The enhanced two micron emission in thulium doped tellurite glasses

Optical Materials xxx (2013) xxx–xxx

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The enhanced two micron emission in thulium doped tellurite glasses Hrvoje Gebavi a,⇑, Stefano Taccheo a, Daniel Milanese b a b

College of Engineering, Swansea University, Singleton Park, SA2 8PP Swansea, UK Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

a r t i c l e

i n f o

Article history: Available online xxxx Keywords: Energy transfer Thulium Fluorotellurite glass

a b s t r a c t In this work, we demonstrated enhanced infrared thulium emission regarding lifetime values in the novel fluorotellurite glasses in comparison with the traditional TeO2–ZnO–Na2O host. The OH concentration reduction in the novel host material established fast-diffusion regime for the 3 F4 ! 3 H6 emission while the same emission in ‘TZN’ host was in the frame of the diffusion-limited regime. The spectroscopic and thermo-mechanical properties of tellurite glasses showed promising features and possibility for fiber drawing. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Fiber lasers operating in the infrared (IR) spectral region have numerous applications in remote sensing and medicine [1,2]. The most utilized and examined glass host material for fiber laser applications is silica glass mostly thanks to great thermo-mechanical properties. Recently, single frequency fiber laser doped with thulium [3] and mode-locked holmium [4] fiber laser were demonstrated. The main advantages of tellurite over silica glasses are in the wide transparency range (up to 4 lm), low phonon energy (700 cm1), high refractive index (2@633 nm), good rare earth solubility and high nonlinearity. Nonetheless, weaker thermomechanical properties limit their applications keeping them out of high power range. In [5], 1 W power at 2 lm emission in double-cladding, tungsten–tellurite fiber laser is claimed. That result certainly encourages further research towards tellurite fiber lasers improvement especially in IR region where silica glass is not competitive. Apart from the thermo-mechanical features improvement, one of the main issues of host glasses is OH content. Ultra dry tellurite glasses and their importance were stressed and recently reported in the literature [6]. Optical fibers fabricated from the glass host with reduced OH content are expected to have 0.8 dB/m optical loss at 2 lm region [7]. The most suitable active ion which can be used for 2 lm laser applications is thulium. The main reasons are relatively wide absorption band at 790 nm (accessible with the commercial pump diodes) and the phonon assisted energy transfer process called ‘cross-relaxation’ (CR: 3 H4 ; 3 H6 ! 3 F4 ; 3 F4 ) which gives two emitted for one pump photon. Thulium 3 F4 ! 3 H6 emission at 2 lm can be obtained in tellurite and silica glass hosts, however, with ⇑ Corresponding author. Tel.: +44 (0)1792 606843. E-mail address: [email protected] (H. Gebavi).

different efficiencies. Besides that, higher dopant concentrations allow short cavity fiber lasers which have beneficial laser beam quality such as background loss reduction and narrow emission linewidth [8]. Short laser cavity requires highly doped fiber core which carries a potential risk of clustering and increase of excited state absorption (ESA) or reverse cross-relaxation processes. Single mode, short cavity, line narrowed fiber laser may be beneficial for sensing application in high-resolution spectroscopy. The spectroscopic investigation of thulium and understanding of all radiative mechanisms is also a prerequisite to investigate photodarkening phenomenon [9], an issue still not very clear for Tm3+ ions [10] and important and unresolved question in neither silica nor tellurite glass matrices. Tellurite glasses are one of the best candidates among soft glasses and could be used in the spectral region where silica glass is not relevant. Nonetheless, glass host for rare-earth (RE) ions still need optimization. In this paper, we presented novel host composition 75TeO2–xZnF2–yZnO–12PbO–3Nb2O5 [x(10  0), y(x  10) mol.%] and compared it with the traditional TeO2–ZnO–Na2O (‘TZN’) composition. This glass composition showed increased lifetime values, but significant crystallization deficiency in respect to previously reported results [11]. The advantages as higher refractive index, lower OH content and longer 3 F4 ! 3 H6 lifetime were demonstrated. The host is characterized as a weak self-quenching material. The aim was to optimize host material for short cavity fiber laser emitting at 2 lm. Because of that reason the wide range of thulium 0.82– 22.1  1020 cm3 ions concentrations were utilized. Rheological and thermo-mechanical tests showed possibility of fiber drawing. 2. Experimental techniques Oxide and fluoride chemicals utilized for glass fabrication and corresponding purity percentages were the following: TeO2 (99 + %), ZnO (99.99%), Na2CO3 (99.995%) ZnF2 (99%), PbO

0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.03.025

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H. Gebavi et al. / Optical Materials xxx (2013) xxx–xxx

(99.99%), Nb2O5 (99.9%), and Tm2O3 (99.99%). Powder batches were homogenized during 12 h mixing process prior to melting inside a glove box under controlled and dry atmosphere. In order to additionally increase glass homogeneity, 4 h melting process at 900 °C in platinum crucible included steering. The fabricated samples were based on two different host glass compositions: (a) 75TeO2–xZnF2–yZnO–12PbO–3Nb2O5 [x(10  0), y(x  10) mol.%] (‘FT’) and (b) 75TeO2–20ZnO–5Na2O (‘TZN’) and doped with increasing amounts of Tm3+ (0.36, 1, 2, 3, 4, 5, 6, 7 mol.%). The relative molar ratio of the host glass constituents was kept the same for all the samples regardless of Tm3+ doping. Differential Scanning Calorimetry (DSC) was utilized for ‘Tg’ (glass transition temperature peak onset value) and ‘Tx’ (crystallization peak onset value) determination. Glass thermal stability given by DT = Tx  Tg plays an important role in fiber fabrication process as well as in any other glass application. Experimental error for Tg, and Tx was ±3 °C. Samples were heated up in aluminum glass pans at 10 °C/min rate under constant argon flow. The refractive index was measured at 633 nm by means of the prism coupling technique. The instrument resolution of ±0.0001 was reached after five scans at the same point. By measuring refractive index at different sample points, an experimental error of ±0.0003 was obtained. Characterization of the glass viscosity was carried out by parallel-plate method i.e. ‘Thermo-mechanical Analysis’ (TMA 7) system. Samples for viscosity measurements were obtained by glass melt quenching into cylindrically shaped brass mold with diameter of 4.5 mm and 4–6 mm in height. During measurements, the probe force of 100 mN and temperature gradient of 5 °C/min were applied. Glass samples with 1–2 mm thickness were used for lifetime measurements. Pump light at 790 nm (exciting 3 H4 thulium manifold at around 12,594 cm1) was focused on the sample edge and the fluorescence light from 3 F4 manifold was collected under 90° angle. Characteristic decay curve was processed by oscilloscope and fitted on single exponential function afterword. Measurements were performed at room temperature.

3. Results and discussion 3.1. Thermo-mechanical, rheological and optical properties Prepared glasses were divided in three groups as shown in Table 1. The first group ‘I’ presents the ‘FT’ host doped with 1, 3, and 5 mol.% Tm3+ while in the second group ‘II’ dopant concentration was the same (5 mol.% Tm3+), but the ZnF2 content was substituted with ZnO, as indicated below. In the third ‘TZN’ group dopant concentration was varying from 0.36 to 10 mol.% while the host composition was the same. Letter ‘T’ in the samples abbreviation signs thulium and the following number displays Tm3+ ions content in mol.%. Thermo-mechanical properties such as glass transition temperature and the glass stability ‘DT’, together with the refractive index at 633 nm are shown in Table 1. The first parameter, ‘Tg’ showed increase as the dopant concentration was increasing for both types of glass hosts. The increase of the ‘Tg’ with the glass modifier concentration indicated a stronger ion–host bonding and TeO4 ? TeO3 structural change [12]. In the case of the group ‘II’, decrease of the ‘Tg’ value was occurring for the ‘ZnF2/ZnO’ ratio increase. This behavior can be explained by the substitution of ZnO with ZnF2 which probably has led to the TeO2 covalent network breakage and viscosity decrease. It is assumed that the values of ‘DT’ greater than 120 °C minimize the risk of crystallization during the fiber drawing process. As demonstrated, ‘DT’ decreases with the Tm3+ concentration and the ‘ZnF2/ZnO’ ratio increase which can be attributed to the conversion

of the basic structural unit of the tellurite glasses or it is due to the O2 and F exchange causing different coordination geometry. It has been possible to overcome the task in question i.e. to increase the ‘DT’ values beyond 120 °C by means of a small GeO2 addition as demonstrated in [11]. Analog to behavior of the parameter ‘DT’, the refractive index ‘n’ decreased for both, the dopant concentration and adduced ‘ZnF2/ ZnO’ ratio increase. Its value showed an increase of 0.1 in ‘FT’ glasses in comparison with the ‘TZN’ glass host which gives the emission cross section increase and leads to advanced fiber laser performances [11]. In order to test whether the novel ‘FT’ glasses can be used for fiber fabrication, rheological characteristics were tested and compared with the traditional ‘TZN’ host composition. It can be expected that the adequate drawing temperature corresponds to the viscosity value of 104 Pa s (log(g)  4) [13]. Gent’s equation [14] was utilized for the viscosity determination: 5

g¼

2pF h
3 3V   ð2  p  h þ VÞ dh dt

ð1Þ

where F is the applied force, g is 9.81 m/s2, h is sample height, V is sample volume. Vogel–Fulcher–Tammann–Hesse (VFTH) equation gives temperature dependence with viscosity for strong, moderate and fragile glass-forming liquids [15]:

logðgÞ ¼ A þ

B T  T0

ð2Þ

where A is the log(g0), B ¼ D  T 0 = ln 10, ‘D’ is the strength parameter, and ‘T0’ is diverging temperature. It is expected that glasses have similar complexity at certain temperatures. Following that assumption and in order to extrapolate viscosity values out of the experimental range and determine the drawing temperature, two boundary conditions were considered: (a) viscosity value at glass transition temperature is 1012 Pa s [16], and (b) A = 5 [17] for Tg/T ? 0. Experimentally obtained viscosity values, extrapolated curves and glass transition temperatures are shown in Fig. 1. Furthermore, the comparison between ‘TZN’ and ‘FTG’ hosts given in Fig. 1 shows that ‘FT’ glasses have higher viscosity than ‘TZN’ glasses for the same dopant concentration. Fitting parameters of VFTH model are given in Table 2. These data showed that the drawing temperature is higher in the case of the ‘FT’ glasses for about 15 °C. Considering crystallization temperatures reported in Table 1, it can be seen that both glass compositions can be used for fiber fabrication. The experimental error caused for example by slightly unparalleled sample bases could influence the results. However, the authors expect negligible deviation of the drawing temperature still remaining far below crystallization temperature. The obtained results highlighted the potential of low doped FT glasses for fiber fabrication. 3.2. Concentration quenching of the 3F4 manifold emission in ‘TZN’ and ‘FT’ hosts The time resolved luminescence provided the lifetime values of the 3F4 manifold. The lifetime values were calculated by decay curves fitting on a single exponential function. The quenching rate RQ is given by the following relationship [18]:

RQ ¼

1



1

sðcÞ s0

¼ K n  cn

ð3Þ

where ‘s0’ is lifetime at low concentration, ‘n’ is equal to 1 or 2 which corresponds to the case of the fast diffusion (weak selfquenching materials), or limited diffusion (strong self-quenching

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H. Gebavi et al. / Optical Materials xxx (2013) xxx–xxx Table 1 Thermal and optical properties of the prepared ‘FT’ and ‘TZN’ Tm-doped tellurite glasses. Group

Sample name

Tm3+ (1020 cm3)

Tg (°C)

DT (°C)

n (633 nm)

I

F(10ZnF2–0ZnO) T1 F(10ZnF2–0ZnO) T3 F(10ZnF2–0ZnO) T5

2.14 6.39 10.55

307 316 321

151 78 46

2.1236 2.1183 2.1057

II

F(10ZnF2–0ZnO) T5 F(7.5ZnF2–2.5ZnO) T5 F(5ZnF2–5ZnO) T5 F(2.5ZnF2–7.5ZnO) T5 F(0ZnF2–10ZnO) T5

10.55 10.59 10.64 10.69 10.72

321 322 328 334 336

46 78 82 86 162

2.1057 2.1120 2.1179 2.1261 2.1373

III

TZN TZN TZN TZN TZN TZN TZN TZN TZN

0.82 2.28 4.87 6.84 9.06 11.3 13.5 15.7 22.1

311 313 317 320 321 324 326 330 333

132 134 146 148 152 153 147 149 133

2.0488 2.0454 2.0412 2.0396 2.0356 2.0306 2.0268 2.0239 2.0116

T0.36 T1 T2 T3 T4 T5 T6 T7 T10

11

12

TZN exp. data FT exp. data TZN fit curve FT fit curve

10 9

10

8

9

ln(RQ)

log (viscosity)

11

8 7 6

TZN, slope = 2.1

7 6 5

5

4

4

FT, slope = 1

3

3 300

330

360

390

2

420

59.5

60.0

60.5

61.0

ln(Tm3+ Fig. 1. Viscosity comparison of ‘TZN’ and ‘FT’ glass hosts with dopant concentration of 1 mol.% Tm3+.

Table 2 VFTH fitting parameters and drawing temperature for FT and TZN hosts with dopant concentration of 1 mol.% Tm3+. Sample

B

T0

* 2

FT1 TZN T1

1848 ± 17 1415 ± 17

182 ± 2 217 ± 2

96.4 95.1

R (%)

**

307 313

Tg

**

Tx

458 447

***

T[log(g)=4] (°C)

388 374

R2 is the square of the correlation coefficient. Tg, Tx ± 3 °C obtained by DSC. *** T[log(g)=4] ± 5 °C. *

**

62.0

62.5

63.0

concentration)

Fig. 2. Logarithmic dependence of 3 F4 ! 3 H6 transition quenching rate and dopant concentration. The curve slopes are 2 and 1 for ‘TZN’ and ‘FT’ glass hosts, respectively.

different models. The first one, related with the fast diffusion has been theoretically given in [19]. It assumes that for a given energy gap, a multiphonon assisted energy transfer is more probable than a non-radiative decay. The model is valid for a multiphonon selfquenching of the first lanthanide excited states in high-purity host. The equation which correlated lifetime values and dopant concentration as dependent parameters is given as:

s¼ materials), respectively. Fig. 2 shows the quenching rate for different dopant concentrations in lnln scale for ‘TZN’ and ‘FT’ host materials. During the measurements, especial attention was dedicated to reabsorption due to set-up geometry. It was assumed that the lifetime deviation of less than 15% could be attributed to real photon reabsorption. Fig. 2 shows significantly different curve slopes in the cases of ‘TZN’ and ‘FT’ hosts. Therefore, the slope equal to 1 can be ascribe to the fast diffusion case characteristic for weak self-quenching materials while the slope of the ‘TZN’ glasses is equal to 2.1 and indicates limited diffusion and strong self-quenching materials. The experimentally obtained lifetime values dependence on dopant concentrations could be theoretically described by two

61.5

s0 1 þ 1:45  ðc=c0 Þ  eN=3

ð4Þ

where ‘s0’ is lifetime at low concentration, ‘c’ is dopant concentration, ‘c0’ is the critical concentration related to the sensitizer energy transfer, and ‘N’ is the number of phonons necessary to bridge the energy gap. Considering the experimental results of Raman spectroscopy obtained on fluorotellurite and tellurite glasses [6,12] the phonon energy of 760 cm1 is expected. Besides that, the 3F4 manifold energy gap is taken as 5882 cm1 which gives the number of phonons N = 7.73. The above equation is valid for 5–8 phonons radiative transition size which is well satisfied in our case. The second assumption was that only dipole–dipole transitions are included. This condition simplifies the model since probably the higher order interactions play significant role as well [20].

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H. Gebavi et al. / Optical Materials xxx (2013) xxx–xxx

In the second case, related with the ‘TZN’ host material, the selfquenching behavior can be described by the formula proposed by Auzel et al. [21]:

c c0

where ‘s’ is measured lifetime at given concentration ‘c’, ‘s0’ is lifetime for low concentrations i.e. radiative lifetime, ‘c0’ is the critical sensitizer concentration related with critical distance between sensitizer and trap as R0 = (3/(4pN0))1/3. The obtained values in the case of the ‘FT’ host were: s0 = (3.61 ± 0.01) ms, c0 = (1.77 ± 0.03)  1020 cm3 while for the ‘TZN’ host: s0 = (3.29 ± 0.07) ms, c0 = (4.2 ± 0.1)  1020 cm3, and R0(3F4) = 8.3 Å. The radiative lifetime value increased 16% and the critical concentration was shifted significantly towards the higher concentrations in the case of novel ‘FT’ glasses. The attention should be paid to the interpretation of the critical concentration since in Eq. (4) it corresponds to 90% of a single ion intrinsic lifetime value while in the Eq. (5) to 41%. The lifetime values fit on the theoretical curves is shown in Fig. 3. Fig. 3 demonstrates longer 3 F4 ! 3 H6 lifetime values of ‘FT’ glasses especially in the case of higher doping concentrations. High doping concentration increases quantum efficiency due to better overlapping of electric fields between sensitizer and acceptor ions, but also a number of concentration quenching processes become dominant over a certain concentration value. Furthermore, the 3 F4 ! 3 H6 time decay was analyzed depending on the ZnF2 content scaled as shown in the glass group ‘II’ (Fig. 4). As Fig. 4 reveals, experimental results showed a tendency of the lifetime value to reach its maximum for the ZnF2 content greater than 10 mol.%. Since the increase of the ZnF2 content impairs the thermo-mechanical properties (Section 3.1), the optimal value of the ZnF2 content for which the lifetime value reaches its maximum will be demonstrated in the future measurements. The lifetime values in these experiments were dependent on the atmosphere purity inside the glove box, initial contamination of the raw materials with OH groups and melting time duration [12,22]. In the present study, the lifetime values of ‘FT’ glasses significantly increased while the self-quenching efficiency is reduced. Since the quenching of the 3F4 manifold cannot be ascribed to the cross-relaxation as in the case of the 3H4 manifold, we supposed that the main factor for the spectroscopic properties improvement is due to OH group reduction. The simplified reaction

3.5

FT

3F 4

lifetime (ms)

3.0 2.5

0.5

1.6 1.4 1.2 experimental values 1.0 0.8 0

1

2

3

4

5

6

7

8

9

10

11

ZnF2 concentration (mol %) Fig. 4. Lifetime values of the ‘FT’ glasses with different ZnF2 content and constant Tm3+ concentration of 5 mol.%.

which was occurring due to ZnF2 presence can be written as: 2(OH) + 2F ? O2 + 2HF. We propose that this OH vibrations reduction is bounded with ion–ion interaction regime change demonstrated in Fig. 2. 4. Conclusions The present study concerns the investigation of two tellurite based host compositions while the emission in IR region at 2 lm is ensured with thulium ions. Development and optimization include better understanding of rare earth ion concentration influence on lifetime values and energy transfer dynamics. Thermal, rheological, optical and spectroscopic properties of the novel ‘FT’ host with the traditional ‘TZN’ host were compared. The drawing temperature of the examined Tm3+ doped glasses was lower than crystallization temperature which guarantees good viscosity for fiber drawing. The first indication of the advance host material was given by the increase of the refractive index which guarantees enhanced emission cross section necessary for good fiber lasers performances. Experimental decay times of the 3F4 manifold was measured as a function of thulium and ZnF2 concentration. The findings of this study indicated that the OH content determines whether the 3F4 manifold self-quenching will be ruled by fast diffusion or limited diffusion processes. These results characterized fluorotellurite glasses as the weak self-quenching materials. The enhanced lifetime value gave significant advantageous over the traditional ‘TZN’ host composition which encourages further improvements.

This project was funded by FP7 LIFT (Leadership in Fiber Technology) Project (Grant #228587).

TZN

1.0

1.8

Acknowledgment

2.0 1.5

2.0

lifetime (ms)

p

ð5Þ

 2

4

1þ

9 2 

2.2

3F

s0

s¼

2.4

TZN exp. data FT exp. data TZN fit curve FT fit curve

1

References

10

Tm3+ concentration 1020 (cm-3) Fig. 3. Experimental data of 3 F4 manifold fitted on theoretical curves in the case of ‘TZN’ and ‘FT’ hosts.

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