Mid-infrared luminescence and energy transfer of Tm3+ in silicate glasses by codoping with Yb3+ ions

Optics and Laser Technology 94 (2017) 106–111

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Mid-infrared luminescence and energy transfer of Tm3+ in silicate glasses by codoping with Yb3+ ions Ruijie Cao a, Yu Lu a, Ying Tian a, Feifei Huang a, Yanyan Guo b, Shiqing Xu a, Junjie Zhang a,⇑ a b

Collage of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China Collage of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China

a r t i c l e

i n f o

Article history: Received 19 December 2016 Received in revised form 24 February 2017 Accepted 17 March 2017

Keywords: Silicate glass Laser Infrared spectrum Rare earth ions

a b s t r a c t A kind of novel silicate glasses doped with Tm3+ sensitized by Yb3+ were prepared by conventional melt quenching method. The optical properties of the synthesized glasses were theoretically and experimentally investigated. Based on the absorption spectra and the Judd-Ofelt theory, the J-O intensity parameters (Xt), radiative transition probability (400.4 s
1), fluorescence lifetime (4.99 ms) and absorption and emission cross sections (re = 2.51  10
20 cm2) were calculated. According to fluorescence spectra, the 1.8 lm emission of Tm3+ could be greatly enhanced by adding proper amount of Yb3+ under the excitation of 980 nm and the optimized concentration ratio of Tm3+ and Yb3+ was found to be 1:3 in the present silicate glass system. Besides, the energy transfer mechanism between Yb3+ and Tm3+ were thoroughly discussed. With the assistance of Yb3+, the lifetime of Tm3+ from 0.54 ms increased to 1.42 ms. The energy transfer efficiency from Yb3+ to Tm3+ could reach 90.94%, and the energy transfer coefficient was 5.43  10
41 cm6/s. The content of OH
was measured. The above results showed that Tm3+/Yb3+ codoping could be expected to a promising way to achieve high efficient 2 lm lasing pumped by a 980 nm LD. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Solid state lasers in mid-infrared spectral region have numerous potential applications including eye-safe radar, high-resolution molecular spectroscopy, biomedical systems, remote sensing and so forth [1–3]. As we all know, rare earth ions Tm3+ and Ho3+ are the perfect candidates for 2 lm lasers owing to the energy level transition of 3F4 ? 3H6 (Tm3+) and 5I7 ? 5I8 (Ho3+). Compared with Ho3+, the absorption peak of Tm3+ is around 800 nm, thus it can be pumped by a commercial 808 nm laser diode (LD) directly. Besides, the so called ‘two-for-one’ cross relaxation process may occur, which means two Tm3+ ions are pumped to the 3F4 level by one pump photon with a 200% pumping quantum efficiency [4]. The emission bandwidth of Tm3+ is wider than that of Ho3+ in 2 lm spectral region therefore it is beneficial to produce shorter laser pulses [5]. Nevertheless, when excited by a commercially high power 808 nm LD the absorption peak of the Tm3+ (3H6 ? 3H4 transition) is shorter and the absorption efficiency is lower, which is harmful to emit 2 lm lasers [4]. Yb3+ owns higher absorption cross section that can efficiently absorb the excitation emitting from commercial 980 nm LD and then transfer energy to Tm3+ (3H5 ⇑ Corresponding author. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.optlastec.2017.03.026 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.

level) via a nonresonant energy transfer process [6]. The optimization of Yb3+ and Tm3+ doping concentration is also significant to further enhance 1.8 lm emission of Tm3+. Heavy metal oxide glasses exhibit a great deal of advantages such as lower maximum phonon energy, higher rare earth ions solubility and easy fabrication [4,7,8]. The Tm3+ doped and Er3+ doped tellurium germanate glass and its double-cladding fiber for 2 lm laser have been published [9,10]. Fluoride glass have developed rapidly depending on the low phonon energy, high doping level, low viscosity, and wide transparency from UV to IR [11]. But the cost and mechanical behavior limit the further development of the above matrix. However, silicate glasses have excellent thermal stability against crystallization and higher glass transition temperature and stable physical chemical properties [6,12]. Above all, it can be easily drawn into the fibers and the slope efficiency is much higher than that in other glass fibers [13]. Up to the present, wattlevel 1.8 lm laser output in Tm3+ doped silicate glass fiber has been demonstrated [14]. In recent, large-mode-area single-modeoutput silicate glass in all-solid photonic crystal fiber has been achieved [15]. In commercial, 2 lm single frequency and Qswitched fiber laser products have been produced by Advalue Photonics Company. As a traditional glass matrix, silicate still has a place in fiber laser field.

R. Cao et al. / Optics and Laser Technology 94 (2017) 106–111

Snitzer E. through a large number of experiments found that alkali metal-alkaline earth metal silicate glass was the most suitable candidate for laser glass [16]. Currently, the most widely used system is K2O-BaO-SiO2, R2O-CaO-SiO2 and Li2O-MgO/CaO-Al2O3SiO2. The properties of Er3+/Ho3+, Er3+/Tm3+, Tm3+/Ho3+ co-doped silicate glass are investigated based on these systems [17–20]. In this work, the advantages of alkali metal and alkaline-earth metal are combined. According to our earlier reports [21–23], it is proved that this matrix is suitable for 2 lm laser emission. In this work, Al3+ obtain non-bridging oxygen to form [AlO4], which are able to enter silica-oxygen net thereby made the structure closely. CaO and BaO can be used to polarize non-bridging oxygen and weaken silicone bond, which can reduce viscosity of the host glass. La2O3 improve chemical stability and reduce the coefficient of thermal expansion. BaF2 and CaF2 are introduced into the glasses to reduce the content of hydroxyl group, which can weaken the emission. Furthermore, in the process of drawing fiber, fluoride can decrease the melt temperature and improve the solutions of rare earth ions availably [24]. According to our knowledge, in recent years, there have been many reports on Tm3+/Yb3+ codoped luminescent materials [25–29], but limited by the efficiency of energy transfer, the slope efficiency and output power are low and thermally damage cannot be avoided. There are few systematic studies focused on 1.8 lm emissions in Tm3+/Yb3+ co-doped silicate glasses [30]. The theoretical study of the optical parameters of Tm3+ are compared with the experimental results and the effects of Yb3+ concentration on the spectroscopic properties of silicate glasses doped Tm3+ are discussed. The energy transfer process and the coefficients are analyzed by the extended spectral overlap method in detail. 2. Experimental details Silicate glasses with the molar compositions of (98-x)(SiO2Al2O3-CaO-CaF2-BaO -BaF2-La2O3)-1Tm2O3-xYb2O3 (x = 0, 1, 2, 3, 4) had been denoted as STY-x. 20 g of the raw materials had been weighted and well mixed, then melted in a platinum crucible at the temperature about 1400 °C for 60 min. The melts had been poured onto a preheated (600 °C) stainless steel plate then further annealed at 600 °C for 4 h, after they had been cooled to room temperature (A large number of experimental results show that when melting temperature is 1350 °C, the glass is not melted completely, limited by equipment requirements the temperature cannot exceed 1450 °C. When annealed under 650 °C, there are crystalline in glass). The cooled samples had been cut and polished to the size of 20  20  1.5 mm3 carefully, prepared for the optical property measurements. The densities (3.58 g/cm3) had been measured by Archimedes drainage method, refractive index (1.623) had been measured by using a Metricon Model 2010/M Prism Coupler. The absorption spectra had been recorded with a Perkin-Elmer Lambda 900UV/ VIS/NIR spectrophotometer from 400 nm to 2100 nm. The emission spectra and the fluorescence decay curves had been obtained using an FLSP 920 instrument (Edinburgh instruments Ltd., UK) with a 980 nm LD as an excitation source. All the measurements had been performed at room temperature.

107

ground state (3H6) to excited states (1G4, 3F2,3, 3H4, 3H5, and 3F4) respectively. In the view of the strong intrinsic absorption band gap in the host glass, energy levels before 1G4 level are not identified totally. For rare earth ions doped glasses, crystals and ceramics, the Judd-Ofelt (J-O) theory [31,32] plays vital role in analyzing the spectroscopic properties (intensity parameters Xt, spontaneous emission probabilities, radiative lifetimes, fluorescence branching ratios and so on). As a part of the J-O theory, the intensity parameters Xt (t = 2, 4, 6) are calculated making use of the absorption value according to the procedure measured above [33]. Table 1 lists the parameters acquired from STY-3 sample and the comparison with other glass hosts. It is shown that X2 in the STY-3 glass is the largest, indicating that in this matrix there are stronger covalent bond between Tm3+ and O2
ions and lower symmetry of the coordination structure surrounding the Tm3+ ions [34,35]. The value of X4/X6 determines the spectroscopy quality of the host materials [36]. According to the intensity parameters obtained from J-O theory, the radiative properties can be worked out by the expressions illustrated in the literature and the results are shown in Table 2 [6,33]. It is seen that the spontaneous emission probability of the Tm3+:3F4 ? 3H6 transition is 400.4 s
1, which is much larger than that in sodium aluminophosphate glass (106.2 s
1) [37], Oxyfluoride glass (127.23 s
1) [35]. Since A is related to the refractive index, higher A can provide more opportunities to obtain 2 lm laser [38].

3.2. Fluorescence spectra and energy transfer mechanism between Yb3+ and Tm3+ The fluorescence spectra of STY-x glasses pumped by 980 nm LD are illustrated in Fig. 2(a). We can found that there is no fluorescence in Tm3+ singly doped glass, while a group of emission band near 1.8 lm emitted from the Tm3+ comes out with the assistance of Yb2O3. This confirms the lack of absorption band near 976 nm for Tm3+ and the existence of energy transfer from Yb3+ to Tm3+. Besides, with the adding of the Yb2O3 concentration, the fluorescence intensity of Tm3+ increases at the same time, which can be accounted for shortening the distance between Yb3+ and Tm3+ to raise the energy transfer probability from Yb3+ to Tm3+. Once the concentration of Yb2O3 reaches the maximum value of 3 mol%, when increase the Yb2O3 concentration further, the fluorescence intensity will be weaken by concentration quenching or other dif-

3. Results and discussion 3.1. Absorption spectra and Judd-Ofelt theory analysis The absorption spectra of Tm3+ doped and Tm3+/Yb3+ co-doped silicate glass samples over the wavelength region of 400–2000 nm are presented in Fig. 1. The five absorption bands of Tm3+ centered around 460, 681, 790, 1210, and 1700 nm corresponds with the

Fig. 1. The absorption spectra of Tm3+ doped and Tm3+/Yb3+ co-doped silicate glasses.

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R. Cao et al. / Optics and Laser Technology 94 (2017) 106–111

Table 1 Judd-Ofelt intensity parameters Xt of Tm3+ in various glasses.

Xt (10
20 cm2)

X2

X4

X6

X4/X6

Reference

Tellurite Germanate Oxyfluoride Silicate STY-3

3.3 4.45 4.6 2.87 4.76

2.38 1.13 3.69 1.71 2.49

1.28 1.05 2.46 0.69 1.56

1.86 1.08 1.50 2.47 1.59

[21] [8] [22] [23] This work

Table 2 The predicted spontaneous transition probability (A), total spontaneous transition probability (RA), branching ratios (b) and radiative life times (s) of silicate glass sample for various selected excited states of Tm3+. Transition

k (nm)

A (s
1)

RA (s
1)

b (%)

s (ms)

3

1634 1209 3429 787 1493 2293 682 1171 1565 5112 641 1055 1364 3455 10662 466 652 758

400.4 346.44 3.01 923.48 73.19 24.72 1747.19 107.77 233.85 3.03 668.10 453.61 231.64 16.45 0.15 720.26 124.60 655.15

400.4 349.45

100 98.79 1.21 90.41 7.17 2.42 83.53 5.15 11.18 0.14 48.71 33.1 16.91 1.20 0.01 40.75 7.05 37.07

4.99 4.01

3

F4 ? H 6 3 H5 ? 3H6 3 F4 3 H4 ? 3H6 3 F4 3 H5 3 F3 ? 3H6 3 F4 3 H5 3 H4 3 F2 ? 3H6 3 F4 3 H5 3 H4 3 F3 1 G4 ? 3H6 3 F4 3 H5

1021.38

2092.56

1369.96

1767.37

0.98

0.48

0.73

0.57

Fig. 2. (a) Fluorescence spectra of STY-x glasses and (b) energy level between Tm3+ and Yb3+.

fusion [39]. That is the reason why we choose the third glass (Tm2O3 concentration is 3%) as the mainly study object. According to the energy level diagram shown in Fig. 2(b), Yb3+ ions are initially stimulated at the 2F7/2 ground state then pumped to the 2F5/2 level excited by a 980 nm LD. Afterwards, one part of Yb3+ ions de-excite radiatively to the 2F7/2 level with 1006 nm emission, another part of them transfer energy to the 3H5 level of Tm3+ through the nonresonant phonon-assisted, which is energy transfer process (ET1), corresponding to 2F5/2(Yb3+)+3H6

(Tm3+) ? 2F7/2(Yb3+)+3H5(Tm3+). Meanwhile, Tm3+ ions on the 3H5 level decay quickly to the 3F4 level. The ions in 3F4 are able to relax to the 3H6 level emitting 1.8 lm emissions. The hydroxyl groups play a decisive role in the lifetime of rare earth ions in midinfrared laser glass [40]. As one of the dominant quenching centers in Tm3+ doped glasses, only a few hydroxyls (2700–3700 cm
1) are required for nonradiative de-excitation through multiphonon relaxation [41], especially for populations of 3F4 and 3H4 levels. Fig. 2(b) is the specific process between OH
and Tm3+.

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R. Cao et al. / Optics and Laser Technology 94 (2017) 106–111

s s0

3.3. Absorption and emission cross sections

g¼1


The absorption and emission cross sections of STY-3 sample are calculated to discuss the energy transfer process between Yb3+ and Tm3+ ions, as shown in Fig. 3. From the Beer-Lambert equation [6], the absorption cross section (ra) is described as:

In this formula, s0 and s are the lifetimes of Yb3+ singly doped and Tm3+/Yb3+ co-doped samples, respectively. For STY-3 glass sample, lifetime of Yb3+ single doped is 629 ls and the co-doped with Tm3+ is 57 ls. Following the above formula, energy transfer efficient is 90.94%, which is much higher than the parameter in fluorophosphate glass (46.3%) [43], lanthanum tungsten tellurite glasses (89%) [44]. Thus Tm3+ ions can well utilize the pump energy absorbed by Yb3+ to enhance the 1.8 lm emission. Between 2F5/2(Yb3+) and 3H5(Tm3+) level, the energy difference is nearly 1985 cm
1, which is almost the twice of the maximum phonon energy (1050 cm
1 corresponding to the vibration stretching energy of Si-O in [SiO4] tetrahedra [45]), means this process is nonresonant and need the assistant to compensate the energy gap and the lattice vibration energy [44]. The probability rate of energy transfer expression is [46]:

ra


 2:303log II0 ¼ Nl

ð1Þ

where N represents the concentration of rare earth ions (here is Tm3+ and Yb3+), l is the sample thickness, I0 and I are the intensities of incident and transmitted light, respectively. The stimulated emission cross sections (re) can be calculated from the absorption cross section using the following McCumber expression [6]:

re ðkÞ ¼ ra ðkÞ

Zl EZl
hck
1 exp Zu kT

!

ð2Þ

where Zu and Zl are the partition functions of the upper and lower level, T represents the temperature (here is the room temperature), k is the Boltzmann constant, and EZL is the zero line energy. From Fig. 3(a), the maximum ra of Yb3+ is 0.99  10
20 cm2 at 973 nm, which is larger than the one in another silicate glass (0.72  10
20 cm2) [36] and fluorophosphate glass (0.92  10
20 cm2) [42], the re of Tm3+ around 1.8 lm as high as 2.51  10
20 cm2 at 1870 nm, which is larger than that in tellurite glass (0.78  10
20 cm2) [34] and the silicate glass (0.2  10
20 cm2) [36] indicating that it is probable to achieve high gain in the present glasses [6].

W D
A ¼

ð3Þ

  2p jHDA j2 SNDA
h

ð4Þ

where |HDA| is the matrix element of the perturbation Hamilton between lower and upper states in the energy transfer process, SNDA is the overlap integral between the m-phonon emission sideband of donor ions (D is Yb3+ here) and the k-phonon absorption sideband of acceptor ions (A is Tm3+ here) and N is the total phonons in the whole energy transfer process (N = m + k). On the measured emission and absorption spectra, Tarelho et al. [47] presented a theory to value the spectral sideband based:

rDeðm
phononsÞ ¼ rDe ðkþm Þ 

3.4. Decay analysis and micro parameter The fluorescence lifetimes of STY-x glass samples are obtained by fitting an exponential to the tail of the measured curve. Fig. 4 (a) shows the lifetime values of the STY-x. Fig. 4(b) presents the lifetimes comparison in STY-0 and STY-3 glass samples. The lifetime of Tm3+ in co-doped is longer than that in Tm3+ single doped silicate glass. This finding indicates the efficient energy transfer from Yb3+ to Tm3+ once again. The two very important parameters of the energy transfer efficiency and coefficient play an important role in revealing the sensitization effects of Yb3+. The energy transfer efficiency (g) is shown as [43]:

rAaðk
phononsÞ ¼ rAa ðk
k Þ 

Sk0 e
S0
Þk rAaðexptÞ ðE þ k
hx0 Þ ðn k!

ð5Þ ð6Þ


¼ 1=ðe
hx0 =kT
1Þ is the average where S0 is the Huang-Rhys factor, n occupancy of phonon mode at temperature T (Here is room temper
1 
1  ature). kþ hx0 and k
hx0 are the wavem ¼ 1= k
m
k ¼ 1= k þ k
lengths of Yb3+ with m-phonon emission and Tm3+ with k-phonon absorption, respectively. Ignoring the k-phonon annihilation process and just focusing on the m-phonon creation process, the energy transfer coefficients of the direct transfer (D ? A) and back transfer (A ? D) can be gained using the following equations:

C D
A ¼

Fig. 3. (a) Absorption cross section and emission cross section of Tm3+ and Yb3+ in NIR (b) absorption cross section and emission cross section of Tm3+ in MIR.


S0 Sm 0 e
þ 1Þm rDeðexptÞ ðE
m
hx0 Þ ðn m!

6cg Dlow

Z 1 X N X P þðN
kÞ P
k Pþk

ð2pÞ4 n2 g Dup N¼0 k¼0

rDe ðkþN ÞraA ðkÞdk

Fig. 4. (a) Decay curve of STY (b) decay curve of ST and STY-3.

ð7Þ

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R. Cao et al. / Optics and Laser Technology 94 (2017) 106–111

Table 3 Energy transfer parameters between Yb3+ and Tm3+ in STY sample. Energy transfer 3+

Coefficient (cm6/s)

N (number of phonons) (% phonon assisted)

3+

Yb ? Yb (migration) (2F5/2 + 2F7/2 ? 2F7/2 + 2F5/2) Yb3+ ? Tm3+(direct transfer) (2F5/2 ? 3H5) Tm3+ ? Yb3+(back transfer) (3H5 ? 2F5/2)

0 97.682 1 75.468 1 98.633

2 19.26

1 2.318 3 5.272 2 1.367

8.52  10
39 5.43  10
41 2.77  10
43

reduce the concentration of hydroxyl groups, which was proposed by Jonathan [49]. Fig. 5 is the transmittance spectra of the silicate glass samples, which compare the content of OH
groups when alkali earth metal oxides are replaced by fluoride. It is shown that the content of OH
groups decreased sharply with the appearance of BaF2 and CaF2. The content of OH
groups can be evaluated by the absorption coefficient a
OH, which is calculated by:

a
OH ¼ lnðT0 =TÞl

ð11Þ

where l is the thickness of the sample equal to 0.15 cm, and the T0 and T are the incident and transmitted transmittance intensities, respectively. According to this formula, the absorption coefficient in Tm3+/Yb3+ co-doped silicate glass (STY) is 0.905 cm
1. The value drops to 0.527 cm
1 when BaF2 and CaF2 are doped to silicate glass samples (SFTY). 4. Conclusion 3+

3+

Fig. 5. The transmittance spectra of Tm /Yb

C A
D ¼

A 6cg low

1 X N X

A ð2pÞ4 n2 g up N¼0 k¼0

P
ðN
kÞ P
k Pþk


þ 1ÞS0  PþðN
kÞ ffi exp½
ð2n


S0  P
k ffi exp½
2n

Z

co-doped silicate glass.

reA ðk
N ÞrDa ðkÞdk

ðN
kÞ

S0
þ 1ÞN
k ðn ðN
kÞ!

Sk0
Þk ðn k!

ð8Þ

ð9Þ

ð10Þ

The gDlow (gAlow ) and gDup (gAup ) are the degeneracies of the respective lower and upper levels of the donor (acceptor), respectively.
Pþ ðN
kÞ and Pk means the probability of (N-k) phonon emission by the donor and k-phonon absorption by the acceptor. It is shown in Table 3 that the energy transfer coefficient of Yb3+ ? Yb3+ transition is the largest because compared with the phonon assisted energy transfer processes, this process is resonant. The direct energy transfer (from Yb3+ to Tm3+) is more efficient than the back energy transfer (from Tm3+ to Yb3+) because the direct energy transfer coefficient is much larger than the back value, and the ratio (CYb-Tm/CTm-Yb) reaches up to 196, it contributes to obtain the enhanced 1.8 lm emission of Tm3+.

3.5. Hydroxyl group analysis There are many ways to remove water from laser glass, such as reaction atmosphere method, introduction of fluoride method, ultra-vacuum melting method, and raw material soaking method and so on. Besides, Jiang [48] found increase the melting temperature and select the appropriate gas phase reactant (CCl4, SOCl2, POCl3) can improve the removal rate of phosphate glass. Besides, ZnF2 was used instead of ZnO and NH4HF pretreatment also can

1.8 lm emissions have achieved in Tm3+/Yb3+ co-doped silicate glass. The energy transfer process between Yb3+ and Tm3+ has been investigated in detail. Considering the spectra properties, the optimal Yb2O3 concentration is 3 mol%. The lifetime of Tm3+ in STY-3 sample is 1.42 ms, which is much larger than ones in Tm3+ single doped silicate glass (0.54 ms) and the energy transfer efficient between Tm3+ and Yb3+ is 90.94%. The energy transfer coefficients between Yb3+ and Tm3+ have been calculated by the extended spectral overlap method, the value is 5.43  10
41 cm6/s corresponding to the Yb3+:2F5/2 ? Tm3+:3H5, the ratio of direct transfer and back transfer is 196. The content of hydroxyl group is low,
1 the absorption coefficient a
when BaF2 and CaF2 OH is 0.527 cm are doped to silicate glass samples. The above value indicate that Tm3+/Yb3+ co-doped silicate glass is a promising way to achieve high efficient 2 lm laser emission. Acknowledgments This research was financially supported by the Chinese National Natural Science Foundation (Nos. 51372235, 51272243, and 51472225), Zhejiang Provincial Natural Science Foundation of China (No. LR14E020003). References [1] G. Galzerano, M. Marano, S. Longhi, E. Sani, A. Toncelli, M. Tonelli, P. Laporta, Opt. Lett. 21 (2003) 2085–2087. [2] J. Geng, Q. Wang, T. Luo, S. Jiang, F. Amzajerdian, Opt. Lett. 22 (2009) 3493– 3495. [3] D. Creeden, P.A. Ketteridge, P.A. Budni, S.D. Setzler, Y.E. Young, J.C. McCarthy, K. Zawilski, P.G. Schunemann, T.M. Pollak, E.P. Chicklis, M. Jiang, Opt. Lett. 4 (2008) 315–317. [4] B. Richards, A. Jha, Y. Tsang, D. Binks, J. Lousteau, F. Fusari, A. Lagatsky, C. Brown, W. Sibbett, Laser Phys. Lett. 3 (2010) 177–193. [5] M. Rico, J. Liu, U. Griebner, V. Petrov, M.D. Serrano, F. Esteban-Betegón, C. Cascales, C. Zaldo, Opt. Exp. 12 (2004) 5362–5367. [6] B. Bochentyn, A. Warych, A. Szreder, A. Mielewczyk-Gryn´, J. Karczewski, M. Przes´niak-Welenc, M. Gazda, B. Kusz, J. Non-Crys. Sol. 439 (2016) 51–56. [7] M. Reben, J. Wasylak, D. Dorosz, Proc. SPIE 7120 (2008) 712001.

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