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Thermo-optical properties measurements in chalcogenide glasses using thermal relaxation and thermal lens methods
Journal of Non-Crystalline Solids 348 (2004) 108–112 www.elsevier.com/locate/jnoncrysol
Thermo-optical properties measurements in chalcogenide glasses using thermal relaxation and thermal lens methods S.M. Lima a,*, A. Steimacher b, A.N. Medina b, M.L. Baesso b, M.N. Petrovich c, H.N. Rutt c, D.W. Hewak c a
Universidade Estadual de Mato Grosso do Sul, Cidade Universita´ria de Dourados, C.P. 351, CEP 79804-970, Dourados, MS, Brazil b Departamento de Fı´sica, Universidade Estadual de Maringa´, CEP 87020-900, Maringa´, PR, Brazil c Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, England, United Kingdom
Abstract In this work we determine the thermo-optical properties of two chalcogenide glasses (in mol%): 65 Ga2S3; 30 La2S3; 5 La2O3 (Ga:La:S) and 72.5 Ga2S3; 27.5 La2O3 (Ga:La:S:O). The thermal relaxation calorimetry and the thermal lens technique were combined so that the samples specific heat, thermal diffusivity, thermal conductivity, and the temperature coefficient of the optical path length change could be measured. Our results indicate that changes in thermal diffusivity (2.7 · 103 cm2/s) and conductivity (4.8 · 103 W/K cm) observed when La2O3 is added in the glass compounds are less than the error in the data. 2004 Elsevier B.V. All rights reserved. PACS: 65.60.+a; 78.20.-e; 42.70.Ce
1. Introduction Chalcogenide glasses based on gallium and lanthanum sulphides and oxi-sulphides (for instance Ga:La:S and Ga:La:S:O) have been investigated recently because of their potential for photonic applications [1,2]. Their glass matrix phonon energy 425 cm1 results in infrared transparency to around 5 lm and low non-radiative decay rates of rare-earth energy levels. The covalency nature of the glass bonds increases the radiative decay rates, the emission and the optical absorption cross sections [3]. Furthermore, the refractive index is correlated with third-order non-linearity, which is attractive for alloptical switching [4]. All these properties together with higher glass transition temperatures, chemical stability and non-toxicity make these glasses suitable for photonic applications. Laser action in Nd3+-doped Ga:La:S
*
Corresponding author. Fax: +55 67 411 9173. E-mail address: [email protected] (S.M. Lima).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.08.134
glass was observed in glass samples [5], fibers [6], and ultraviolet-written waveguides [7]. Besides, minimum losses of about 1 and 4 dBm1 at 4 lm have been achieved in unclad Ga:La:S [1] and Ga:La:S:O [2] fibers, respectively. Knowledge of the figures of merit of these glasses is useful in applications such as all-optical switching. For rare-earth doped lasers, for instance, it is important to know the thermalization time within the host material, the thermally induced distortion of a laser beam through the host, and the heat diffusion coefficients of the host material. These are determined by measuring the thermal diffusivity, D, the temperature dependence of the optical path length change, ds/dT, and the specific heat, cp [8,9]. In the last few years, these properties have been measured in glasses, crystals, and polymers, also as a function of temperature, by using the thermal lens (TL) technique and thermal relaxation calorimetry (TRC) [10–13]. The aim of this work is to combine the TRC and TL techniques to determine the thermo-optical properties
S.M. Lima et al. / Journal of Non-Crystalline Solids 348 (2004) 108–112
(D, cp, ds/dT, and the thermal conductivity, K) of two chalcogenide glasses of composition (in mol%): 65 Ga2S3; 30 La2S3; 5 La2O3 (Ga:La:S) and 72.5 Ga2S3; 27.5 La2O3 (Ga:La:S:O). In addition, the temperature dependence of the specific heat was also measured for both glass compositions from 300 to 450 K. The aim for this study is to verify the effect of the oxide content on the thermo-optical properties of the glass.
2.1. Thermal relaxation calorimetry (TRC) In the TRC, the setup is composed of a sample holder (or substrate) and a temperature sensor. The substrate is supported by wires, which are attached to a thermal reservoir. A thermal link between the system and the thermal bath is also provided by these wires, as well as the gas of the surrounding atmosphere and the thermal radiation. This thermal link brings the system to a thermal equilibrium at a temperature T0. When a constant power, P, of a laser radiation is supplied to the substrate, which is in contact with the sample, part of this energy is transferred to the reservoir via the thermal links, so that the heat balance equation is given by [8,9] dðDT Þ P ¼C þ K eff DT ; dt
theory, an analytical expression can be obtained for the probe beam intensity change, I(t), [10] h IðtÞ ¼ Ið0Þ 1 tan1 2 " #)2 2mV ; ð2Þ 2 ½ð1 þ 2mÞ þ V 2 tc =2t þ 1 þ 2m þ V 2 where 2 wp m¼ ; we
2. Theory
ð1Þ
where C is the system heat capacity, Keff is the effective thermal conductance, DT = DTmax[1 exp(t/s)] is the temperature difference between the system and the thermal bath, DTmax = P/Keff is the final temperature difference in the steady state, and s = C/Keff is the external relaxation time. When the laser power is turned off, the system temperature decays exponentially to T0 following the equation DT = DTmax exp(t/s). By monitoring the time dependence of the temperature rise or decay it is possible to determine the final temperature difference and the relaxation time. Therefore, the heat capacity of the sample is obtained subtracting the substrate heat capacity, which is previously determined. 2.2. Thermal lens (TL) technique In the dual-beam mode-mismatched TL technique, the sample is illuminated by two TEM00 Gaussian laser beams, of which one is used for exciting the sample to produce a local temperature increase, creating a lens-like element in the heated region, and the other to probe the thermal effect. When the probe beam passes through the created lens, its optical path length undergoes a temporal change that can be observed by measuring the beam center intensity in the far field. Using Fresnel diffraction
109
V ¼
Z1 Z cp
with Z cp Z 2 :
ð3Þ
In which, wp and we are the probe and excitation laser beams radius at the sample, respectively, Z1 is the distance between the probe beam focal plane and the sample, Zcp is the confocal distance of the probe beam, Z2 is the distance between the sample and the detector and I(0) is I(t) when the transient time t or h is zero. The characteristic TL signal response time, tc, is given by [10] w2e ; ð4Þ 4D where D = K/qcp is the thermal diffusivity, K is the thermal conductivity, q is the density and cp is the sample specific heat. The probe beam phase shift, h, induced by TL is given by [10] tc ¼
h¼
PAe Leff ds : u dT Kkp
ð5Þ
In which P is the excitation laser power, kp is the probe beam wavelength, Leff = (1 eAeL)/Ae is the effective length, L is the sample thickness, Ae is the absorption coefficient, and u is the fraction of absorbed energy converted into heat per photon. In the case of non-luminescent samples, such as the samples studied in this work, all absorbed energy is converted into heat, so u = 1 in Eq. (5).
3. Experimental procedures The chalcogenide glasses were prepared with nominal molar composition 65% Ga2S3; 30% La2S3; 5% La2O3 (Ga:La:S) and 72.5% Ga2S3; 27.5% La2O3 (Ga:La:S:O) by standard powder melting. The sulphides were synthesized in-house and their purity, as measured by glow discharge mass spectroscopy was approximately 99.9999%. The lanthanum oxide was commercial purity grade, 99.999%. Batches with the chosen compositions were weighed inside a glovebox, under dry and inert atmosphere, loaded in vitreous carbon crucibles and then transferred to a quartz-lined furnace. Melting was performed at 1150 C for about 24 h under a stream of argon; glass quenching was accomplished by pushing the crucible into a water-cooled jacket. Glass ingots were annealed, cut in a rectangular shape of approximately
S.M. Lima et al. / Journal of Non-Crystalline Solids 348 (2004) 108–112
2 · 1 cm · 1 mm thick and then the larger surfaces were optically polished. In our calorimeter, a constant heat power is supplied to the system by illuminating the sample using a diode laser. As mentioned before, when the heat is applied to the system, the temperature difference between the substrate and the thermal bath increases exponentially (see Eq. (1)) with time until a steady state is reached. A shutter then blocks the laser beam and the temperature decay is observed. The increase or decrease of temperature difference is monitored by thermocouple (cooper– constantan) in differential configuration, which sends the signal into a nanovoltmeter (Keithley, model 2182). The output of the microvoltmeter is then digitized and stored in a computer for posterior analysis. A temperature controller (LakeShore Cryotronics Inc., model 340), stabilizes the temperature of the bath using a calibrated sensor (PT-100) and a resistive heater attached to the thermal reservoir. Further details of the experimental procedure can be found elsewhere [11]. In the dual-beam mode-mismatched TL experimental setup used in this work, the sample is placed at the minimum beam diameter, the waist, of the cw excitation laser beam (Ar+ laser at 514 nm), and the probe laser beam (a HeNe laser at 632.8 nm) is arranged so that its beam waist is shifted by Z1 1.7Zcp with respect to the beam waist of the excitation beam. In a typical experimental setup the excitation beam is focused by a 15 cm focal length lens. A mechanical shutter or a chopper controls the exposure time of the sample to the excitation beam. The probe beam is focused by a 20 cm focal length lens and is aligned at an angle smaller than 1.5 with respect to the excitation beam. For more details about the experimental setup see Refs. [10,12].
4. Results Fig. 1 shows the TL transient signal obtained for the two chalcogenide glasses by exciting at 514 nm with the same excitation power (around 1.9 mW). By fitting these curves with Eq. (2), h = (0.193 ± 0.001) rad and tc = 1.08 ± 0.01 ms were obtained for the Ga:La:S and h = (0.0802 ± 0.0005) rad and tc = 1.16 ± 0.01 ms for the Ga:La:S:O. Using these tcs in Eq. (4) jointly with the known excitation beam waist, we = 3.5 · 103 cm,
Normalized Transmittance
110
1.20 1.15 1.10 1.05 1.00 0
75 150 Time / ( ms )
225
Fig. 1. TL transient signal for Ga:La:S (triangle) and Ga:La:S:O (circle) and the theoretical curve obtained by Eq. (2). The glasses were excited at 514 nm with the same excitation power (around 1.9 mW). The correlation coefficients of the fit function to the data are: RD = 0.9992 and R = 0.9989.
the thermal diffusivity was obtained and the results are shown in Table 1. By performing the TL measurements as a function of the excitation power, the linear dependence of h normalized by the fraction of absorption, AeLeff, could be found for the samples, which are shown in Fig. 2. The solid lines are linear curves fits for which the correlation coefficient are: RD = 0.9997 and R = 0.9922. From the fit, the angular coefficient 114 ± 2 W1 for Ga:La:S and 157 ± 3 W1 for Ga:La:S:O were obtained. From Eq. (5), it is interesting to notice that the angular coefficient is exactly (kpK)1ds/dT. However, before calculating ds/ dT it is necessary to know the thermal conductivity of the samples. To obtain K, we performed TRC measurements to determine the sample specific heat. Fig. 3 shows the temperature dependence of the specific heat for both samples. By fitting linear function to the data, the temperature dependence was found to be cp(T)(J/g K) = (0.312 ± 0.002) + (3.71 ± 0.04) · 104T(K) and c p (T)(J/g K) = (0.339 ± 0.002) + (2.87 ± 0.04) · 104T(K) for Ga:La:S and Ga:La:S:O, respectively. The correlation coefficients of the fits are: Rm = 0.9755 and R = 0.9801. At room temperature, cp was calculated by these expressions and since q was measured by buoyancy method [14] based on the ArchimedesÕs principle with CCl4 as the immersion liquid to be 4.24 and 4.18 g/cm3 for Ga:La:S and Ga:La:S:O, respectively, the product qcp could be calculated (see Table 1). Therefore, noting that K=qcpD, the thermal conductivity could
Table 1 Thermal lens signal normalized by the absorbed excitation power, H = h/PAeLeff = (kpK)1 · ds/dT, the thermal diffusivity, D, the product densityspecific heat, qcp, the thermal conductivity, K and the temperature coefficient of the optical path length change, ds/dT, at 632.8 nm, to the Ga:La:S and Ga:La:S:O Samples
H (W1)
D (103 cm2/s)
qcp (J/cm3 K)
K (103 W/K cm)
ds/dT (105 K1)
Ga:La:S Ga:La:S:O
114 ± 2 157 ± 3
2.8 ± 0.1 2.6 ± 0.1
1.78 ± 0.08 1.80 ± 0.08
5.1 ± 0.4 4.6 ± 0.4
3.7 ± 0.3 4.6 ± 0.5
S.M. Lima et al. / Journal of Non-Crystalline Solids 348 (2004) 108–112
θ/AeLeff ( rad )
0.6 0.4 0.2 0.0 0.0
0.5 1.0 1.5 Excitation Power P/( mW )
2.0
Fig. 2. TL signal, h, normalized by the product between the absorption coefficient and the effective thickness, AeLeff, as a function of the excitation power, P, for Ga:La:S (triangle) and Ga:La:S:O (circle). The curve fits of the linear function, (h/PAeLeff)D = (114 ± 2) · P and (h/PAeLeff) = (157 ± 3) · P. The correlation coefficients of the fit function to the data are: RD = 0.9997 and R = 0.9922.
Specific heat cp( J/gK )
0.55 0.50 0.45 0.40 0.35
300
350 400 450 Temperature / ( K )
500
Fig. 3. Temperature dependence of the specific heat, cp, for Ga:La:S (closed triangle) and Ga:La:S:O (circle). The solid lines are their linear curve fittings. The correlation coefficients of the fit function to the data are: Rm = 0.9755 and R = 0.9801.
be calculated at T = 300 K and the obtained results are also shown in Table 1. So, by using kp = 632.8 nm and K in the product (kpK)1 · ds/dT, ds/dT could be determined for both glasses, as shown in Table 1.
5. Discussion Our experimental results show that by increasing the percentage of La2O3 in the glass composition no detectable changes in thermal diffusivity are observed. The average D for these chalcogenide glass samples (2.7 · 103 cm2/s) is approximately the same as that obtained in a similar chalcogenide glass recently measured with the TL method [14,15]. The comparison with other glasses shows that the thermal diffusivity of chalcogenides is similar to that of fluorides [12], but it is 50% of the diffusivities obtained for low calcium aluminosilicate glasses [16], and approximately two times larger than that of a chalcohalide glass [17].
111
At room temperature, the specific heats for the samples are within the experimental error of approximately 3%. The average 0.43 J/g K is less than those of other chalcogenide glasses, such as As2S3, in which cp is 0.653 J/g K [11]. In the temperature range considered in this study (i.e., from 300 to 480 K), the specific heat for Ga:La:S increases approximately 14% while for Ga:La:S:O cp increases 11%. In the experimental uncertainty for the thermal conductivity measurements (around 8% for these samples) we assert that both glasses have the same Ks. However, these Ks are approximately 18% less than that obtained recently in a 70 mol% Ga2S3; 30 mol% La2S3 chalcogenide glass [14]. The explanation for this difference is that the product, qcp, for our samples is around 18% less than that of the cited chalcogenide glass [14]. By comparing with fluoride glasses, the Ga:La:S and Ga:La:S:O samples have thermal conductivities 1.6 times less [10]. The explanation is that the fluoride density and specific heat results are respectively 5% and 36% greater than for our samples [12]. It is also interesting to mention that the K for chalcogenide glasses are just 33% of the thermal conductivity measured in aluminate glasses [16]. The Ga:La:S:O sample shows an intense reversible photodarkening effect that makes difficult the exact determination of the optical absorption coefficient at 514 nm, and consequently the calculation of the change in the temperature coefficient of the optical path length, ds/dT. This effect was smaller in the Ga:La:S glass and therefore its ds/dT value could be determined more accurately. The Ga:La:S glass has a ds/dT around 50% less than that previously obtained for the 70 mol% Ga2S3; 30 mol% La2S3 [14]. This difference may be explained by the fact that the inclusion of La2O3 in the glass composition reduces the thermal expansion coefficient, and consequently decreases ds/dT. This observation is supported by the smaller thermal expansion coefficients of oxide glasses, between 5 · 106 and 8 · 106 K1 [18], in comparison to that of chalcogenides (around 30 · 106 K1) [19]. The ds/dTÕs values for chalcogenide glasses are positive, similarly to those of oxide glasses, while for fluorides, ds/dTÕs values are negative [12]. It is important to mention that ds/dTÕs values for chalcogenides are one order of magnitude larger than obtained for fluorides. This difference makes the optical path in chalcogenide glasses more susceptible to temperature changes.
6. Conclusions By combining the thermal lens technique and the thermal relaxation calorimetry it was possible to determine the thermal diffusivity, thermal conductivity, specific heat, and the temperature coefficient of the
112
S.M. Lima et al. / Journal of Non-Crystalline Solids 348 (2004) 108–112
optical path length change for two chalcogenides samples (in mol%): 65% Ga2S3; 30% La2S3; 5% La2O3 (Ga:La:S) and 72.5% Ga2S3; 27.5% La2O3 (Ga:La:S:O). Our results indicate that changes in thermal diffusivity (2.7 · 103 cm2/s) and conductivity (4.8 · 103 W/ K cm) observed when La2O3 is added in the glass compounds are less the error in the data. These D and K are in good agreement with those reported in the literature for a similar chalcogenide glass [14]. The ds/dT obtained in the present work for Ga:La:S is smaller by 50% than that recently measured in a 70% Ga2S3; 30% La2S3 glass [14]. Finally, the smaller K and larger ds/dT of these chalcogenide glasses, as compared to those of fluorides, are responsible for the observed larger thermal lens effects [5,6].
Acknowledgments The authors are thankful to CNPq, FAPESP and Fundac¸a˜o Arauca´ria for the financial support of this work. We are grateful to Dr T. Catunda for permitting the TL measurements in his Laboratory.
References [1] Y.D. West, T. Schweizer, D.J. Brady, D.W. Hewak, Fiber Integr. Opt. 19 (2000) 229. [2] T. Schweizer, D.J. Brady, D.W. Hewak, Opt. Exp. 1 (1997) 102. [3] T. Schweizer, in: D. Hewak (Ed.), Properties, Processing and Applications of Glass and Rare Earth Doped Glasses for Optical Fibres, INSPEC, Southampton, 1998, p. 320.
[4] I. Kang, T.D. Krauss, F.W. Wise, B.G. Aitken, N.F. Borrelli, J. Opt. Soc. Am. B 12 (1995) 2053. [5] D.W. Hewak., R.C. Moore, T. Schweizer, J. Wang, B. Samson, W.S. Brocklesby, D.N. Payne, E.J. Tarbox, Electron. Lett. 32 (1996) 384. [6] T. Schweizer, B.N. Samson, R.C. Moore, D.W. Hewak, D.N. Payne, Electron. Lett. 33 (1997) 414. [7] A.K. Mairaj, A.M. Chardon, D.P. Shepherd, D.W. Hewak, IEEE J. Sel. Topics Quant. Electron. 8 (2002) 1381. [8] E. Courtens, M. Foret, B. Hehlen, R. Vacher, Solid State Commun. 117 (2001) 187. [9] Y.S. Touloukian, R.W. Powell, Y.C. Ho, M.C. Nicalasu, Thermal Diffusivity, Plenum, New York, 1973. [10] 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. [11] A.N. Medina, A.M.F. Caldeira, A.C. Bento, M.L. Baesso, J.A. Sampaio, T. Catunda, F.G. Gandra, J. Non-Cryst. Solids 304 (2002) 299. [12] S.M. Lima, T. Catunda, R. Lebullenger, A.C. Hernandes, M.L. Baesso, A.C. Bento, L.C.M. Miranda, Phys. Rev. B 60 (1999) 15173. [13] J.H. Rohling, A.N. Medina, A.C. Bento, J.R.D. Pereira, A.F. Rubira, M.L. Baesso, L.C.M. Miranda, J. Phys. D: Appl. Phys. 34 (2001) 407. [14] S.M. Lima, J.A. Sampaio, T. Catunda, A.S.S. de Camargo, L.A.O. Nunes, M.L. Baesso, D.W. Hewak, J. Non-Cryst. Solids 284 (2001) 274. [15] S.M. Lima, T. Catunda, M.L. Baesso, L.D. Vila, Y. Messaddeq, E.B. Stucchi, S.J.L. Ribeiro, J. Non-Cryst. Solids 247 (1999) 222. [16] M.L. Baesso, A.C. Bento, A.R. Duarte, A.M. Neto, L.C. Miranda, J.A. Sampaio, T. Catunda, S. Gama, F.C.G. Gandra, J. Appl. Phys. 85 (1999) 8112. [17] S.M. Lima, A.A. Andrade, T. Catunda, R. Lebullenger, F. Smektala, Y. Jestin, M.L. Baesso, J. Non-Cryst. Solids 284 (2001) 203. [18] R.M. Waxler, G.W. Cleek, J. Res. 77A (1973) 755. [19] E.M. Marmolejo, E. Granado, O.L. Alves, C.L. Cesar, L.C. Barbosa, J. Non-Cryst. Solids 247 (1999) 189.
Thermo-optical properties measurements in chalcogenide glasses using thermal relaxation and thermal lens methods S.M. Lima a,*, A. Steimacher b, A.N. Medina b, M.L. Baesso b, M.N. Petrovich c, H.N. Rutt c, D.W. Hewak c a
Universidade Estadual de Mato Grosso do Sul, Cidade Universita´ria de Dourados, C.P. 351, CEP 79804-970, Dourados, MS, Brazil b Departamento de Fı´sica, Universidade Estadual de Maringa´, CEP 87020-900, Maringa´, PR, Brazil c Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, England, United Kingdom
Abstract In this work we determine the thermo-optical properties of two chalcogenide glasses (in mol%): 65 Ga2S3; 30 La2S3; 5 La2O3 (Ga:La:S) and 72.5 Ga2S3; 27.5 La2O3 (Ga:La:S:O). The thermal relaxation calorimetry and the thermal lens technique were combined so that the samples specific heat, thermal diffusivity, thermal conductivity, and the temperature coefficient of the optical path length change could be measured. Our results indicate that changes in thermal diffusivity (2.7 · 103 cm2/s) and conductivity (4.8 · 103 W/K cm) observed when La2O3 is added in the glass compounds are less than the error in the data. 2004 Elsevier B.V. All rights reserved. PACS: 65.60.+a; 78.20.-e; 42.70.Ce
1. Introduction Chalcogenide glasses based on gallium and lanthanum sulphides and oxi-sulphides (for instance Ga:La:S and Ga:La:S:O) have been investigated recently because of their potential for photonic applications [1,2]. Their glass matrix phonon energy 425 cm1 results in infrared transparency to around 5 lm and low non-radiative decay rates of rare-earth energy levels. The covalency nature of the glass bonds increases the radiative decay rates, the emission and the optical absorption cross sections [3]. Furthermore, the refractive index is correlated with third-order non-linearity, which is attractive for alloptical switching [4]. All these properties together with higher glass transition temperatures, chemical stability and non-toxicity make these glasses suitable for photonic applications. Laser action in Nd3+-doped Ga:La:S
*
Corresponding author. Fax: +55 67 411 9173. E-mail address: [email protected] (S.M. Lima).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.08.134
glass was observed in glass samples [5], fibers [6], and ultraviolet-written waveguides [7]. Besides, minimum losses of about 1 and 4 dBm1 at 4 lm have been achieved in unclad Ga:La:S [1] and Ga:La:S:O [2] fibers, respectively. Knowledge of the figures of merit of these glasses is useful in applications such as all-optical switching. For rare-earth doped lasers, for instance, it is important to know the thermalization time within the host material, the thermally induced distortion of a laser beam through the host, and the heat diffusion coefficients of the host material. These are determined by measuring the thermal diffusivity, D, the temperature dependence of the optical path length change, ds/dT, and the specific heat, cp [8,9]. In the last few years, these properties have been measured in glasses, crystals, and polymers, also as a function of temperature, by using the thermal lens (TL) technique and thermal relaxation calorimetry (TRC) [10–13]. The aim of this work is to combine the TRC and TL techniques to determine the thermo-optical properties
S.M. Lima et al. / Journal of Non-Crystalline Solids 348 (2004) 108–112
(D, cp, ds/dT, and the thermal conductivity, K) of two chalcogenide glasses of composition (in mol%): 65 Ga2S3; 30 La2S3; 5 La2O3 (Ga:La:S) and 72.5 Ga2S3; 27.5 La2O3 (Ga:La:S:O). In addition, the temperature dependence of the specific heat was also measured for both glass compositions from 300 to 450 K. The aim for this study is to verify the effect of the oxide content on the thermo-optical properties of the glass.
2.1. Thermal relaxation calorimetry (TRC) In the TRC, the setup is composed of a sample holder (or substrate) and a temperature sensor. The substrate is supported by wires, which are attached to a thermal reservoir. A thermal link between the system and the thermal bath is also provided by these wires, as well as the gas of the surrounding atmosphere and the thermal radiation. This thermal link brings the system to a thermal equilibrium at a temperature T0. When a constant power, P, of a laser radiation is supplied to the substrate, which is in contact with the sample, part of this energy is transferred to the reservoir via the thermal links, so that the heat balance equation is given by [8,9] dðDT Þ P ¼C þ K eff DT ; dt
theory, an analytical expression can be obtained for the probe beam intensity change, I(t), [10] h IðtÞ ¼ Ið0Þ 1 tan1 2 " #)2 2mV ; ð2Þ 2 ½ð1 þ 2mÞ þ V 2 tc =2t þ 1 þ 2m þ V 2 where 2 wp m¼ ; we
2. Theory
ð1Þ
where C is the system heat capacity, Keff is the effective thermal conductance, DT = DTmax[1 exp(t/s)] is the temperature difference between the system and the thermal bath, DTmax = P/Keff is the final temperature difference in the steady state, and s = C/Keff is the external relaxation time. When the laser power is turned off, the system temperature decays exponentially to T0 following the equation DT = DTmax exp(t/s). By monitoring the time dependence of the temperature rise or decay it is possible to determine the final temperature difference and the relaxation time. Therefore, the heat capacity of the sample is obtained subtracting the substrate heat capacity, which is previously determined. 2.2. Thermal lens (TL) technique In the dual-beam mode-mismatched TL technique, the sample is illuminated by two TEM00 Gaussian laser beams, of which one is used for exciting the sample to produce a local temperature increase, creating a lens-like element in the heated region, and the other to probe the thermal effect. When the probe beam passes through the created lens, its optical path length undergoes a temporal change that can be observed by measuring the beam center intensity in the far field. Using Fresnel diffraction
109
V ¼
Z1 Z cp
with Z cp Z 2 :
ð3Þ
In which, wp and we are the probe and excitation laser beams radius at the sample, respectively, Z1 is the distance between the probe beam focal plane and the sample, Zcp is the confocal distance of the probe beam, Z2 is the distance between the sample and the detector and I(0) is I(t) when the transient time t or h is zero. The characteristic TL signal response time, tc, is given by [10] w2e ; ð4Þ 4D where D = K/qcp is the thermal diffusivity, K is the thermal conductivity, q is the density and cp is the sample specific heat. The probe beam phase shift, h, induced by TL is given by [10] tc ¼
h¼
PAe Leff ds : u dT Kkp
ð5Þ
In which P is the excitation laser power, kp is the probe beam wavelength, Leff = (1 eAeL)/Ae is the effective length, L is the sample thickness, Ae is the absorption coefficient, and u is the fraction of absorbed energy converted into heat per photon. In the case of non-luminescent samples, such as the samples studied in this work, all absorbed energy is converted into heat, so u = 1 in Eq. (5).
3. Experimental procedures The chalcogenide glasses were prepared with nominal molar composition 65% Ga2S3; 30% La2S3; 5% La2O3 (Ga:La:S) and 72.5% Ga2S3; 27.5% La2O3 (Ga:La:S:O) by standard powder melting. The sulphides were synthesized in-house and their purity, as measured by glow discharge mass spectroscopy was approximately 99.9999%. The lanthanum oxide was commercial purity grade, 99.999%. Batches with the chosen compositions were weighed inside a glovebox, under dry and inert atmosphere, loaded in vitreous carbon crucibles and then transferred to a quartz-lined furnace. Melting was performed at 1150 C for about 24 h under a stream of argon; glass quenching was accomplished by pushing the crucible into a water-cooled jacket. Glass ingots were annealed, cut in a rectangular shape of approximately
S.M. Lima et al. / Journal of Non-Crystalline Solids 348 (2004) 108–112
2 · 1 cm · 1 mm thick and then the larger surfaces were optically polished. In our calorimeter, a constant heat power is supplied to the system by illuminating the sample using a diode laser. As mentioned before, when the heat is applied to the system, the temperature difference between the substrate and the thermal bath increases exponentially (see Eq. (1)) with time until a steady state is reached. A shutter then blocks the laser beam and the temperature decay is observed. The increase or decrease of temperature difference is monitored by thermocouple (cooper– constantan) in differential configuration, which sends the signal into a nanovoltmeter (Keithley, model 2182). The output of the microvoltmeter is then digitized and stored in a computer for posterior analysis. A temperature controller (LakeShore Cryotronics Inc., model 340), stabilizes the temperature of the bath using a calibrated sensor (PT-100) and a resistive heater attached to the thermal reservoir. Further details of the experimental procedure can be found elsewhere [11]. In the dual-beam mode-mismatched TL experimental setup used in this work, the sample is placed at the minimum beam diameter, the waist, of the cw excitation laser beam (Ar+ laser at 514 nm), and the probe laser beam (a HeNe laser at 632.8 nm) is arranged so that its beam waist is shifted by Z1 1.7Zcp with respect to the beam waist of the excitation beam. In a typical experimental setup the excitation beam is focused by a 15 cm focal length lens. A mechanical shutter or a chopper controls the exposure time of the sample to the excitation beam. The probe beam is focused by a 20 cm focal length lens and is aligned at an angle smaller than 1.5 with respect to the excitation beam. For more details about the experimental setup see Refs. [10,12].
4. Results Fig. 1 shows the TL transient signal obtained for the two chalcogenide glasses by exciting at 514 nm with the same excitation power (around 1.9 mW). By fitting these curves with Eq. (2), h = (0.193 ± 0.001) rad and tc = 1.08 ± 0.01 ms were obtained for the Ga:La:S and h = (0.0802 ± 0.0005) rad and tc = 1.16 ± 0.01 ms for the Ga:La:S:O. Using these tcs in Eq. (4) jointly with the known excitation beam waist, we = 3.5 · 103 cm,
Normalized Transmittance
110
1.20 1.15 1.10 1.05 1.00 0
75 150 Time / ( ms )
225
Fig. 1. TL transient signal for Ga:La:S (triangle) and Ga:La:S:O (circle) and the theoretical curve obtained by Eq. (2). The glasses were excited at 514 nm with the same excitation power (around 1.9 mW). The correlation coefficients of the fit function to the data are: RD = 0.9992 and R = 0.9989.
the thermal diffusivity was obtained and the results are shown in Table 1. By performing the TL measurements as a function of the excitation power, the linear dependence of h normalized by the fraction of absorption, AeLeff, could be found for the samples, which are shown in Fig. 2. The solid lines are linear curves fits for which the correlation coefficient are: RD = 0.9997 and R = 0.9922. From the fit, the angular coefficient 114 ± 2 W1 for Ga:La:S and 157 ± 3 W1 for Ga:La:S:O were obtained. From Eq. (5), it is interesting to notice that the angular coefficient is exactly (kpK)1ds/dT. However, before calculating ds/ dT it is necessary to know the thermal conductivity of the samples. To obtain K, we performed TRC measurements to determine the sample specific heat. Fig. 3 shows the temperature dependence of the specific heat for both samples. By fitting linear function to the data, the temperature dependence was found to be cp(T)(J/g K) = (0.312 ± 0.002) + (3.71 ± 0.04) · 104T(K) and c p (T)(J/g K) = (0.339 ± 0.002) + (2.87 ± 0.04) · 104T(K) for Ga:La:S and Ga:La:S:O, respectively. The correlation coefficients of the fits are: Rm = 0.9755 and R = 0.9801. At room temperature, cp was calculated by these expressions and since q was measured by buoyancy method [14] based on the ArchimedesÕs principle with CCl4 as the immersion liquid to be 4.24 and 4.18 g/cm3 for Ga:La:S and Ga:La:S:O, respectively, the product qcp could be calculated (see Table 1). Therefore, noting that K=qcpD, the thermal conductivity could
Table 1 Thermal lens signal normalized by the absorbed excitation power, H = h/PAeLeff = (kpK)1 · ds/dT, the thermal diffusivity, D, the product densityspecific heat, qcp, the thermal conductivity, K and the temperature coefficient of the optical path length change, ds/dT, at 632.8 nm, to the Ga:La:S and Ga:La:S:O Samples
H (W1)
D (103 cm2/s)
qcp (J/cm3 K)
K (103 W/K cm)
ds/dT (105 K1)
Ga:La:S Ga:La:S:O
114 ± 2 157 ± 3
2.8 ± 0.1 2.6 ± 0.1
1.78 ± 0.08 1.80 ± 0.08
5.1 ± 0.4 4.6 ± 0.4
3.7 ± 0.3 4.6 ± 0.5
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θ/AeLeff ( rad )
0.6 0.4 0.2 0.0 0.0
0.5 1.0 1.5 Excitation Power P/( mW )
2.0
Fig. 2. TL signal, h, normalized by the product between the absorption coefficient and the effective thickness, AeLeff, as a function of the excitation power, P, for Ga:La:S (triangle) and Ga:La:S:O (circle). The curve fits of the linear function, (h/PAeLeff)D = (114 ± 2) · P and (h/PAeLeff) = (157 ± 3) · P. The correlation coefficients of the fit function to the data are: RD = 0.9997 and R = 0.9922.
Specific heat cp( J/gK )
0.55 0.50 0.45 0.40 0.35
300
350 400 450 Temperature / ( K )
500
Fig. 3. Temperature dependence of the specific heat, cp, for Ga:La:S (closed triangle) and Ga:La:S:O (circle). The solid lines are their linear curve fittings. The correlation coefficients of the fit function to the data are: Rm = 0.9755 and R = 0.9801.
be calculated at T = 300 K and the obtained results are also shown in Table 1. So, by using kp = 632.8 nm and K in the product (kpK)1 · ds/dT, ds/dT could be determined for both glasses, as shown in Table 1.
5. Discussion Our experimental results show that by increasing the percentage of La2O3 in the glass composition no detectable changes in thermal diffusivity are observed. The average D for these chalcogenide glass samples (2.7 · 103 cm2/s) is approximately the same as that obtained in a similar chalcogenide glass recently measured with the TL method [14,15]. The comparison with other glasses shows that the thermal diffusivity of chalcogenides is similar to that of fluorides [12], but it is 50% of the diffusivities obtained for low calcium aluminosilicate glasses [16], and approximately two times larger than that of a chalcohalide glass [17].
111
At room temperature, the specific heats for the samples are within the experimental error of approximately 3%. The average 0.43 J/g K is less than those of other chalcogenide glasses, such as As2S3, in which cp is 0.653 J/g K [11]. In the temperature range considered in this study (i.e., from 300 to 480 K), the specific heat for Ga:La:S increases approximately 14% while for Ga:La:S:O cp increases 11%. In the experimental uncertainty for the thermal conductivity measurements (around 8% for these samples) we assert that both glasses have the same Ks. However, these Ks are approximately 18% less than that obtained recently in a 70 mol% Ga2S3; 30 mol% La2S3 chalcogenide glass [14]. The explanation for this difference is that the product, qcp, for our samples is around 18% less than that of the cited chalcogenide glass [14]. By comparing with fluoride glasses, the Ga:La:S and Ga:La:S:O samples have thermal conductivities 1.6 times less [10]. The explanation is that the fluoride density and specific heat results are respectively 5% and 36% greater than for our samples [12]. It is also interesting to mention that the K for chalcogenide glasses are just 33% of the thermal conductivity measured in aluminate glasses [16]. The Ga:La:S:O sample shows an intense reversible photodarkening effect that makes difficult the exact determination of the optical absorption coefficient at 514 nm, and consequently the calculation of the change in the temperature coefficient of the optical path length, ds/dT. This effect was smaller in the Ga:La:S glass and therefore its ds/dT value could be determined more accurately. The Ga:La:S glass has a ds/dT around 50% less than that previously obtained for the 70 mol% Ga2S3; 30 mol% La2S3 [14]. This difference may be explained by the fact that the inclusion of La2O3 in the glass composition reduces the thermal expansion coefficient, and consequently decreases ds/dT. This observation is supported by the smaller thermal expansion coefficients of oxide glasses, between 5 · 106 and 8 · 106 K1 [18], in comparison to that of chalcogenides (around 30 · 106 K1) [19]. The ds/dTÕs values for chalcogenide glasses are positive, similarly to those of oxide glasses, while for fluorides, ds/dTÕs values are negative [12]. It is important to mention that ds/dTÕs values for chalcogenides are one order of magnitude larger than obtained for fluorides. This difference makes the optical path in chalcogenide glasses more susceptible to temperature changes.
6. Conclusions By combining the thermal lens technique and the thermal relaxation calorimetry it was possible to determine the thermal diffusivity, thermal conductivity, specific heat, and the temperature coefficient of the
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optical path length change for two chalcogenides samples (in mol%): 65% Ga2S3; 30% La2S3; 5% La2O3 (Ga:La:S) and 72.5% Ga2S3; 27.5% La2O3 (Ga:La:S:O). Our results indicate that changes in thermal diffusivity (2.7 · 103 cm2/s) and conductivity (4.8 · 103 W/ K cm) observed when La2O3 is added in the glass compounds are less the error in the data. These D and K are in good agreement with those reported in the literature for a similar chalcogenide glass [14]. The ds/dT obtained in the present work for Ga:La:S is smaller by 50% than that recently measured in a 70% Ga2S3; 30% La2S3 glass [14]. Finally, the smaller K and larger ds/dT of these chalcogenide glasses, as compared to those of fluorides, are responsible for the observed larger thermal lens effects [5,6].
Acknowledgments The authors are thankful to CNPq, FAPESP and Fundac¸a˜o Arauca´ria for the financial support of this work. We are grateful to Dr T. Catunda for permitting the TL measurements in his Laboratory.
References [1] Y.D. West, T. Schweizer, D.J. Brady, D.W. Hewak, Fiber Integr. Opt. 19 (2000) 229. [2] T. Schweizer, D.J. Brady, D.W. Hewak, Opt. Exp. 1 (1997) 102. [3] T. Schweizer, in: D. Hewak (Ed.), Properties, Processing and Applications of Glass and Rare Earth Doped Glasses for Optical Fibres, INSPEC, Southampton, 1998, p. 320.
[4] I. Kang, T.D. Krauss, F.W. Wise, B.G. Aitken, N.F. Borrelli, J. Opt. Soc. Am. B 12 (1995) 2053. [5] D.W. Hewak., R.C. Moore, T. Schweizer, J. Wang, B. Samson, W.S. Brocklesby, D.N. Payne, E.J. Tarbox, Electron. Lett. 32 (1996) 384. [6] T. Schweizer, B.N. Samson, R.C. Moore, D.W. Hewak, D.N. Payne, Electron. Lett. 33 (1997) 414. [7] A.K. Mairaj, A.M. Chardon, D.P. Shepherd, D.W. Hewak, IEEE J. Sel. Topics Quant. Electron. 8 (2002) 1381. [8] E. Courtens, M. Foret, B. Hehlen, R. Vacher, Solid State Commun. 117 (2001) 187. [9] Y.S. Touloukian, R.W. Powell, Y.C. Ho, M.C. Nicalasu, Thermal Diffusivity, Plenum, New York, 1973. [10] 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. [11] A.N. Medina, A.M.F. Caldeira, A.C. Bento, M.L. Baesso, J.A. Sampaio, T. Catunda, F.G. Gandra, J. Non-Cryst. Solids 304 (2002) 299. [12] S.M. Lima, T. Catunda, R. Lebullenger, A.C. Hernandes, M.L. Baesso, A.C. Bento, L.C.M. Miranda, Phys. Rev. B 60 (1999) 15173. [13] J.H. Rohling, A.N. Medina, A.C. Bento, J.R.D. Pereira, A.F. Rubira, M.L. Baesso, L.C.M. Miranda, J. Phys. D: Appl. Phys. 34 (2001) 407. [14] S.M. Lima, J.A. Sampaio, T. Catunda, A.S.S. de Camargo, L.A.O. Nunes, M.L. Baesso, D.W. Hewak, J. Non-Cryst. Solids 284 (2001) 274. [15] S.M. Lima, T. Catunda, M.L. Baesso, L.D. Vila, Y. Messaddeq, E.B. Stucchi, S.J.L. Ribeiro, J. Non-Cryst. Solids 247 (1999) 222. [16] M.L. Baesso, A.C. Bento, A.R. Duarte, A.M. Neto, L.C. Miranda, J.A. Sampaio, T. Catunda, S. Gama, F.C.G. Gandra, J. Appl. Phys. 85 (1999) 8112. [17] S.M. Lima, A.A. Andrade, T. Catunda, R. Lebullenger, F. Smektala, Y. Jestin, M.L. Baesso, J. Non-Cryst. Solids 284 (2001) 203. [18] R.M. Waxler, G.W. Cleek, J. Res. 77A (1973) 755. [19] E.M. Marmolejo, E. Granado, O.L. Alves, C.L. Cesar, L.C. Barbosa, J. Non-Cryst. Solids 247 (1999) 189.