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Thin Solid Films 516 (2008) 3094 – 3097 www.elsevier.com/locate/tsf
Erbium-activated silica–zirconia planar waveguides prepared by sol–gel route Rogéria R. Gonçalves a,⁎, Younes Messaddeq b , Alessandro Chiasera c , Yoann Jestin c , Maurizio Ferrari c , Sidney J.L. Ribeiro b a
Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo-Av., Bandeirantes 3900, cep 14040-901, Ribeirão Preto-SP, Brazil b Instituto de Química, UNESP-Rua Prof. Francisco Degni, s/n, Quitandinha, cep 14800-900 Araraquara-SP — Brazil c CNR-IFN, Istituto di Fotonica e Nanotecnologie, CSMFO group, via Sommarive 14, 38050 Povo, Trento, Italy Received 15 August 2006; received in revised form 30 June 2007; accepted 20 July 2007 Available online 3 August 2007
Abstract Er3+ doped (100 − x)SiO2 − xZrO2 planar waveguides were prepared by the sol–gel route, with x ranging from 10 up to 30 mol%. Multilayer films doped with 0.3 mol% Er3+ ions were deposited on fused quartz substrates by the dip-coating technique. The thickness and refractive index were measured by m-line spectroscopy at different wavelengths. The fabrication protocol was optimized in order to confine one propagating mode at 1.5 μm. Photoluminescence in the near and visible region indicated a crystalline local environment for the Er3+ ion. © 2007 Elsevier B.V. All rights reserved. Keywords: Planar waveguides; Sol–gel; Zirconia; Silica; Erbium; Optical properties; Photoluminescence
1. Introduction Er3+-activated silicate glasses have attracted great interest from several scientific and technological areas, and they are known for their applications in the C telecommunication band (1530–1565 nm) [1–4]. The sol–gel process, on the other hand, has emerged as one of the cheapest and most versatile routes for the fabrication of SiO2-based planar waveguides, with valuable prospects for photonic application [5]. Among the silicate-based systems, the SiO2–MO2 binary systems (M = Ti, Hf) have been extensively used to produce Er-doped planar waveguides by the sol–gel route [2,3,6–15]. Er3+-activated SiO2–HfO2 planar waveguides have been shown to be a promising material for 1.5 μm applications, mainly because of their efficient luminescence quantum yield, effective broad bandwidth, and attenuation coefficient as low as 0.8 dB/cm at 1.5 μm [12–14]. The optical and morphological properties of SiO2–ZrO2 systems prepared by
⁎ Corresponding author. Tel.: +55 16 36024851; fax: +55 16 36338151. E-mail address:
[email protected] (R.R. Gonçalves). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.07.183
the sol–gel process have also been recently reported in the literature [15]. Zirconium and Hafnium oxides, called twin oxides, are similar in terms of many chemical and physical properties. In fact, like HfO2, ZrO2 is transparent over a wide range of wavelengths, namely from 300 nm to 8 μm. ZrO2 has a high refractive index and low cut-off phonon energy of 650 cm− 1 [16]. Refractive index values between 1.79 and 1.97 in the visible and near infrared regions have been measured for zirconium oxide films, and values between 2.13 and 2.20 have been recorded for compact materials, depending on the preparation methodology [16–18]. Such properties of ZrO2 allow the development of rare earth-activated planar waveguides with competitive optical and spectroscopic properties, not to mention the fact that, zirconium oxide represents a qualified material for low cost production. This paper reports on the preparation and characterization of sol–gel-derived (100 − x)SiO2 − xZrO2, (x = 10, 20, 30) planar waveguides doped with 0.3 mol% Er3+ ions. The optical and spectroscopic properties of these materials are discussed in terms of the zirconium oxide content.
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2. Experimental details The sol precursor was obtained by mixing tetraethylorthosilicate (TEOS), ethanol (EtOH) and deionized water, having hydrochloric acid as catalyst, and the resulting mixture was prehydrolyzed for 1 h at 65 °C. The TEOS:HCl:EtOH:H2O molar ratio was 1:0.01:37.9:2. An ethanolic colloidal suspension was prepared using ZrOCl2·8H2O as a precursor. This suspension, with zirconium concentrations varying of 0.090, 0.179, and 0.269 mol L− 1, was then added to the TEOS solution resulting in a Si/Zr molar ratio of 70/30, 80/20 or 90/10 (sol concentration was 0.448 mol L− 1). Erbium was added as an ErCl3 aqueous solution, at an Er/(Si + Zr) molar concentration of 0.3 mol%. The sol was kept under stirring at room temperature for 16 h. Multilayers were deposited on clean pure fused quartz substrates by dip-coating, at a dipping rate of 40 mm/min. Before further coating, each layer was annealed in air at 900 °C for 50 s. After a 10 dipping cycle, the film was heated at 900 °C for 2 min. Film densification was achieved by a final heat treatment performed in air at 900 °C, where the annealing time was chosen as a function of the waveguide composition (see Table 1). The waveguide thickness and the refractive indices at 632.8, 543.5, 1319 and 1542 nm were measured by an m-line apparatus (Metricon model 2010) based on the prism coupling technique [12]. To assess the planar waveguide propagation losses, light power scattered out of the waveguide plane (which is proportional to the guided power) was recorded by a scanning optical fiber probe moving along the length of the propagating light streak. The attenuation coefficient was obtained by fitting the data to an exponentially decaying function, assuming a homogeneous longitudinal distribution of the scattering centers and a constant loss parameter of the planar waveguide. In the case of multimode waveguides, in-plane diffraction effects were ignored, together with those of any existing mode coupling. The measurements were performed by excitation of the planar waveguide lowest order Transverse Electric mode (TE0) at 632.8 and at 1319 nm. The TE0 mode waveguiding excitation was used for photoluminescence measurements. The visible photoluminescence, obtained upon excitation at 514.5 nm, was selected by a double monochromator and analyzed by a photon counting system. Photoluminescence measurements in the region of the 4 I13/2 → 4I15/2 transition were obtained using the 980 nm line of a titanium sapphire laser as the excitation source. The luminescence light was dispersed by a 320 nm single grating monochromator with a 2 nm resolution. The light was detected using an InGaAs photodiode and a lock-in technique. Decay Table 1 Nominal composition and fabrication protocol parameters for the silica–zirconia planar waveguides Waveguide labelling
Nominal composition (100 − x)SiO2 − xZrO2
Number of layers
Time of the final heat treatment at 900 °C (min)
SZ9/1–30 SZ8/2–28 SZ7/3–25
90SiO2–10ZrO2 80SiO2–20ZrO2 70SiO2–30ZrO2
30 28 25
540 60 5
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Table 2 Optical parameters measured at 632.8 nm, 514.5 nm, and 1550 nm (TE0 mode) for the Erbium doped SiO2–ZrO2 planar waveguides Waveguide
SZ9/1–30
SZ8/2–28
SZ7/3–25
Thickness (±0.05 μm) Total heat treatment time (min) Refractive index at 632.8 nm (±0.005) Refractive index at 514.5 nm (±0.005) Refractive index at 1319 nm (±0.005) Refractive index at 1550 nm (±0.005) Number of modes at 632.8 nm Number of modes at 514.5 nm Number of modes at 1319 nm Number of modes at 1550 nm Attenuation coefficient at 632.8 nm (± 0.3 dB/cm) Attenuation coefficient at 1.3 μm (± 0. 3 dB/cm)
1.11 540 1.508 1.512 1.495 1.492 1 2 1 1 0.6
0.78 60 1.562 1.566 1.546 1.544 2 2 1 1 0.7
0.90 5 1.628 1.634 1.612 1.609 2 3 1 1 1.5
0.4
0.2
–
curves were obtained by chopping the excitation beam with a mechanical chopper and recording the signal with a digital oscilloscope. All the measurements were performed at room temperature. 3. Results and discussion Homogeneous and crack free films were obtained for all the compositions. Films did not exhibit any texture under optical microscope inspection and were transparent in the visible region. Thicknesses ranging from 0.8 to 1.1 μm were measured for the planar waveguides (see Table 2). The film thickness was adjusted according to the composition and refractive index, so that it would be enough to support one mode at 1.5 μm. All the planar waveguides displayed one propagating mode at 1542 and 1319 nm, and 2 or 3 propagating modes at 632.8 and 543.5 nm. The refractive index increased as a function of the zirconium oxide content. Values ranging from 1.508 up to 1.628 measured at 632.8 nm in the TE polarization are summarized in Table 2 for the specific final annealing suitable for full film densification. The propagation losses of the films were low enough to consider this material suitable for optical planar waveguide application. The refractive index of the waveguides was compared with those calculated at 633 nm by the Lorentz–Lorenz equation using n = 1.4575 (measured) and n = 2.13 [18] for the refractive indices of SiO2 and ZrO2, respectively (see Fig. 1). The measured refractive index for the SZ9/1–30 (annealed for 540 min), SZ8/2–28 (annealed for 60 min), and SZ7/3–25 (annealed for 5 min) planar waveguides agreed with the calculated values, suggesting that a high densification degree was achieved for all the compositions. It is worthy to note that the waveguides still had the same refractive index one month after fabrication. Comparing the heat treatments to which the samples with different ZrO2 contents were submitted (see Table 2), the introduction of zirconium dioxide decreased the total annealing
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Fig. 1. Calculated refractive index values obtained by Lorentz–Lorenz equation (at 632.8 nm solid line; and 1550 nm dotted line), as a function of the ZrO2 molar fraction; and the measured refractive index values for all the waveguide compositions (( ) at 632.8 nm and (●) at 1550 nm).
Fig. 3. Photoluminescence spectra in the visible range of the Er3+-activated planar waveguide SZ8/2–28, annealed as reported in Table 2. Excitation was performed at 514.5 nm, in the TE0 mode.
time necessary for full densification to be achieved in air at 900 °C from 540 to 5 min, ongoing from a ZrO2 concentration of 10 mol% to a ZrO2 concentration of 30 mol%. Fig. 2 shows the photoluminescence spectra of the Er3+doped planar waveguides, annealed as reported in Table 2, under excitation at 980 nm. Luminescence from the excited 4I13/2 state was observed with a broadened band with maximum at about 1531 nm, and the spectral bandwidth, measured at 3 dB from the intensity maximum, was 27 ± 1 nm for all the samples. The Stark structures at 1462, 1511, 1573, and 1611 nm were well defined. In comparison with others similar sol–gel based materials, a decrease in the emission bandwidth from 48 to 27 nm was observed [12,13]. Fig. 3 shows the visible region luminescence spectra of the planar waveguide SZ8/2–28, under excitation at 514.5 nm. For the SZ9/1–30 and SZ7/3–25 waveguides, similar spectra were obtained. The luminescence bands, with maxima centered around 525 nm and 517 nm, were assigned to the 2H11/2 → 4I15/2
transition. The bands with maxima located around 545 nm and 562 nm were assigned to the 4S3/2 → 4I15/2 transition. The presence of narrow structures and the important reduction in the inhomogeneous line width in the luminescence spectra indicate a crystalline local environment for the Er3+ ion [8,17,19]. The narrowing is more evident in the visible photoluminescence spectra than in the 1.5 μm one. In fact, the bandwidth reduction effect for the 4I13/2 → 4I15/2 transition is limited by the accidental coincidence between the numerous Stark components of the initial and final multiplets. So the 4S3/2 → 4I15/2 transition, which involves one small J number multiplet, is a better probe to evidence and estimate the line narrowing [19]. Moreover, the 2 H11/2 → 4I15/2 transition is hypersensitive and therefore particularly suitable to detect the change in the environment immediately around the Er3+ ion [19]. The crystal-field strength experienced by the optically active ions directly reflects the maximum energy splitting of the Stark components of the electronic level. So the presence of nanocrystals is the factor responsible for the reduction
Fig. 2. Photoluminescence spectra relative to the 4I13/2 → 4I15/2 transition of the Er3+ ions in the planar waveguides (A) SZ9/1–30, (B) SZ8/2–28, and (C) SZ7/ 3–25, annealed as reported in Table 2. Excitation was performed at 980 nm, in the TE0 mode.
Fig. 4. Decay curve of the luminescence from the 4I13//2 metastable state of Er3+ ions in the SZ8/2–28 planar waveguide, upon excitation at 980 nm.
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in the inhomogeneous linewidth, as observed in the present case [19]. The 4I13/2 level decay curves presented a single-exponential profile for all the samples, with a lifetime of 9.0 ms for the samples with 10 mol% ZrO2 and 8.0 ms for the waveguides containing 20 and 30 mol% ZrO2. As an example, Fig. 4 shows the 4I13/2 level decay curve for the waveguides containing 20 mol% ZrO2. Any comment regarding the relaxation dynamics is insignificant at this stage of the research, specially because the nanocrystal volume fraction in the different samples is unknown. Further structural and spectroscopic measurements as a function of the erbium content and thermal annealing are in progress to obtain detailed information about the crystallization of the silica–zirconia system and about the possibility of developing transparent glass ceramic planar waveguides. 4. Conclusions Erbium-activated (100 − x)SiO2 − xZrO2 planar waveguides were prepared using the sol–gel methodology. Multilayer films were optimized so that they would confine one propagating mode at 1542 nm. Emission in the C-telecom band was observed for the planar waveguides. The 4I13/2 level decay curves presented a single-exponential profile, with a lifetime of 9.0 ms for the samples with 10 mol% ZrO2 and 8.0 ms for the waveguides containing 20 and 30 mol% ZrO2. High stability in terms of optical and structural properties was observed for all the compositions. The optical parameters as well as the spectral features were found to be dependent on the zirconium content. The measured refractive indices were compatible with a fully densified system, and the introduction of zirconium dioxide decreased the annealing time necessary for full densification to be achieved. The observation of narrow structures and the important reduction in the inhomogeneous line width in the luminescence spectra indicate that a crystalline local environment for the Er3+ ion is present in the SiO2–ZrO2 waveguides. Further structural and spectroscopic measurements as a function of the erbium content and thermal annealing are in progress, to obtain detailed information about the crystallization of the silica–zirconia system and about the possibility for developing transparent glass ceramic planar waveguides.
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Acknowledgements This research was partially supported by the Short Term Mobility Program CNR 2005 and by the PAT 2004–2006 FAPVU project. Gonçalves, RR acknowledges FAPESP and CNPq for financial support. The invaluable technical support of Enrico Moser is greatly acknowledged. References [1] A.J. Kenyon, Prog. Quantum Electron. 26 (2002) 225. [2] X. Orignac, D. Barbier, X.M. Du, R.M. Almeida, O. McCarty, E. Yeatman, Opt. Mater. 12 (1999) 1. [3] W. Huang, R.R. Syms, E.M. Yeatman, M.M. Ahmad, T.V. Clapp, S.M. Ojha, IEEE Photonics Technol. Lett. 14 (2002) 959. [4] A. Polman, SPIE 3942 (2000) 2. [5] S. Sakka (Ed.), Handbook of Sol–Gel Science and Technology Processing, Characterization and Applications, Kluwer Acad. Publ., 2005 [6] R.M. Almeida, A.C. Marques, S. Portal, Opt. Mater. 27 (2005) 1718. [7] M. Montagna, A. Chiasera, E. Moser, F. Visintainer, M. Ferrari, L. Zampedri, R.R. Gonçalves, S.J.L. Ribeiro, A. Martucci, M. Guglielmi, M. Ivanda, R.M. Almeida, Mat. Sci. Forum 455–456 (2004) 520. [8] C. Strohhöfer, J. Fick, H.C. Vasconcelos, R.M. Almeida, J. Non-Cryst. Solids 226 (1998) 182. [9] E.M. Yeatman, M.M. Ahmad, O. McCarthy, A. Martucci, M. Guglielmi, J. Sol–Gel Sci. Technol. 19 (2000) 231. [10] M.A. Forastiere, S. Pelli, G.C. Righini, M. Guglielmi, M.M. Ahmad, O. McCarthy, E. Yeatman, A. Vannucci, Fiber Integr. Opt. 20 (2001) 29. [11] E.M. Yeatman, M.M. Ahmad, O. McCarthy, A. Vannucci, P. Gastaldo, D. Barbier, D. Mongardien, C. Moronvalle, Opt. Commun. 164 (1999) 19. [12] R.R. Goncalves, G. Carturan, L. Zampedri, M. Ferrari, M. Montagna, A. Chiasera, G.C. Righini, S. Pelli, S.J.L. Ribeiro, Y. Messaddeq, Appl. Phys. Lett. 81 (2002) 28. [13] R.R. Gonçalves, G. Carturan, M. Montagna, M. Ferrari, L. Zampedri, S. Pelli, G.C. Righini, S.J.L. Ribeiro, Y. Messadeq, Opt. Mater. 25 (2004) 131. [14] L. Zampedri, G.C. Righini, H. Portales, S. Pelli, G. Nunzi Conti, M. Montagna, M. Mattarelli, R.R. Goncalves, M. Ferrari, A. Chiasera, M. Bouazaoui, C. Armellini, J. Non-Cryst. Solids 345&346 (2004) 580. [15] M. Crisan, M. Gartner, L. Predona, R. Scurtus, M. Zaharescu, J. Sol–Gel Sci. Technol. 32 (2004) 167. [16] G. Ehrhart, B. Capoen, O. Robbe, P. Boy, S. Turrell, M. Bouazaoui, Thin Solid Films 496 (2006) 227. [17] C. Urlacher, C. Marco De Lucas, J. Mugnier, Synth. Met. 90 (1997) 199. [18] M. Jerman, Z. Qiao, D. Mergel, Appl. Opt. 44 (2005) 3006. [19] M. Mortier, Philos. Mag., B 82 (2002) 745.
