J. Phys. IV France 125 (2005) 193-196 Ó EDP Sciences, Les Ulis DOI: 10.1051/jp4:2005125045
Thermal lens determination of fluorescence quantum efficiency of 3F4 level of Tm3+ ions in solids S.L. Oliveira1,2,*, S.M. Lima1,3, T. Catunda1, H. Vargas2, L.A.O. Nunes1, J.H. Rohling4, A.C. Bento4 and M.L. Baesso4 1
Instituto de Física de São Carlos, Universidade de São Paulo, CP 369 CEP 13560 - 970, São Carlos, SP, Brazil. 2 Universidade Estadual do Norte Fluminense, Av. Alberto Lamego 2000, CEP 28013-600, Campos dos Goytacazes, RJ, Brazil 3 Universidade Estadual do Mato Grosso do Sul – UEMS, Grupo de Espectroscopia Óptica e Fototérmica, CP 351, Dourados, MS, Brazil. 4 Departamento de Física, Universidade Estadual de Maringá, Av. Colombo 5790, CEP 87020-900, Maringá, PR, Brazil Abstract. In this work, a method based on thermal lens spectrometry is proposed to determine the fluorescence quantum efficiency (h) of the 3F4 level (~1.8mm emission) of Tm-doped solids. It was applied to Tm-doped water free low silica calcium aluminate glasses. The obtained h value for the glasses with high Tm content was approximately 30 %, which is in agreement with the achieved h values from ratio between radiative and experimental lifetimes.
1. INTRODUCTION The knowledge of the fluorescence quantum efficiency (h) is important for characterization of luminescent ion-doped materials used in photonic devices [1]. Several methods based on the pure optical apparatus, for instance by comparing the experimental and radiative lifetimes (hLT=texp/trad), have been used to measure it. On the other hand, the photothermal techniques among them, thermal lens (TL) spectrometry, which are related to the fraction of absorbed radiative energy converted into heat by material (j), have also contributed for determination of h parameter due to the intrinsic relation among them [1]. The TL spectrometry has been extensively used in the evaluation of h in doped materials. Different approaches based on TL effect were proposed to evaluate the h of the 4 F3/2 level (1.06 mm emission) of Nd3+-doped solids. Among the rare earth ions, Tm3+ ions have wake up interest of the researches due to its infrared emission around eye-safe 2.0 mm, which can be used in several technological applications including medicine, nonlinear frequency mixing, etc [2]. This emission can be achieved using high power diode laser operating at 0.79 mm [3]. In this investigation, we present an approach based on TL technique to measure h of the 3F4 level (~1.8 mm emission) of Tm-doped materials. The method was applied to Tm-doped low silica calcium aluminate (LSCA) glasses. ______________________________ * Author to whom correspondence should be addressed; e-mail:
[email protected]
Article published by EDP Sciences and available at http://www.edpsciences.org/jp4 or http://dx.doi.org/10.1051/jp4:2005125045
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2. EXPERIMENTAL DETAILS The LSCA glass samples with different Tm concentrations were melted under vacuum conditions with high purity powders as described previously [1]. The TL measurements were performed in the dual-beam mode-mismatched TL configuration using an Ar+ (1.09 mm) and He:Ne (0.63 mm) as excitation and probe laser beam, respectively. Optical absorption measurements were made using a Perkin Elmer Lambda 900. Luminescence measurements were carried out using Kr+ laser operating around 0.66 mm and a monochromator coupled with a cooled InAs detector. Lifetime measurements were done with a OPO pumped by Nd:YAG. The lifetime signals dispersed by the monochromator were detected by a cooled detector (InAs). The signals were recorded in a digital oscilloscope. All measurements were done at room temperature. 3. RESULTS AND DISCUSSIONS Figure 1.a shows the absorption spectrum of the LSCA glass with 5.0 wt.% of Tm3+. Figure 1.b displays a partial energy level diagram of Tm3+ in LSCA glass, showing only energy level up to 15000 cm-1. (a)
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Figure 1. (a) Absorption spectrum for 5 wt. % Tm2O3 in LSCA glasses; (b) Simplified energy levels diagram for Tm3+ ions. The excitation wavelengths and relaxation processes are indicated.
The fluorescent spectra of the samples with 0.5 and 5 wt. % Tm2O3 are exhibited in Fig. 2.a. Starting from these results we can observe an increase (decrease) of the intensity of emission around 1.8 mm (0.8 mm) when the Tm3+ concentration increase. This behavior is related to the energy transfer mechanism called cross relaxation (CR), described as following: an excited ion at the 3H4 level can transfer part of its energy to a nearby ion at 3H6 state promoting it to the 3F4 level while it decay to the 3 F4 level (Fig. 1.b). The CR process can improve the pumping efficiency of 3F4 by a factor 2, mainly under laser excitation around 3H4 level. In luminescent materials with a main radiative emission channel and energy transfer mechanisms that causes only an increase of thermal loading, a part of the absorbed excitation photon energy (hnexc) is converted into heat and the remaining energy is converted in radiative emission, generating a photon with average energy h. In these materials, j is related to h through the expression j=1-h(nexc)-1 (Eq. 1). From this, 1.09 mm was chosen as excitation wavelength to evaluate the h value of 3F4 level because it is close to the 3H5 energy level (Fig. 1.b) and it satisfies the conditions mentioned above. Therefore, from the j values, which can be achieved from TL measurements, the h parameter can be evaluated since (nexc)-1 is known.
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Figure 2. (a) Luminescence spectra of LSCA glasses with 0.5 and 5 wt. % Tm2O3 under pumping at 0.66 mm; (b) 1.8 mm fluorescence intensity as a function of Tm3+ concentration.
The TL technique is based on the induced TL effect in a partially transparent medium when an excitation laser beam passes through the sample. The absorbed energy from the excitation laser converted into heat changes the optical path length (s) in solid sample. The propagation of a probe beam through the TL can be spread (ds/dT < 0) or focalized (ds/dT > 0), depending mainly on the temperature coefficients of the thermal expansion and electronic polarizability of the sample[1]. By measuring the probe beam on-axis intensity in the far field, j values can be obtained by means of the transient signal amplitude which is proportional to the normalized phase parameter given by Q = -(ds/dT)(Klp)-1j , where lp is the probe beam wavelength, K is the thermal conductivity and ds/dT is the temperature coefficient of the optical path length change at lp. The procedure used for h determination of the 3F4 level is based on the comparison between the TL signal of the ion-doped (Qd) and undoped (Qund) samples under 1.09 mm excitation. Considering that K-1ds/dT is approximately the same for doped and undoped samples, and the undoped sample presents no fluorescence (hund = 0 and jund = 1), j d values can be obtained from the ratio Qd/Qund=jd. Consequently, h values of 3F4 level can be calculated from Eq. 1. The jd values were obtained for the samples with 4 and 5 wt.% Tm2O3. These samples were chosen because of the intense fluorescence emission around 1.8 mm (Fig. 2.b). The TL measurements were done using excitation wavelength at 1.09 mm. The jd values are similar (~ 84 %) for these glasses. Using these data and = 1.8 mm in Eq. 1, the h values of 3F4 level were achieved, Table 1. In the same table are also exhibited h values determined from the expression hLT = texp/trad. The radiative lifetime (trad) of the 3F4 level calculated from Judd-Ofelt (JO) theory is 6.7 ms. Table 1. Lifetime, texp, and the fluorescence quantum efficiency, h, measured by Thermal Lens (TL) method and determined from the lifetimes (LT) for 4 and 5 (wt. %) Tm2O3 doped LSCA glasses. Tm2O3 (wt. %) 4 5
texp (ms) 2.3 ± 0.1 2.2 ± 0.1
hTL 0.25 ± 0.09 0.29 ± 0.09
hLT 0.34 ± 0.04 0.33 ± 0.04
3. CONCLUSIONS In conclusions, we reported an approach to measure h of the 3F4 level in Tm-doped materials using TL spectrometry. The method was applied in Tm-doped LSCA glasses. The good agreement between the results obtained from the TL measurements and ratio between radiative and experimental lifetimes indicates it a reliable method.
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Acknowledgements The authors are thankful to FAPESP, CNPq and Fundação Araucária for the financial support of this work.
References [1] M. L. Baesso, A. C. Bento, A. A. Andrade, J. A. Sampaio, E. Pecoraro, L. A. O. Nunes, T. Catunda and S. Gama, Phys. Rev. B. 57, 10545 (1998). [2] A. F. El-Sherif and T. A. King, IEEE J. Quantum Electron. 39, 759-765 (2003). [3] R. A. Hayward, W. A. Clarkson, P. W. Turner, J. Nilsson, A. B. Grudinin and D. C. Hanna, Electron. Lett. 36, 711 (2000).
