Fluorescence quantum efficiency of Er3+ in low silica calcium aluminate glasses determined by mode-mismatched thermal lens spectrometry

Journal of Non-Crystalline Solids 351 (2005) 1594–1602 www.elsevier.com/locate/jnoncrysol

Fluorescence quantum efficiency of Er3+ in low silica calcium aluminate glasses determined by mode-mismatched thermal lens spectrometry J.A. Sampaio a

a,*

, S. Gama a, M.L. Baesso b, T. Catunda

c

Instituto de Fı´sica Gleb Wataghin, Universidade Estadual de Campinas, CP. 6165, CEP 13083-970, Campinas, SP, Brazil Departamento de Fı´sica, Universidade Estadual de Maringa´, Av. Colombo, 5790, CEP 87020-900, Maringa´, PR, Brazil c Instituto de Fı´sica de Sa˜o Carlos, Universidade de Sa˜o Paulo, CP 369, CEP 13560-970, Sa˜o Carlos, SP, Brazil

b

Received 8 October 2004; received in revised form 22 March 2005

Abstract The fluorescence quantum efficiency of Er3+ in low silica calcium aluminate glasses, with nominal composition 58 CaO, 27.1
x Al2O3, 6.9 MgO, 8 SiO2, x Er2O3, 0.1 6 x 6 1.5 (mol%), melted in air and under vacuum, has been measured using the mode-mismatched thermal lens spectrometry, with excitation beam wavelength at 488 nm and 804 nm. From 0.1 up to 1.1 mol% of Er2O3 the quantum efficiency is around 0.65 for vacuum-melted samples, and around 0.46 for air-melted ones. The quenching of the quantum efficiency appears above 1.3 mol% of Er2O3. The thermal diffusivity was also obtained, with results showing a decrease from 5.69 · 10
3 cm2/s (undoped sample) to 4.78 · 10
3 cm2/s (for the highest doped sample, 1.5 mol% of Er2O3).
2005 Elsevier B.V. All rights reserved. PACS: 42.70.Hj; 78.20.Nv; 78.40.Pg

1. Introduction The rare earth (RE) ions are natural candidates to be used as active ions in solid-state laser materials because they present a great amount of sharp fluorescent transitions, representing almost every region of the visible and near infrared portion of the electromagnetic spectrum [1]. Among RE laser ions, Er3+ is the most extensively studied after Nd3+ [2], and its most important lasing transition is the 4I11/2 ! 4I13/2 one, around 2.7–2.9 lm, which finds applications in medicine [3]. This laser line matches with the fundamental OH absorption band stretch vibration, essential to perform extremely precise cutting and ablation of water-containing tissues. The *

Corresponding author. E-mail addresses: jsampaio@ifi.unicamp.br, [email protected] (J.A. Sampaio). 0022-3093/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.03.047

first erbium doped glass laser was demonstrated by Snitzer and Woodcock [4] with emission around 1.5 lm, which corresponds to the 4I13/2 ! 4I15/2 Er3+ transition, and since that time several crystal and glass systems have been studied as Er3+ host [5]. Until recently, the emission of light around 2.8 lm had been observed only in Er3+ doped crystals [6] (yttrium aluminum garnet – YAG and yttrium lithium fluoride – YLF) and fluoride glasses [7]. But not long ago, de Souza et al. [8] showed that the emission of light around this wavelength is also possible in Er3+ doped oxide glass, namely low silica calcium aluminosilicate glasses (LSCA). This fine outcome is attributed to the relatively low phonon energy (800 cm
1) of LSCA glasses [9], that is smaller than those of phosphate and silicate glasses [10], 1100 and 1000 cm
1, respectively. The vacuum melting condition is pointed out [11] as crucial to remove the strong OH optical absorption, avoiding fluorescence quenching

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processes. Besides, the ytterbium co-doping provided an enhancement of the pumping efficiency around 0.98 lm, of about twice greater than in those doped only with Er3+. In previous works [12–15] we showed that LSCA glasses are candidates for solid-state laser media because they have optimized thermal, optical and mechanical properties. For example, LSCA glasses have thermal shock resistance comparable to silicate glasses, and about four times greater than fluoride glasses, making them suitable to operate under a hostile environment such as a laser cavity [13]. In addition to these features, the spectroscopic properties play the major role in the choice of a laser media host. The fluorescence quantum efficiency and lifetime of fluorescence are important parameters, and they are strongly dependent on the host as well as on the rare earth doping concentration [2]. Several quenching processes, such as energy transfer, cross-relaxation and cooperative up-conversion are related to interaction among the rare earth ions in the host [2,16,17]. The absolute value of the fluorescence quantum efficiency, g, of doped solids is difficult to obtain. For this reason several methods have been proposed in the literature in order to measure it. These experiments include: integrating sphere method [18], laser calorimetry measurements [19], fluorescence lifetime approach [20,21], and more recently, the photothermal techniques [22,23]. In the early seventies the Thermal Lens Spectrometry (TLS) appeared as a tool to measure the fluorescence quantum efficiency of organic dyes [24]. Since then, the TLS has been used to measure the fluorescence quantum efficiency of fluorescent solutions and polymers, and of transition metal complexes [25–27]. Recently, the TLS in the mode-mismatched configuration was used to measure the absolute value of g of Nd3+ doped LSCA glasses [28]. However, investigations about the influence of other rare earth ions in LSCA glasses are scarce. In order to fill out this lack of information, in this work we investigate the fluorescence quantum efficiency of Er3+ in LSCA glasses using the twobeam mode-mismatched TLS. An investigation of the influence of the melting atmosphere (vacuum and air) as well as different rare earth doping concentration on the quantum efficiency and thermal diffusivity is presented.

2. Theory TLS is a non-contacting technique and can be performed in a low-frequency range (time resolved measurements) and in the steady-state mode. In the thermal lens (TL) measurements, the sample is placed in a TEM00 Gaussian laser beam and non-radiative decay processes following optical energy absorption produce a temperature rise. Since the refractive index of the sample changes with temperature, a refractive

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index gradient is produced, creating a lenslike optical element at the sample. This explains the origin of the name Thermal Lens. The wave front of the TEM00 Gaussian laser beam from the excitation laser beam itself or another laser beam (the probe beam) passing through the TL will be slightly distorted, resulting in a change of the laser beam Gaussian profile, and also in a variation of its intensity at the beam center in far field. Snook and Lowe [29] reported a good review on the subject, and more recently Lima et al. [30] reviewed the use of the mode-mismatched TLS for thermo-optical properties measurements in optical glasses. There are several experimental configurations for the TL spectrometry. The two-beam mode-mismatched experimental arrangement, showed in Fig. 1, has proved to be the most sensitive one [31]. In this experimental configuration a CW TEM00 laser beam, excitation beam, illuminates a weakly absorbing sample thereby generating the thermal lens. The TL effect is treated as an optical path length change to the probe laser beam, which can be expressed as an additional phase shift on the probe beam wave front after passing through the sample. By measuring the beam on-axis intensity in the far field and using this theoretical model the thermo-optical properties of the sample can be obtained. The theoretical model for the TL effect in this configuration has been developed, and an analytical expression to treat it quantitatively was obtained [32]. The variation of the intensity in the center of the probe beam at the detector caused by the thermal lens can be expressed as [32,33] 8 < h IðtÞ ¼ Ið0Þ 1
tan
1 : 2 2 3 92 = 2mV 5 i 4h 2 ð1 þ 2mÞ þ V 2 ðt =2tÞ þ 1 þ 2m þ V 2 ; c

ð1Þ with
m¼

 x1p ; xe

V ¼

Z1 when Z c  Z 2 ; Z2

tc ¼

x2e ; 4D

ð2Þ

h ¼
Plub; where tc is the characteristic thermal lens time constant, xe is the excitation laser beam radius at the sample, x1p is the probe beam radius at the sample, Zc is the confocal distance of the probe beam, Z1 is the distance between the probe beam waist and the sample, Z2 is the distance between the sample and the detector, h is approximately the p phase difference of the probe beam at r = 0 and ffiffiffi r ¼ 2xe induced by the thermal lens, P is the excitation laser beam power, a is the optical absorption coefficient at the excitation beam wavelength (cm
1), l is the sample thickness, D is the thermal diffusivity (cm2/s),

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Fig. 1. Mode-mismatched thermal lens experimental setup.

b = (Kkp)
1ds/dT, where kp is the probe beam wavelength, K is the thermal conductivity (J s
1 cm
1 K
1) and ds/dT is the thermal change in the optical path length and I(0) = I(t) when the transient time t or h is zero. The u parameter is introduced into the h expression to take into account the energy emitted in detriment of that converted into heat. That is, for fluorescent samples the factor u = 1
g(ke/hkemi), where g is the fluorescence quantum efficiency, ke is the excitation beam wavelength and hkemi is the average wavelength of the fluorescence. In order to obtain the absolute value of the quantum efficiency, it is supposed that b is the same either for the doped sample as well as for the undoped one. It follows that we can write the quantum efficiency as
 Hd hkem i g¼ 1
; ke Hu

ð3Þ

where Hd = (h/Pal)d concerning to doped sample and Hu = (h/Pal)u to undoped one. An alternative method was proposed recently [34] for situations in which the undoped sample is not available, as well as ds/dT and K are unknown. In these experiments, the TL measurements are carried out as a function of the wavelength excitation beam.

3. Experiment In order to investigate the influence of the melting atmosphere on the fluorescence quantum efficiency of Er3+ in LSCA glasses, two series of samples with

nominal composition 58 CaO, 27.1
x Al2O3, 6.9 MgO, 8 SiO2, x Er2O3, 0.1 6 x 6 1.5 (mol%) were prepared. The reagent grade powders CaCO3 (99%), Al2O3 (99.1%), SiO2 (99%), MgO (97%) and Er2O3 (99.99%), in 30 g quantities were mixed in a ball mill. Afterwards the batches were melted below 1500 C, at first, under vacuum (10
3 mbar) using graphite crucible, during 2 h for fining. The quenching was made switching off the heater and moving up the crucible to a cooled vacuum chamber. The samples of the second series were melted in air, in an electric furnace, using Pt–Rh crucible. These samples were refined during 30 min and then poured out in a pre-heated (400 C) graphite mould, and let cooling to room temperature. The vacuum and airmelted samples were annealed close to their glass transition temperature, Tg, using a heating rate of 10 C/min, and remained near Tg during 12 h, and cooled slowly to room temperature. Afterwards they were examined for crystallinity with optical microscopy and X-ray diffraction. The samples were cut in a slow speed saw in plate shape, 3 mm · 3 mm · 10 mm. However, due to the strong optical absorption coefficient of the high doped samples, the thickness was adjusted to give a good thermal lens signal. The samples were investigated using the mode-mismatched thermal lens spectrometry, where the excitation beam lasers were an argon laser (Coherent Innova 90 plus) operating at k = 488 nm, and a Ti:Sapphire laser (Coherent) at k = 804 nm. In both cases, the lens 1 with a focal length f = 20 cm focused the laser beam, with the sample placed at its focal plane. The exposure of the sample to the excitation beam was controlled by means of a chopper, frequency of 10 Hz. The probe laser was a He–Ne laser (Melles Griot, 5 mW) at

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k = 632.8 nm, which was focused by lens 2 with focal length f = 20 cm, at an angle c = 1.5 with respect to the excitation beam. The sample was placed at lens 2 confocal plane. The probe beam and excitation beam spot sizes at the sample were: (1) for measurements performed at k = 488 nm, 104 lm and 42 lm (samples melted under vacuum), respectively, and 90.5 lm and 62.6 lm (samples melted in air), respectively; (2) for measurements performed at 804 nm, 88 lm and 62 lm (samples melted under vacuum and in air), respectively. The signal was recorded for about 80 ms during the development of the thermal lens. The optical absorption coefficients were determined using the same experimental configuration applied for TL measurements. In order to take into account the thermal lens signal as a function of the excitation beam powers, TL measurements were carried out in the steady-state mode. In this case the thermal lens signal is given by h ¼ 2½1
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 IðtÞ=Ið0Þ½arctanð2mV =ð1 þ 2m þ V 2 ÞÞ , where I(0) is power intensity observed in photodiode 2 before the sample is exposed to the excitation beam, and I(t) is the intensity after the exposure, with tc ! 1. Each h obtained was plotted versus the incident power, and from linear fitting we obtained the hÕs normalized to the excitation beam power. Measurements of visible and infrared transmission were also recorded using a spectrophotometer (Perkin–Elmer and Lambda 9) at room temperature.

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a

b Fig. 2. Optical transmission of the 2-mol% doped Er2O3 LSCA glass: in (a) visible region and in (b) mid-infrared region. The dot line concern to the air-melted sample, with l = 0.248 cm, and the straight line to the vacuum melted, with l = 0.320 cm.

4. Results The results of optical microscopy and X-ray diffraction confirmed the amorphous structure of the samples. Fig. 2(a) and (b) show the optical transmission in the visible and infrared region for a 2-mol% of Er2O3 doped LSCA glass. The influence of the melting atmosphere is remarkable. The optical transmission in the visible region up to 500 nm, Fig. 2(a), is below 60% for air-melted sample and only after 800 nm there is an approximation between the transmission curves of the vacuum and airmelted samples. A shift of the transmission edge of about 52 nm is observed; in the case of the vacuummelted ones the transmission start at 261 nm, and for air-melted ones at 313 nm. It is worthy noting that the color of the vacuum and air-melted samples are quite different: for example, the undoped sample has a slight blue–green light color when melted under vacuum, whereas the air melted has an amber color; the Er2O3 doped samples have a pink color when melted in vacuum and a mix of amber–pink color when melted in air. The mid-infrared optical transmission curves of the vacuum and air-melted samples, Fig. 2(b), present also a remarkable difference: whereas the transmission of the vacuum-melted samples is around 85% with cutoff around 5500 nm, the air-melted ones have a strong

absorption band that initiate at 2700 nm and goes up to 4000 nm. This band is attributed [35] to the OH group, related to the absorption from 2700 up to 3000 nm of the O–H valence vibration, and from 3400 up to 3600 nm related to the H bridge bond to singly bonded O, having similar cut-off as the vacuum-melted ones. Fig. 3 shows a linear increasing of the optical absorption coefficient, either for air-melted as well as for vacuum-melted samples. However, in Fig. 3(a) we observe that the a curve of the air-melted samples has a greater slope than that of vacuum sample, viz. 5.0 ± 0.3 and 1.65 ± 0.05, respectively. On the other hand, in Fig. 3(b) the a curve slopes of air and vacuum-melted samples are similar, i.e., 0.47 ± 0.02. The correlation factor for all cases is 0.99 ± 0.01. Fig. 4(a) shows the time-resolved thermal lens signal with an excitation beam power of 52 mW for the 1 mol% Er2O3 doped sample, melted in air, while in Fig. 4(b) is showed the transient for the 1.3 mol% Er2O3 doped sample prepared under vacuum, with an excitation beam power of 127 mW. From the curve fitting (Eq. (1)) we found h =
0.053 ± 0.001 and tc = 1.85 ± 0.01 ms, and h =
0.114 ± 0.001 and tc = 2.02 ± 0.01 ms, respectively. Using these tcÕs we calculate the thermal diffusivity, D.

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a

b Fig. 3. Optical absorption coefficients as a function of the Er2O3 concentration. In (a) is showed the measurements performed at 488 nm and in (b) those performed at 804 nm. The melting atmosphere influence is indicated in the figure.

The error bars for D depend on the errors in the determination of tc and xe, which in this work presented be less than ±5%. From Fig. 5 we can see that D decrease as a function of the Er2O3 concentration. The normalized thermal lens signal h/Pl obtained in the steady-state mode and their respective linear fit curves are shown in Fig. 6. The normalized hÕs are summarized in Tables 1 and 2. The values obtained either in the steady-state mode as well as in the transient mode were within the error measurements, as expected. For instance, in the case of 0.5 mol% Er2O3 doped sample the h/P in the steadystate mode was 0.524 ± 0.004, whereas in the transient mode was 0.527 ± 0.003. Fig. 7 shows the normalized thermal lens signal h as a function of the optical absorption coefficient. In (a) are presented the results obtained for measurements performed at 804 nm, and in (b) those carried out at 488 nm. The TL signal at 488 nm is about 10 times greater for air-melted samples than for those melted under vacuum, on the other hand the data measured at 804 nm are independent of the melting atmosphere. The thermal lens signal of the samples with high Er2O3 concentration has a more prominent increasing than of those with low erbium concentration, which suggests a quenching of the fluorescence for these samples.

a

b Fig. 4. Time-resolved experimental data and its best-fit curve. In (a) for the 1.0 mol% Er2O3 doped sample melted in air, with excitation beam power of 52 mW, and in (b) for the 1.3 mol% Er2O3 doped sample melted under vacuum, with excitation beam power of 127 mW. In both cases de excitation beam at 804 nm and probe beam at 632.8 nm.

Fig. 5. Thermal diffusivity as a function of the Er2O3 concentration.

5. Discussion As discussed previously, the samples melted in air and under vacuum presented different colors. Shelby [36] shows that the optical properties of air-melted calcium aluminate glasses change as function of the CaO concentration. When the CaO content is >48 mol% (in

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a

b

c

d

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Fig. 6. Normalized thermal lens signal h/l, for different excitation beam power, P: in (a) and (b) are showed the measurements performed at 488 nm and, in (c) and (d) those performed at 804 nm. The probe beam wavelength is 632.8 nm. The melting atmosphere is indicated for each case.

Table 1 Thermo-optical parameters obtained at 488 nm Er2O3 (mol%)

l (cm)

a (cm
1)

h/P (rad W
1)

H (rad W
1)

D (·10
3 cm2/s)

Vacuum 0.0 0.1 0.2 0.4 0.7 1.1 1.3 1.5

0.100 0.284 0.305 0.271 0.356 0.225 0.170 0.253

0.116 ± 0.002 0.310 ± 0.002 0.533 ± 0.008 0.793 ± 0.009 1.28 ± 0.02 1.93 ± 0.03 2.31 ± 0.03 2.55 ± 0.04

0.128 ± 0.002 0.537 ± 0.006 0.96 ± 0.01 1.00 ± 0.01 1.90 ± 0.01 2.54 ± 0.01 2.38 ± 0.01 4.25 ± 0.01

11.03 6.10 5.91 4.65 3.37 5.84 6.06 6.59

5.73 ± 0.07 5.58 ± 0.07 5.51 ± 0.07 5.44 ± 0.07 5.25 ± 0.06 5.01 ± 0.06 4.96 ± 0.06 4.74 ± 0.05

Air 0.0 0.5 0.7 1.0 1.1 1.4

0.253 0.169 0.168 0.132 0.139 0.073

1.46 ± 0.04 4.83 ± 0.09 5.5 ± 0.1 6.5 ± 0.2 7.4 ± 0.2 8.3 ± 0.2

2.84 ± 0.08 19.8 ± 0.2 22.9 ± 0.2 20.2 ± 0.2 24.2 ± 0.4 19.0 ± 0.2

7.70 24.24 24.91 23.60 37.69 44.81

5.76 ± 0.02 – – – – –

our samples 58 mol%) a yellowish coloration is observed, whereas those with less CaO are colorless. This

feature is not related to a specific optical absorption band, but rather resulted from an increase in absorption

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Table 2 Thermo-optical parameters obtained at 804 nm Er2O3 (mol%)

l (cm)

a (cm
1)

h/P (rad W
1)

H (rad W
1)

D (·10
3 cm2/s)

g

Vacuum 0.0 0.2 0.7 1.1 1.3 1.5

0.362 0.370 0.283 0.232 0.167 0.255

0.071 ± 0.001 0.174 ± 0.002 0.39 ± 0.01 0.58 ± 0.01 0.69 ± 0.01 0.77 ± 0.01

0.263 ± 0.002 0.438 ± 0.002 0.755 ± 0.005 0.905 ± 0.08 0.88 ± 0.01 1.42 ± 0.03

10.23 6.80 6.84 6.75 7.61 7.25

5.69 ± 0.03 – 5.11 ± 0.01 – 4.76 ± 0.01 4.80 ± 0.01

– 0.64 0.63 0.65 0.49 0.56

Air 0.0 0.4 0.5 1.0 1.1 1.3 1.5

0.253 0.170 0.170 0.096 0.140 0.091 0.113

0.120 ± 0.001 0.301 ± 0.005 0.41 ± 0.01 0.59 ± 0.01 0.66 ± 0.01 0.75 ± 0.1 0.80 ± 0.1

0.299 ± 0.001 0.410 ± 0.003 0.524 ± 0.004 0.424 ± 0.006 0.69 ± 0.01 0.50 ± 0.01 0.74 ± 0.02

9.85 8.01 7.55 7.49 7.48 7.40 8.16

5.70 ± 0.01 5.34 ± 0.01 5.31 ± 0.01 5.20 ± 0.03 5.13 ± 0.01 5.03 ± 0.08 4.78 ± 0.01

– 0.36 0.45 0.46 0.46 0.48 0.32

a

b Fig. 7. Normalized thermal lens signals vs optical absorption coefficients of Er2O3 doped LSCA glasses. The drawn circles/ellipses indicate the region in which the fluorescence quenching occurs for both series of samples.

over the entire visible region, which gradually decreases with increasing wavelength. The mode-mismatched TLS arrangement allows the measurement of optical absorption coefficients as low as 10
7 cm
1, higher than conventional spectrometry 10
3 [32]. Besides, the determination of a using the TLS experimental setup guarantees that the position of the sample is the same where the

measurement is carried out, usually a region of the sample free of striae, cords, or microbubbles. The choice of the best region is made by passing the probe beam laser through the sample and observing in a far field the projection of its spot. On the other hand, undoped sample aÕs are more difficult to obtain precisely, both using TLS and spectrometry, due to their high transparency. A solution to overcome this problem is to add to the glass composition of the standard sample small amounts (< 1 wt%) of transition metal ions such as Fe3+ or Co2+. In the case of fluoride glasses this artifice works well, since the structural change due to these ions is minute [37]. TLS measurements [32] performed in Fe3+ doped soda-lime glasses showed that K and ds/dT are, respectively, 10 · 10
3 W cm
1 K
1 and 2.1 · 10
6 K
1, for the undoped sample, as compared with 12 · 10
3 W cm
1 K
1 and 4.7 · 10
6 K
1, respecting values of the 2 wt% Fe2O3 doped one. In both cases the measurements were performed at 632.8 nm. These values resulted in b = 3.32 W
1 and 6.19 W
1, respectively, undoped and doped sample, showing a difference of about 53%. We conducted a similar analysis for LSCA glasses and we found a variation of about 300%, i.e., b is 11.3 ± 0.4 W
1 for undoped sample and 3.7 ± 0.1 W
1 for 1 wt% Fe2O3 doped one. Thus for oxide glasses, as in the case of our samples, this strategy is not appropriate, mainly because the transition metal atoms act as glass network-formers. However, in the case of LSCA the rare earth atoms act as modifiers, for example, it was showed [34] that a sample doped with 0.4 mol% (about 1 wt%) of Nd2O3 presented b = 12.4 ± 0.6 W
1, i.e., within the uncertainties of measurement, of about 5%, when compared with b obtained without using the undoped sample. This result support the assumption that b can be considered the same, either for undoped sample as well as for rare earth doped ones. The decrease of the thermal diffusivity is attributed to

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the modifier character of the rare earth [14]. This can be explained taking into account the fact that the introduction of the rare earth in the glass composition decreases the network connectivity, as corroborated with the decrease observed in the glass transition temperature and in the hardness, as reported recently [12]. From Fig. 6(a) we observe that, except for the undoped sample, h/l reach greater values than in (b)–(d), although the excitation beam power is very low, <15 mW. From these curves we can observe that normalized hÕs (to the sample thickness and excitation beam power) of the air-melted samples are more susceptible to small changes in the excitation beam power than those of vacuum-melted ones. Accordingly to Hewak [38], the 4I13/2 ! 4I15/2 transition (kem  1.5 lm) is more important in oxide glasses due its largest quantum efficiency. The other transitions decay mostly by multiphonon cascade, i.e., 4F7/2 ! 4 S3/2 ! 4F9/2 ! 4I9/2 ! 4I11/2 ! 4I13/2, which the quantum efficiencies are 0.0327, 0.015 and 0.021, respectively [39]. For this reason we hypothesis that the transition at 1.5 lm is prevalent in our experiments. Using Eq. (3) we can calculate the fluorescence quantum efficiency, g. However this was not successful in the case of the measurements performed at 488 nm. In a previous work [40] on the thermo-optical properties of OH-free Er doped LSCA we showed that it was possible only to obtain the fraction of energy converted into heat, u. From Table 1, we obtain, for instance, for the 0.7 mol% Er2O3 sample melted under vacuum, u = 0.305, resulting g = 2.19, yet, following the same reasoning, now taking the sample with the same erbium concentration, but melted in air, we obtain u = 3.24, which is greater than unit, resulting in a negative g, which does not make sense. Taking into account this fact we can conclude

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that TLS experiments carried out at 488 nm is not a good choice to determine g. This difficulty is due to the low sensitivity of the Er3+ ion at this wavelength, and to the high optical absorption coefficient of the matrix glass, disguising the results. On the other hand, the measurements performed in the mid infrared presented reliable results. For example, in the case of 1.1 mol% Er2O3 doped LSCA glass, g = 0.65 ± 0.03 for the sample melted under vacuum and g = 0.46 ± 0.02 for that melted in air. The results are given in Table 2 and plotted in Fig. 8. The quantum efficiency difference of vacuum and air-melted samples is attributed to enhancement of the multiphonon relaxation of the excited states of Er3+ due to the OH presence in the airmelted samples. The quenching of quantum efficiency is observed for samples doped with concentration greater than 1.3 mol% Er2O3.

6. Conclusion In this work we show the application of the thermal lens spectrometry as a tool to measure the fluorescence quantum efficiency of Er3+ doped low silica calcium aluminate glasses. The method showed to be useful for measurements performed at 804 nm. The strong optical absorption band in the visible region disguises the Er3+ weak optical absorption at 488 nm, and measurements performed at this wavelength are strongly influenced by the matrix glass optical absorption coefficient. For vacuum and air-melted samples the quantum efficiency appears to be constant for Er2O3 concentrations 61.1 mol%. The hydroxyl impurity plays an important role in this property, since the interactions of OH
with the rare earth ion reduces the quantum efficiency. The quenching of the quantum efficiency was apparent beyond 1.3 Er2O3 mol%. The thermal diffusivity decreases about 20% as the RE doping increases of about 1.5 Er2O3 mol%. This result is an indication that the RE ions act as network modifiers.

Acknowledgment This work was supported under auspices of the Brazilian agencies CNPq, CAPES and FAPESP.

References

Fig. 8. Fluorescence quantum efficiency vs different Er2O3 concentration determined by thermal lens method. The melting atmosphere is indicated in the figure.

[1] W. Koechner, Solid State Laser Engineering, 4th Ed., Springer, Berlin, 1996. [2] M.J.F. Digonnet (Ed.), Rare Earth Doped Fiber Lasers and Amplifiers, Marcel Dekker, New York, 1993. [3] R.M. Dwyer, M. Bass, Lasers in Medicine, vol. 3, Academic, New York, 1977. [4] E. Snitzer, R. Woodcock, Appl. Phys. Lett. 6 (1965) 45.

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[5] F. Gan, Laser Materials, World Scientific, Singapore, 1995. [6] G.J. Kintz, R. Allen, L. Esterowitz, Appl. Phys. Lett. 50 (1987) 1553. [7] S.A. Pollack, M. Robinson, Electron. Lett. 24 (1988) 320. [8] D.F. de Sousa, L.F.C. Zonetti, M.J.V. Bell, J.A. Sampaio, L.A.O. Nunes, M.L. Baesso, A.C. Bento, L.C.M. Miranda, Appl. Phys. Lett. 74 (1999) 908. [9] S. Tanabe, T. Ohyagi, T. Hanada, N. Soga, J. Ceram. Soc. Jpn. 101 (1993) 78. [10] C.B. Layne, W.H. Lowdermilk, M.J. Weber, Phys. Rev. B 16 (1977) 10. [11] D.F. de Sousa, J.A. Sampaio, L.A.O. Nunes, M.L. Baesso, A.C. Bento, L.C.M. Miranda, Phys. Rev. B 62 (2000) 3176. [12] M.L. Baesso, A.C. Bento, A.R. Duarte, A.M. Neto, L.C.M. Miranda, J.A. Sampaio, T. Catunda, S. Gama, F.C.G. Gandra, J. Appl. Phys. 85 (1999) 8112. [13] J.A. Sampaio, M.L. Baesso, S. Gama, A.A. Coelho, J.A. Eiras, I.A. Santos, J. Non-Cryst. Solids 304 (2002) 293. [14] J.A. Sampaio, T. Catunda, A.A. Coelho, S. Gama, A.C. Bento, L.C.M. Miranda, M.L. Baesso, J. Non-Cryst. Solids 273 (2000) 239. [15] J.A. Sampaio, T. Catunda, F.C.G. Gandra, S. Gama, A.C. Bento, L.C.M. Miranda, M.L. Baesso, J. Non-Cryst. Solids 247 (1999) 196. [16] W.M. Yen, P.M. Selzer (Eds.), Laser Spectroscopy of Solids, in: Topics in Applied Physics, vol. 49, Springer, Berlin, 1981. [17] K. Arai, H. Namikawa, K. Kumata, T. Honda, Y. Ishii, T. Handa, J. Appl. Phys. 59 (1986) 3430. [18] J.C. de Mello, H.F. Wittmann, R.H. Friend, Adv. Mater. 9 (1997) 230. [19] A.J. Ramponi, J.A. Caird, J. Appl. Phys. 63 (1988) 5476. [20] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511.

[21] F. Auzel, D. Meichenin, A. Mendorioz, R. Balda, J. Fernandez, J. Lumin. 72–74 (1997) 152. [22] R.S. Quimby, W.M. Yen, Opt. Lett. 3 (1978) 181. [23] R.C. Powell, D.P. Neikirk, D. Sardar, J. Opt. Soc. Am. 70 (1980) 486. [24] C. Hu, J.R. Whinnery, Appl. Opt. 12 (1973) 72. [25] J.H. Brannon, D. Magde, J. Phys. Chem. 82 (1979) 705. [26] J. Shen, R.D. Snook, Chem. Phys. Lett. 155 (1989) 583. [27] J. Degen, K. Reincke, H.H. Schmidte, Chem. Phys. 162 (1992) 419. [28] M.L. Baesso, A.C. Bento, A.A. Andrade, J.A. Sampaio, E. Pecoraro, L.A.O. Nunes, T. Catunda, S. Gama, Phys. Rev. B 57 (1998) 10545. [29] R.D. Snook, R.D. Lowe, Analyst 120 (1995) 2051. [30] S.M. Lima, J.A. Sampaio, T. Catunda, A.C. Bento, L.C.M. Miranda, M.L. Baesso, J. Non-Cryst. Solids 273 (2000) 215. [31] F.J. Power, E.D. Salin, Anal. Chem. 60 (1988) 838. [32] M.L. Baesso, J. Shen, R.D. Snook, J. Appl. Phys. 75 (1994) 3232. [33] J. Shen, R.D. Lowe, R.D. Snook, Chem. Phys. 165 (1992) 385. [34] A.A. Andrade, S.M. Lima, V. Pilla, J.A. Sampaio, T. Catunda, M.L. Baesso, Rev. Sci. Instrum. 74 (2003) 857. [35] W. Vogel, Glass Chemistry, 2nd Ed., Springer, Berlin, 1994. [36] J.E. Shelby, J. Am. Ceram. Soc. 68 (1985) 155. [37] S.M. Lima, J.A. Sampaio, T. Catunda, R. Lebullenger, A.C. Hernandes, M.L. Baesso, A.C. Bento, F.C.G. Gandra, J. NonCryst. Solids 256&257 (1999) 337. [38] D. Hewak, Glass and Rare-Earth Doped Glasses for Optical Fibers, Short Run Press, London, 1993. [39] X. Zou, T. Izumitani, J. Non-Cryst. Solids 162 (1993) 68. [40] J.A. Sampaio, T. Catunda, S. Gama, M.L. Baesso, J. Non-Cryst. Solids 284 (2001) 216.