669 34.298 40.642 0.791

0 0 4.725 2.042

0.903 0.04 0.053 0.0033

S3/2 → 4I15/2 4 I13/2 4 I11/2 4 I9/2 4 F9/2

543.3 886.2 1226.8 1685.1 3234.3

847.232 342.17 26.251 38.982 0.401

0 0 0 0 0

0.675 0.273 0.021 0.031 ∼0

H11/2 → 4I15/2 4 I13/2 4 I11/2 4 I9/2 4 F9/2 4 S3/2

520.2 791.4 1114.9 1480.9 2557.5 12220.6

1329 45.613 24.018 28.956 3.672 0.018

0 47.592 6.114 0.502 0.104 0

0.895 0.063 0.02 0.0025 0.0025 ∼0

F7/2 → 4I15/2 4 I13/2 4 I11/2 4 I9/2 4 F9/2 4 S3/2 2 H11/2

486.9 716.9 972.5 1239.8 1914.4 4691.2 7614.1

1919 211.068 119.331 81.104 2.142 0.012 0.336

0 0 0 8.782 9.325 0 0

0.816 0.09 0.051 0.038 0.0049 ∼0 ∼0

Transition

4

4

4

4

4

2

4

10.45

1.17

0.797

0.673

0.425

Fig. 5. Fluorescence spectrum of Er3+ doped BaF2 single crystal at room temperature under an excitation wavelength of 376 nm. 107


A. Bitam et al.

Fig. 6. Room temperature time-resolved emission spectra of Er3+ ions doped BaF2 single crystals under 376 nm as excitation wavelength. Table 5 Radiative lifetime, measured lifetime and radiative quantum efficiency of Er3+ ions in different hosts. Sample

Radiative lifetime (ms)

Measured lifetime (ms)

Radiative quantum efficiency (%)

Ref.

BaF2

0.797

0.560

70

LiYF4 KY3F10 CdF2-ZnF2

0.580 0.595 0.700

0.460 0.295 0.450

79 49 65

This work [27] [28] [29]

7. Cross-section emission and optical gain parameter It is well known that emission cross-section is an important key parameter influencing the potential laser performance. Using the Füchtbauer–Ladenburg method expressed by the equation:

β

σem (λ ) =

λ2

8πn2cτrad ∫ λI (λ ) dλ λ1

(10)

where I(k) is the emission spectral intensity at each value of the wavelength λ, τrad is the radiative lifetime of the upper laser level and β is the branching ration between the upper and the lower level of the transition, n the refractive index and c the velocity of light. We have computed the emission cross-section of the green transition (Fig. 8). The obtained value is 1.425 × 10−20cm2 is in the same order of magnitude than those obtained for other potential laser hosts based on Er3+ visible luminescence [30–32]. The experimental calculations leads also to determine the quantum efficiency of a transition defined as η = τexp/τrad. The main emission, i.e. the green emission has quantum efficiency 70%. We use also the optical gain parameter defined as: Fig. 7. Fluorescence decay curves of red (a) and green (b) emission of BaF2: Er3+(2% mol.) upon excitation at λ = 376 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

G = σem Texp

(11)

to select a laser host material with high stability [33]. In this investigation, the optical gain parameter for the green is equal to: Ggreen = 8.0 × 10−24 cm2s units. It appears from this study that the green emission has very advantageous optical parameters to serve as laser emission.

find that our sample has very similar performances in terms of dynamic parameters compared with other fluoride samples and this is valid for crystalline and vitreous samples.

108


A. Bitam et al.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

Fig. 8. Green emission cross-section of BaF2: Er3+(2% mol.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

[18] [19] [20]

8. Conclusion

[21]

The first successful growth of Er3+ doped BaF2 crystal is reported. The JO model has been applied to the room temperature absorption spectra of the Er3+: BaF2 single crystal. The JO parameters were: Ω2 = 0.949 × 10−20 cm2, Ω4 = 0.975 × 10−20 cm2 and Ω6 = 1.258 × 10−20 cm2. The radiative lifetime of the transitions from 4 I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2 and 4F7/2 states to the lower-lying manifolds are 9.47, 9.18, 10.45, 1.17, 0.797, 0.673 and 0.425 ms, respectively. The radiative lifetime and the fluorescence decay lifetime are: 0.797 ms, 0.56 ms, respectively, leading to a quantum efficiency of 70% which is similar to the values found in other fluoride matrices. With such spectroscopic parameters for the barium fluoride BaF2 doped with Er3+, we may conclude that Er3+:BaF2 is a serious contender for laser systems.

[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

109

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