Pr3 +–Yb3 +‐codoped lanthanum fluorozirconate glasses and waveguides for visible laser emission

Pr3 +–Yb3 +‐codoped lanthanum fluorozirconate glasses and waveguides for visible laser emission

Journal of Non-Crystalline Solids 358 (2012) 2695–2700 Contents lists available at SciVerse ScienceDirect Journal of Non-Crystalline Solids journal ...

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Journal of Non-Crystalline Solids 358 (2012) 2695–2700

Contents lists available at SciVerse ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol

Pr 3 +–Yb 3 +‐codoped lanthanum fluorozirconate glasses and waveguides for visible laser emission B. Dieudonné a, B. Boulard a,⁎, G. Alombert-Goget b, 1, Y. Gao a, A. Chiasera b, S. Varas b, M. Ferrari b a b

Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, Equipe Fluorures, LUNAM Université, Université du Maine, Av. Olivier Messiaen, 72085 Le Mans cedex, France Institute for Photonics and Nanotechnologies, CNR, CSMFO Laboratory, via alla Cascata 56/C Povo, 38123 Trento, Italy

a r t i c l e

i n f o

Article history: Received 26 March 2012 Received in revised form 25 June 2012 Available online 20 July 2012 Keywords: Fluorozirconate; Glasses; Rare earth; Waveguide; Luminescence

a b s t r a c t Pr3+–Yb3+‐codoped fluoride glass waveguides have been synthesized by Physical Vapor Deposition (PVD). A study of the evaporation of ternary mixture of rare earth fluorides LaF3–PrF3–YbF3 has been necessary to control the doping of the evaporated glass. Optical and spectroscopic studies have been performed in both bulk and waveguide configuration. Red, orange, green and blue emissions in Pr3+–Yb3+-codoped lanthanum flurozirconate glasses called ZLAG have been investigated, by exciting in the blue or in the infra-red at 980 nm. Bulk samples with different dopant concentrations (0.25–3 mol% for Pr3+ and 0–5 mol% for Yb3+) have been studied in order to optimize the Pr3+ emission. It has been shown than the luminescence is similar in bulk and waveguide upon excitation at 980 nm. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Frequency up-conversion of infra-red to visible light in rare-earth (RE) doped materials has attracted much interest because of the large number of potential applications already known. One of major interests is the operation of visible laser pumped with infra-red commercially available diode laser. The fluoride glasses are well adapted for this kind of application, because of their wide optical transmission from 0.2 to 8 μm, their low phonon energy (600 cm−1) and the ability to accept high RE content [1]. In particular, the Pr3+–Yb 3+‐codoped fluoride system is interesting, since laser action in the blue, green, orange and red have been obtained by up-conversion in ZBLAN glass fiber, BaY2F8 and LiYF4 hosts using the same pumping scheme at 850 nm [2]. An energy level diagram is shown on Fig. 1; the interesting properties of the system are the strong infra-red absorption of Yb 3+, the efficient energy transfer (ET) from Yb3+ to Pr3+ [3,4] and the low non-radiative probability for the 1G4 → 3F3,4 transition of Pr3+. Among the different fluoride glasses with different network formers the new family of lanthanum fluorozirconate glass (ZLAG and ZLA) [5,6] excluding glass modifier (i.e. NaF, BaF2) emerged as a promising host since this glass is also the precursor of transparent glass ceramics, obtained after an adequate thermal treatment. These glasses are characterized by a high LaF3 content (23 mol%) in comparison to the well

known ZBLAN or ZBLA fluoride glasses with 5 mol% LaF3 [7,8]. Last but not the least, RE‐doped ZLAG glasses (with RE=Er, Yb, Ce…) have been obtained as thin films and deposited on CaF2 and SiO2/Si substrates [9,10]. Thin film waveguides made from up-converting fluoride glasses may offer a way to combine the attractive aspect of light producing single crystal or fibers while offering a very compact light source. Moreover, the ability to deposit these waveguides on semiconductor substrates would allow also direct integration with the pump source. In this scope, the ZLAG glass is a fluoride material of main interest. Prior to this work, other techniques have been used to obtain RE-doped fluorozirconate glass planar waveguides: ionic F/Cl exchange for Pr3+-ZBLA [11] and pulsed laser deposition for Pr3+-ZBLAN [12]; only the ionic exchange gave waveguides with the required optical properties. In this paper, we present the optical and spectroscopic characterizations of Pr3+–Yb3+-codoped ZLAG bulk glasses for visible emission of Pr3+ in the blue, green and red; direct pumping in the 3P2 level of Pr3+ and undirect pumping via ET are compared. The results of the first blue and infrared pumped Pr3+–Yb 3+-codoped glassy waveguides that produce visible light are also reported.

2. Experimental 2.1. Bulk glass synthesis

⁎ Corresponding author. Tel.: +33 2 43 83 33 70. E-mail address: [email protected] (B. Boulard). 1 Laboratoire de Physico-Chimie des Matériaux Luminescents, UMR CNRS 5620, Université Claude Bernard Lyon1, bâtiment Alfred Kastler, 10 rue Ada Byron, 69622 Villeurbanne cedex, France. 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.06.030

Two series of bulk ZLAG glass — with composition: – 70ZrF4(23.5 − x)LaF30.5AlF36GaF3xPrF3 with 0 b x b 3 mol% – 70ZrF4(23 − y)LaF30.5AlF36GaF30.5PrF3yYbF3 with 0 b y b 5 mol%

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B. Dieudonné et al. / Journal of Non-Crystalline Solids 358 (2012) 2695–2700 3

4

P2 3

P 0, 1I6, 3P 1

2

20000

0

D2 log (P mmHg)

1

15000

1

10000

3 3

5000

F 5/2

ZrF4 GaF3 AlF3 NaF BaF2 LaF3 YbF3

-6

F4 F3 F2

3

-8

ET

-10 500

H6

3

700

H5

3

0

G4

-4

900

1100

1300

Temperature (°C) 2

H4

Pr

3+

Yb

F 7/2

3+

Fig. 1. Energy level diagram of Yb3+ and Pr3+ ions showing possible energy transfer (ET).

were prepared as follows; stoichiometric quantities of high purity fluorides (purity > 99.9%) – 4 g – were melted at 875 °C for 15 min in inert atmosphere. The temperature was shortly taken to 900 °C before casting into a brass mold preheated at 240 °C. The heating at 900 °C reduces the viscosity of the melt while the short duration of this step minimizes the loss of ZrF4 that sublimates at 900 °C. Chemical analysis allowed estimating the loss of ZrF4 and GaF3 at ~ 2 mol% [6]. The real concentration of the dopant is thus slightly higher than the nominal one; the ratio is 1.08. 2.2. Waveguide fabrication Amorphous waveguides ZLA (ZrF4–LaF3–AlF3) codoped with Pr 3+ and Yb 3+ were fabricated by Physical Vapor Deposition (PVD). Two separated crucibles containing ZrF4 and rare earth (RE) fluoride mixture (RE = La, Pr, Yb) were heated at different temperatures in order to counter-balance the vapor pressure gap between the ZrF4 and REF3 (Fig. 2). To avoid the sublimation of ZrF4, the ZBNA (52ZrF4– 24BaF2–20NaF–4AlF3 in mol%) glass with a low melting point was used instead of pure ZrF4 to better control the evaporation rate. Moreover, a few % of the glass stabilizer AlF3 could be incorporated in the deposit. Contrary to bulk glass, we found that the stabilizer GaF3 was not necessary to get stable glassy films. Although the vapor pressures of RE fluorides are close, the evaporation of the REF3 physical mixture is not congruent. To achieve Yb 3+–Pr 3+-codoped ZLA glassy deposits, the behavior of mixture of LaF3, YbF3 and PrF3 powders toward evaporation has to be known. Actually the role of crystal chemistry (ionic radius of RE 3+, crystal structure of REF3) is essential to understand the evaporation of REF3 mixtures [13]; the crystal form of LaF3 and PrF3 is hexagonal because the ionic radii are close (1.03 Å for La 3 + and 0.99 Å for Pr 3+) while the crystal form of YbF3 is orthorhombic with 0.87 Å for the ionic radius of Yb 3+ (radii taken from [14]). When a solid or liquid solution is formed between RE, the vapor pressure P of each fluoride is given by the Raoult law: P = x · P°, where P° is the vapor pressure of pure REF3 and x is the molar fraction in the mixture. When no solution is formed (x = 1), the vapor pressure of each fluoride is P°. The relative vapor pressures between the RE fluorides have been estimated after chemical analysis of the deposit obtained for different starting mixtures. The evaporation of binary (1 − y)LaF3–yYbF3 with

Fig. 2. Vapor pressure curves for fluoride entering ZLAG, ZBNA glasses and some RE fluorides.

0.30 b y b 0.45 gives the same La/Yb molar ratio in the deposit. LaF3 and YbF3 thus evaporate from the starting mixture as if they were pure because not extended solid-solution can be made between them, owing to the large difference in ionic radii. The vapor pressure ratio P∘LaF obtained is ∘ 3 ¼ 2:5  0:1; this means that the 0.7LaF3–0.3YbF3 P YbF 3

mixture can be evaporated congruently. In the case of ternary (1−x−y) LaF3–yYbF3–xPrF3, the composition of the deposit is consistent with the Raoult law for ideal solutions formed between LaF3 and PrF3 as shown on Fig. 3, because of close ionic radii. In this case, the vapor pressure of each fluoride is proportional to its molar fraction; the La/Pr molar ratio is thus given by: La 1−x P∘PrF3 ¼ : Pr x P∘LaF3 The slope on Fig. 3 gives an estimation of the vapor pressure ratio P∘PrF of pure RE fluorides, i.e. ∘ 3 ¼ 1:2  0:1; PrF3 appears more volatile P LaF3 than LaF3. To prepare the Pr 3+–Yb 3 +‐codoped ZLA glass with Zr/RE = 7/3, the composition with x = 0.015 and y = 0.3 was selected for the

3.0

100 x/(1-x-y) in the deposit

3

2

-2

2.5 2.0 1.5 1.0 0.5 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

100 x/(1-x-y) in starting composition Fig. 3. Comparison between starting composition and deposit for the evaporation of P∘PrF the ternary mixture (1 − x− y)LaF3–yYbF3–xPrF3. The slope gives the ratio ∘ 3 . P LaF 3

B. Dieudonné et al. / Journal of Non-Crystalline Solids 358 (2012) 2695–2700 Table 1 Thermal properties of bulk ZLAG and ZLAGB glasses.

ZLAG 0.5Pr3+–xYbF3

ZLAGB 0.5Pr3+–xYbF3

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Table 2 Composition, thickness (±0.1 μm), refractive index n of Pr3+/Yb3+‐codoped ZLA films.

YbF3 (mol%)

Tg (°C)

Tc (°C)

ΔT (°C)

0 1 2 3 5 0 10

404 404 403 407 410 375 377

472 474 476 482 476 443 423

68 70 73 75 66 68 46

Sample

W3 W7

Thickness (μm)

n at 633 nm (±0.0005)

Composition (mol%) ZrF4

PrF3

YbF3

1.9 2.1

1.5288 1.5268

81 59

0.4 0.8

5.3 11.5

X-ray analysis Amorphous Amorphous

(1.9 mm) were set at the same place in the experimental setup; the beam was injected in the glass 1 mm far from the surface. 3. Results and discussions

mixture of RE fluorides to be evaporated; the expected doping achieved is 0.5Pr 3+ and 8.5Yb 3+ in mol%. The ternary mixture of RE fluoride and the ZBNA glass were heated simultaneously at respectively 1330 °C and 530 °C in vacuum with a pressure around 10−4 mbar. Two quartz crystal microbalances were used to control the thickness and the film composition (i.e. Zr/RE ratio). Single crystal CaF2 substrate was put in rotation at the intersection of the two evaporation cones and heated close to the glass transition temperature Tg to reduce columnar microstructure and increase packing density. This way, the films are prevented from hydrolysis in air. 2.3. Characterization techniques Thermal properties of bulk samples were obtained by Differential Thermal Analysis (DTA SDT-Q600) and Differential Scanning Calorimetry (DSC Setaram 92) under argon atmosphere with a heating rate of 10 K/min. X-ray diffraction (XRD) analysis was carried out to control the vitreous state of the deposited film. The thickness and refractive index of the waveguides were determined in the visible and IR at 543.5, 633, 1304 and 1540 nm using m-line prism coupling technique (Metricon model 2010/M instrument). The refractive index of bulk glasses were measured with the same apparatus. Transmittance spectra of glasses were recorded at room temperature from 300 to 2500 nm using a spectrophotometer (Cary 5000). The photoluminescence in the visible were performed using the 476 nm line of an Ar+ laser for direct excitation of Pr3+ and 980 nm diode laser for upconversion excitation. The TE0 waveguiding mode of the samples was excited by prism coupling. Lifetime measurements on bulk samples were performed with a spectrofluorimeter (FLS920 Edinburgh Instruments Ltd). In order to compare the relative fluorescence intensity between the studied glasses, the conditions of excitation and detection were fixed, and the samples with the same thickness

WG3 WG7 ZLAG bulk ZLAGB bulk

1.53

Table 1 gathers the thermal characteristics of the glasses; the Tg slightly increases with YbF3 content. The synthesis of ZLAG glass with high YbF3 content (>5 mol%) was unsuccessful, probably because La3+ and Yb3+ ions are too different in atomic radius. In order to improve the glass stability, we added small quantity of glass modifier BaF2 to ZLAG composition. To find the optimal composition that gives ability to prepare transparent GC, we chose to mix the ZLAG glass with ZBLA glass (57ZrF434BaF24LaF35AlF3 in mol%) which is one of the most stable heavy metal fluoride glass. The molar ratio ZLAG:ZBLA equal to 9:1 allowed to achieve a high Yb3+ doping (up to 10 mol%) while keeping the ability to prepare a glass ceramic. As predicted the Tg decreases with the addition of the glass modifier BaF2. The refractive index at 633 nm is n = 1.511 and 1.506 for ZLAG and ZLAGB glass respectively, and does not change significantly with Yb3+ doping. As shown on Fig. 4, the dispersion curve follows the Cauchy distribution: n = A+ B × λ −2 with B = 4.3 · 10 3 nm2. 3.2. Planar waveguides synthesis ZLA planar waveguides codoped with Pr3+ and Yb 3+ have been prepared by PVD. Deposition rate varied from 8 to 11 Å/s, the thickness was close to 2 μm. X-ray analysis of films as deposited with substrate temperature close to Tg showed no discernable structure indicating their amorphous state. Table 2 gathers some characteristics of the waveguides. Fluctuation in composition between the waveguides, although prepared in the same conditions, is due to instability in the heating of the REF3 mixture. 3.3. Spectroscopic study of bulk The absorption cross section of Pr3+–Yb 3+-codoped ZLAG glass in the visible range (400–650 nm) and IR range (850–1050 nm) is given in Fig. 5. The bands observed in the visible are identified with the Pr3+ transitions while the broad band located in the IR is due to Yb 3+. The most intense absorption band in the visible is assigned to

15

1.52

σabs (10-21 cm2)

Refractive index

1.54

3.1. Bulk glass synthesis

1.51

1.50 400

600

800

1000

1200

1400

1600

Wavelength (nm)

3

3

3

3

H4

Yb3+ : 2F7/2

P2

2

F5/2

P1 ,3I 6

H4

10

H4

5

Pr 3+

3

3

P0

3

H4

Pr 3+ : 3 H4

1

D2

1

G4

0 400

500

600

900

1000

Wavelength (nm) Fig. 4. Comparison of dispersion curve for bulk glass (ZLAG and ZLAG codoped with 0.5Pr3+–1Yb3+) and ZLA waveguides. The curves correspond to the Cauchy distribution n = A+ B × λ−2.

Fig. 5. Absorption cross sections of Pr3+–Yb3+-codoped ZLAG glass (3%Yb) (dark line) and singly Pr3+ doped glass (light line).

B. Dieudonné et al. / Journal of Non-Crystalline Solids 358 (2012) 2695–2700

the 3H4 → 3P2 transition of Pr3+ around 442 nm which is suitable for direct pumping using a GaN blue laser diode. As for the band around 975 nm related to the 2F7/2 → 2F5/2 transition of Yb 3+, it is suitable for Pr3+ upconversion pumping through energy transfer from Yb3+ to Pr3+ ions. Pr 3+ has a weak absorption band close to 980 nm as seen on the absorption spectrum of the singly doped glass. The emission spectra with the two pumping schemas have been recorded. The Fig. 6a presents the emission spectra obtained in glass by direct pumping into the 3P0 level of Pr 3+ with 440 nm line of the Ar + laser. The typical emission lines of Pr3+ associated with the transition from the 3P0 and thermalized 1I6 and 3P1 to 3H4, 3H5, 3H6 and 3F2 levels around 478, 519/531/542, 603 and 634 nm, respectively are observed. Diode laser excitation at 980 nm produces a strong upconverted, green luminescence (510–570 nm) which is attributed to Er3+ impurities. This impurity comes not only from YbF3 and PrF3 but also from LaF3 raw material since emission is still observed without Yb3+. P. Goldner et al. have observed [15] similar pollution in Yb 3+ doped ZBLAN glass and estimated the Er3+ impurity level to a few ppm. The luminescence spectra of singly and codoped glasses in the range 450–650 nm is shown in Fig. 6b. For the singly doped glass, there is no luminescence coming from Pr3+; only the luminescence of erbium due to an excitated state absorption is observed. For codoped glass, the intensity of Pr 3+ emission is considerably weaker than that of Er3+ impurities. This observation, taking into account the large difference in concentration for Pr3+ and Er3+ impurities, indicates that the efficiency of energy transfer form Yb3+ to Pr3+ is low. In comparison to the spectra obtained by direct excitation, blue emission appears stronger at 478 nm in comparison to orange (603 nm) and red (634 nm); the green emission of Pr3+ is hidden by the strong Er3+ luminescence.

3

a

3

P0

H4 3

Intensity (a.u.)

P0

3

H6 3

3

P0

3

3

3

525

550

P0

475

3000

2000

1000

0 0

1

2

3

500

Fig. 7 shows the dependence of the up-conversion luminescence intensity as function of Yb 3+ concentration at fixed concentration of Pr 3 +. A linear behavior for small Yb 3+ concentration that is lower than 1% [4] is actually expected for energy transfer (ET) process involving Pr 3+–Yb 3+ pairs, in which the ytterbium sensitizer participates with just one transfer in the process [3,16]. The shape of curve evidences a deviation from this behavior, especially for concentration larger than 2 mol% Yb 3+. The deviation is stronger for the blue emission in comparison to the red and orange ones, although they all arise from the same electronic level. This can be explained by reabsorption of the blue emission because of the superimposing of absorption and emission bands of Pr 3+ around 478 nm (see Figs. 5 and 6). The ET rate reduction could be due to energy back transfer from Pr 3+ to Yb 3+ or a decrease of the Yb 3+ lifetime due to concentration quenching. In order to assess the former mechanism, we recorded the decay curve of 3P0 under blue excitation for singly doped and codoped samples, keeping the Pr 3 + concentration at 0.5%. The results are shown on Fig. 8. Reduction of the fluorescence lifetime as well as well as non-exponentially of the decay at short time are indicative of

0 Yb 1 Yb 2 Yb 3 Yb 5Yb

H5

575

600

625

650

Intensity (arb. units)

b

0.2 P0

3

H4

Er3+ 3

P0

P1

450

5

Fig. 7. Dependence of the Pr3+ emissions as function of the Yb3+ concentration in ZLAG at fixed Pr3+ concentration (0.5 mol%). The lines are a guide for the eye.

Wavelength (nm)

3

4

Yb3+ concentration (mol%)

H5

H4

450

478.5 nm 634 nm 602.5 mn

4000

3

P1

3

P1

F2

5000

Intensity (a.u)

2698

3

H6 3 P0

3

F2

3

H4

475

500

525

550

575

600

625

650

Wavelength (nm) Fig. 6. Visible luminescence spectra for 0.5Pr3+–1Yb3+-codoped ZLAG glass (a) under 440 nm excitation (b) under 980 nm excitation. The spectrum for singly Pr3+ doped glass (——) is shown to assess the energy transfer from Yb3+ to Pr3+.

0

50

100

150

200

Fig. 8. Decay curves of the luminescence from the 3P0 metastable state of Pr3+ ions at 478 nm under blue excitation at 440 nm as function of the Yb3+ concentration.

B. Dieudonné et al. / Journal of Non-Crystalline Solids 358 (2012) 2695–2700

a Emission (a.u.)

the presence of energy transfer form Pr 3+ to Yb 3+. The efficiency of this process has been evaluated to be 82% for 5 mol% Yb 3+ [17]. Although back energy transfer is contributing in the ET rate reduction, Yb 3+ quenching effect cannot be excluded. Yb 3+ lifetime should be recorded as function of ytterbium concentration to evaluate the interaction between Yb 3+ ions. To complete the study and see the effect of Pr3+ concentration, the lifetime τ from the 3P0 state of Pr3+ under direct excitation has been measured on singly doped glass with concentrations ranging from 0.25 to 3 mol% Pr3+. At 0.5% Pr, the lifetime is 37 μs, similar to the value obtained in ZBLA glass [11] and close to the one obtained in Pr3+ doped crystals [18]. From Fig. 9, the luminescence of Pr3+ from the 3P0 level shows a pronounced concentration quenching above 0.25 mol%. This quenching can be due to cross-relaxation between pairs of Pr3+ like (3P0, 3H4) → (1G4, 1G4) and (3P0, 3H4) → (3H4, 1D2) or to energy transfer migration among Pr3+ ions to quenching centers. The linear dependence of 1/τ with Pr3+ concentration up to 2 mol% is consistent with a cross-relaxation mechanism (see inset of Fig. 9). At higher doping level, energy transfer migration most probably occurs.

lifetime

WG3 WG7 bulk

500

550

600

650

700

750

Wavelength (nm)

b

Er 3+ impurities

Emission (a.u.)

3.4. Spectroscopic study of waveguides The luminescence spectra of the waveguides, obtained by the prism coupling technique are compared to the one obtained for the bulk glass, after excitation at 476 nm and 980 nm. In Fig. 10a, a broad and intense band with at maximum around 530 nm is noticeable for the waveguides excited at 476 nm, due to luminescence from defect centers, likely F color centers created during the film growing. Apart from this band, the overall spectrum of the waveguide and its relative intensities are the same. The luminescence of the waveguide doped with 0.4% Pr3+ appears more intense than the luminescence of the waveguide doped with 0.8% Pr3+, in agreement with the concentration quenching observed in bulk glasses. The waveguides excited at 980 nm exhibit up-conversion luminescence of Pr 3+ in the blue (478 nm), orange (604 nm) and red (634 nm). Fig. 10b shows the spectra of both waveguide and bulk glass normalized at 634 nm far from re-absorption that may occur in the blue because of the superposition of absorption and emission bands of Pr 3+ around 478 nm. The spectra are identical in the orange and red but the emission in the blue is more intense in the bulk, probably because of stronger re-absorption. A green luminescence due to Er3+ impurities similar to the bulk but less intense is also found. We explain this result by the fact that ErF3 is less volatile than P∘LaF LaF3 ( ∘ 3 ≈2:3 [9]). P ErF3

2699

WG3 bulk

450

500

550

600

650

Wavelength (nm) Fig. 10. Comparison of luminescence from Pr3+–Yb3+-codoped ZLAG bulk glass and ZLA waveguide a) under 476 nm excitation and b) under 980 nm excitation.

4. Conclusion

40

Acknowledgments

1/

50

We have investigated the visible luminescence in Pr 3+–Yb3+codoped lanthanum fluorozirconate glasses. Blue, green, orange and red emissions have been obtained by direct pumping in the blue or through frequency up-conversion by using a diode laser operating at 980 nm. Our result show that energy transfer occurs from Yb3+ to Pr3+ after absorption of Yb3+ at 980 nm. Planar waveguides of 2 μm thickness have been fabricated by PVD with Pr3+–Yb 3+‐codoping. Visible emissions similar to the bulk have been observed with the two pumping schemes. Luminescent defect centers have been detected when the waveguides are excited in the blue. But no defect center emission is observed using upconversion pumping, making this scheme more suitable for applications.

30

0

1

2

3

20

10 0.0

0.5

1.0

Pr3+ 3

1.5

2.0

2.5

3.0

concentration (mol%) 3+

Fig. 9. Variation of lifetime τ of P0 with Pr concentration in singly doped ZLAG glasses. The variation of 1/τ with concentration is reported in the inset, to assess the cross-relaxation mecanism.

We are grateful to Jean-Luc Adam and Virginie Nazabal (Sciences Chimiques Rennes — Equipe Verres et Céramiques, Université de Rennes) for making available their spectroscopic facilities, and Melinda Olivier for her help in lifetime measurements. The technical assistance and the special skill of Alessandro Carpentiero (IFN-CNR) are gratefully acknowledged.

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