P-14-043
Proceedings of the XX ICG in Kyoto, Sep.27th-Oct.1st(2004)
ERBIUM-ACTIVATED SOL GEL - DERIVED SILICA-HAFNIA PLANAR WAVEGUIDES Rogeria R. Gonçalves, Sidney J.L. Ribeiro & Younes Messaddeq Institute of Chemistry-UNESP, P.O. Box 355, 14801-970 Araraquara, SP, Brazil.
Alessandro Martucci* INSTM, Dipartimento Ingegneria Meccanica S. Materiali, Università di Padova, via Marzolo 9, I-35131 Padova, Italy.
[email protected]
Stefano Pelli, Gualtiero Nunzi Conti & Giancarlo C. Righini CNR-IFAC Istituto di Fisica Applicata, “Nello Carrara”, Optoelectronics and Photonics Department, via Panciatichi 64, I-50127 Firenze, Italy.
Vittorio Foglietti & Antonio Minotti CNR-IFN, Istituto di Fotonica e Nanotecnologie, MEMS group, Via Cineto Romano 42, I-00156 Roma, Italy.
Khiem Tran Ngoc & Luca Zampedri Dipartimento di Ingegneria dei Materiali, Università di Trento, via Mesiano 77, I-38050 Trento, Italy.
Andrea Chiappini, Maurizio Mattarelli, Maurizio Montagna, Enrico Moser, Hervé Portales & Cristiana Tosello Dipartimento di Fisica and INFM, CSMFO group, Università di Trento, via Sommarive 14, I-38050 Povo-Trento, Italy.
Cristina Armellini, Alessandro Chiasera, Maurizio Ferrari & Yoann Jestin CNR-IFN Istituto di Fotonica e Nanotecnologie, CSMFO group, via Sommarive 14, I-38050 Povo-Trento, Italy.
Sol gel – derived SiO2-HfO2 planar waveguides of different molar composition and activated by different content of Er3+ ions have been prepared by dip-coating technique. In order to assess the system, its spectroscopic, optical, and structural properties have been investigated as a function of both the HfO2/ SiO2 molar ratio and the erbium content by using photoluminescence, m-line, and Raman spectroscopy. Particular attention has been devoted to the study of upconversion processes upon 980 nm excitation as well as to the evaluation of the luminescence quenching concentration. Finally, an example of channel waveguides on silicaon-silicon substrates is reported. (Key words: sol-gel method, planar waveguides, luminescence, upconversion, EDWA) 1. Introduction
During the last years, the spreading of optical networks toward the final customers has triggered the research in innovative materials and processes for development of compact and integrated optical devices [1-4]. A growing activity in this field is aimed at developing optical amplifiers in planar format based on rare-earth-activated glasses, the so called Erbium-Doped Waveguide Amplifiers (EDWAs), mainly because they can compensate for signal attenuation resulting from insertion losses of branching or Wavelenght Division Multiplexing (WDM) components [5,6]. Among several techniques the sol-gel process has emerged as one of the cheapest and most versatile routes for the fabrication of SiO2-based planar waveguides with valuable prospects for EDWAs fabrication [7-10]. The SiO2-TiO2 binary system has been extensively used to produce Er-activated planar waveguides by sol-gel route [6,7,11]. However, in the silica-titania system, there is a tendency for separation between
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P-14-043
Proceedings of the XX ICG in Kyoto, Sep.27th-Oct.1st(2004)
SiO2-rich and TiO2-rich phases. Undesirable phase precipitation can take place as a consequence of the thermal treatment, necessary to achieve full densification of the film, or during the reflow process [12,13]. In previous papers we showed that SiO2-HfO2 is an innovative binary system which presents valuable optical, spectroscopic and structural features for successful applications in the telecommunication field. In particular crystallization effect can be strongly reduced following suitable preparation protocol [9,14-16]. This paper reports on the preparation and the characterization of sol gel-derived (100-x)SiO2 - xHfO2 , (x = 10, 20, 30, 40) planar waveguides activated by Er3+ ions. The spectroscopic results are discussed, with the aim of assessing the role of hafnia concentration on the structural and optical properties of these waveguides and improving their spectroscopic features for successful Er-doped waveguide amplifier fabrication.
2. Experimental
Two different sets of waveguides were doped with different erbium content: 0.01 and 0.3 mol%. Each series was constituted by four waveguides with different Si/Hf molar ratios: 60/40, 70/30, 80/20, 90/10. The starting solution obtained by mixing tetraethylorthosilicate (TEOS), ethanol, deionised water and hydrochloric acid as a catalyst, was pre-hydrolysed for 1 hour at 65 °C. The molar ratio of TEOS : HCl : H2O was 1: 0.01 : 2 . The quantity of EtOH was chosen in order to keep constant the 20 cc volume of the solution. Ethanolic colloidal suspension was prepared using as a precursor HfOCl2 and then added to the TEOS solutions, in such a way to obtain the solutions with the different Si/Hf molar ratios. The quantity of ethanol was adjusted for each solution in order to obtain a final total concentration of 0.448 mol/l. Erbium was introduced as Er(NO3)3 ⋅ 5H2O with an Er/(Si+Hf) molar concentration of each set equal to 0.01 and 0.3 mol%, respectively. The final mixture was let to react under stirring for 16 h at room temperature. Erbium-doped silica-hafnia films were deposited on cleaned pure silica substrates by dip-coating, with a dipping rate of 40 mm/min. Before further coating, each layer was annealed in air for 50 s at 900 °C. After a 10-dip cycle, the film was heated for 2 min at 900 °C. Finally, the waveguides were subject to a further annealing at 900 °C, whose duration was different for each waveguide. Table I gives compositional, optical and deposition parameters of the Er3+-activated silica-hafnia planar waveguides. Waveguide label W100.3 W200.3 W300.3 W400.3 W100.01 W200.01 W300.01 W400.01 SiO2 : HfO2 molar ratio 90 : 10 80 : 20 70 : 30 60 : 40 90 : 10 80 : 20 70 : 30 60 : 40 0.3 0.3 0.3 0.01 0.01 0.01 0.01 Er3+ mol% 0.3 Refractive index 1.543 1.600 1.663 1.494 1.562 1.605 1.664 @ 632.8 nm (± 0.005) (TE) 1.470 1.537 1.588 1.646 1.494 1.562 1.601 1.655 (TM) 1.468 1.547 1.605 1.669 1.497 1.566 1.611 1.670 @ 543.5 nm (± 0.005) (TE) 1.474 1.472 1.541 1.593 1.652 1.496 1.565 1.605 1.660 (TM) 2.05 1.27 1.10 0.96 1.16 0.69 0.69 0.53 Thickness (± 0.05 µm) 30 30 25 35 20 20 15 Number of dips 50 210' 5' 5' 30 h 210’ 5’ 5’ Final heat treatment at 900°C 30 h Table I. Compositional, optical and preparation parameters of the Er3+-activated (100-x) SiO2 – xHfO2 planar waveguides.
The thickness of the waveguides and their refractive indexes n at 543.5 and 632.8 nm were measured by an m-line apparatus based on prism coupling technique. The TE0 mode waveguiding excitation was used for Raman, photoluminescence (PL), and upconversion measurements, by detecting the light coming-out from the waveguide surface. PL measurements in the region of the 4I13/2 → 4I15/2 transition and decay curves from the 4I13/2 level were obtained with the experimental set-up described in previous paper [9], by using the 980 nm line of a Ti:Sapphire laser and the 514.5 nm line of an Argon laser as excitation sources. All measurements were performed at room temperature.
3. Results
Figure 1 shows the Raman spectra of the set of (100-x) SiO2 - x HfO2 planar waveguides: (a) W400.3, (b) W300.3, (c) W200.3, and (d) W100.3. The Raman spectrum of SiO2 (Fig.1(e)) is also reported for
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Proceedings of the XX ICG in Kyoto, Sep.27th-Oct.1st(2004)
comparison. The Raman spectra of the waveguides doped with 0.01 mol% of Er3+are not reported here, since they do not present any significant difference with respect to those shown in Fig.1. 8
Luminescence lifetime [ms]
Intensity [arb. units]
x 100 (a) (b) (c) (d)
(e) -400
0
400 800 12001600
3000
6
4
2
0
3500
0.01
0.1
1
3+
-1
Er concentration [mol %]
Raman shift [cm ] Figure 1: Raman spectra of (a) W400.3, (b) W300.3, (c) W200.3, and (d) W100.3 planar waveguides, collected in VV polarization with excitation of the TE0 mode. The Raman spectrum of the v-SiO2 (e) is also reported for comparison.
Figure 2. Luminescence lifetime of the 4I13/2 metastable state of the Er3+ ions as a function of Er3+ concentration for the 70SiO2-30HfO2 planar waveguides doped with 0.01, 0.03, 0.1, 0.3, 0.5, 1, 2, and 4 mol %. The black line represents the result of the fit of the data to concentration quenching empirical formula (see text).
Table II reports the spectroscopic parameters of the 4I13/2 → 4I15/2 transition of the Er 3+ -activated (100x) SiO2 – x HfO2 planar waveguides. All the PL spectra exhibit a main emission peak at 1.53 µm and a spectral width of about 50 nm measured at 3dB from the maximum of the intensity. The similarity of PL spectra for all the waveguides seems to indicate that a small inhomogeneous broadening is present independently from the HfO2 content. All the decay curves of the 4I13/2 metastable state of the Er3+ ions present a single exponential profile shape. Waveguide label W100.3 W200.3 W300.3 W400.3 W100.01 W200.01 W300.01 W400.01 6.5 6.0 5.8 8.5 7.5 7.0 6.6 I13/2 lifetime (± 0.5 ms) 7.1 4 4 51 51 51 50 50 50 50 49 I13/2 → I15/2 Bandwidth (± 2 nm) 3+ 4 4 Table II. Spectroscopic parameters of the I13/2 → I15/2 transition of the Er -activated (100-x) SiO2 – x HfO2 planar waveguides. 4
As a consequence of the electric dipole-dipole interactions between the different Erbium ions, a decrease of the luminescence lifetime of the metastable 4I13/2 state as a function of the increasing Er3+ concentration occurs as described in the following empirical formula: τobs = τ0/[1+(r/Q)p] where τobs is the observed luminescence lifetime, τ0 the ideal luminescence lifetime in the limit of zero concentration, r is the Er3+ ion concentration, Q is the quenching concentration and p a phenomenological parameter characterizing the steepness of the corresponding quenching curve. The concentration quenching was investigated analyzing the luminescence lifetime of the 4I13/2 metastable state of the Er3+ ions as a function of Er3+ concentration for 70SiO2-30HfO2 planar waveguides doped with 0.01, 0.03, 0.1, 0.3, 0.5, 1, 2, and 4 mol %. The data are reported in Fig. 2. The observed luminescence lifetime decreases with increasing the ions concentration. By fitting the experimental data, the following parameters were obtained: τ0 = 6.8 ms, Q = 0.81 mol%, and p = 1.3.
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P-14-043
Proceedings of the XX ICG in Kyoto, Sep.27th-Oct.1st(2004)
10 4
4
Intensity [arb. units]
S3/2
x2 5
2
x 200 H11/2 2
4
I15/2
F9/2
H9/2
0 14000 16000 18000 20000 22000 24000 26000 -1
Wavenumber [cm ] Figure 3. Upconversion emission spectra after CW excitation at 980 nm, with a power of 400mW, in the 70SiO230HfO2 planar waveguides doped with 0.3 mol % Er3+. The 2H9/2→4I15/2 and 4F9/2→4I15/2 transitions bands have been magnified respectively by a factor 200 and 2.
Figure 3 shows the room temperature anti-Stokes luminescence spectra of the 70SiO2-30HfO2 waveguide doped with 0.3 mol% Er3+ obtained upon CW excitation at 980 nm for an excitation power of 400 mW. The spectra show three main groups of SiO2 substrate bands. The band located at 19120 cm-1 is assigned to the 2H11/2→4I15/2 transition and the band at 18300 cm-1 with a shoulder at 18030 cm-1 is assigned to the 4S3/2 (A) →4I15/2 transition. The band at around 15150 cm-1 is due to the 4F9/2→4I15/2 transition. The emission band centered at 24500 cm-1, which is detected for excitation power > 260 mW, is ascribed to the 2 H9/2→4I15/2 transition [15]. A 70SiO2-30HfO2 planar waveguide activated by 0.3 mol% of Er3+, deposited on Silica-on-Silicon (SOS) substrate, was prepared. This composition was (B) chosen in order to have the best compromise Figure 4. (A) Schematic cross section of the rib between signal intensity, lifetimes and optical silica-hafnia waveguide. The dashed line represent properties such as thickness and refractive index. the 2/3 mode intensity contour. (B) SEM The waveguide having thickness of 1.0 µm, and a microphoto of a channel waveguide obtained by a refractive index of 1.60 at 632.8 nm, supports a well dry-etching process on the previously described confined single propagation mode at 1.5 µm planar waveguides. (confinement coefficient of 0.81). An attenuation coefficient of 0.8 dB/cm at 632.8 nm was measured. The lifetime of the 4I13/2 metastable state of the Er3+ ions was 5.9 ± 0.5 ms. Starting from this planar sample, channel waveguides were prepared by using a dry-etching process on the active layer. The geometrical parameters of the channel waveguides have been tailored in order to maintain the single mode propagation at 1.5 µm. A computer code based on the Wave-Matching Method [17] was used to simulate the behavior of the channels waveguides (Figure 4(A)). Figure 4(B) shows a SEM microphoto of a channel waveguide obtained by a dryetching process on the 70SiO2-30HfO2 planar waveguide activated by 0.3 mol% of Er3+, deposited on SOS. Active film
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Proceedings of the XX ICG in Kyoto, Sep.27th-Oct.1st(2004)
4. Discussion
A detailed discussion about the Raman spectra of these waveguides is given in refs. [9,14,16]. It is worthwhile to compare the Raman spectra of the (100-x) SiO2 - x HfO2 planar waveguides with that of fused silica. It appears evident that the introduction of hafnium oxide promotes a strong modification of the silica structure. Although the disruption-role of the hafnia on the silica structure becomes more evident with the increase of the Hf/Si ratio, it is effective even at HfO2 content as low as 10 mol%. A complete densification is achieved for all waveguides. In fact, the bands characteristic of the OHgroups in silicate glasses, generally observable at 3670 and 3750 cm-1 [14], are absent in the region of the Raman spectra between 2800 and 3800 cm-1, shown with a magnification of 100. Comparing the different heat treatments of the samples (Table I), it appears evident that the introduction of hafnium dioxide favors the densification of the glasses, decreasing the annealing time necessary to eliminate the residual OH- group still present in the as-deposited waveguides. At very low Er3+ concentration, the energy transfer processes should be very improbable. Moreover, considering that in completely densified silicate glasses, the multiphonon decay does not affect significantly the 4I13/2 level lifetimes [18], we can assume that the lifetimes measured a 0.01 mol%, is very close to the radiative one. Under this hypothesis, we can estimate that a radiative quantum efficiency of about 85% is obtained in all the samples doped with 0.3 mol% of erbium. Upconversion process, reducing the degree of inversion population of the 4I13/2 state at a given pump power, limits the performance of optical amplification at 1.5 µm. Upconversion can involve a singleion process, the so called excited state absorption (ESA) and multi-ions cooperative mechanism indicated as energy transfer upconversion (ETU) [15,19].The upconversion intensity I, was measured as a function of the incident pump power P, and the experimental data were fitted to a power law, I∝Pn. The following values of the gradient n were obtained: 1.9 ± 0.1 for the 4S3/2 →4I15/2 transition; 1.5 ± 0.1 for the 4F9/2→4I15/2 transition and 2.9 ± 0.1 for the 2H9/2→4I15/2 transition. The n values indicate that two photons are involved in the infrared-to-green and three photons are involved in the infrared-to-blue upconversion processes. Regarding the red emission at 15150 cm-1 the gradient n = 1.5 can be explained by a process involving three absorbed photons which gives two emitted photons [15]. In conclusion, a fabrication protocol of sol-gel derived silica-hafnia planar waveguides has been developed. The 70SiO2-30HfO2 planar waveguide activated by 0.3 mol% of Er3+ exhibits optical, spectroscopic and structural properties which appear fully suitable for the development of integrated optical amplifiers to be used in optical telecommunication systems.
5. Acknowledgments
Authors acknowledge the financial support of MIUR-FIRB “Miniaturized systems for electronics and photonics”(RBNE012N3X-005), PAT 2004-2006 FAPVU “Fabrication of ultratransparent glass ceramics-based planar optical amplifiers”, MIUR-COFIN 2002 “Preparation of hybrid organicinorganic materials by assembling of nanostructured molecular units”, and MIUR-COFIN 2002 “Nanostructured materials for integrated optics”.
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Proceedings of the XX ICG in Kyoto, Sep.27th-Oct.1st(2004)
[9] Gonçalves R.R., Carturan G., Zampedri L., Ferrari M., Armellini C., Chiasera A., Mattarelli M., Moser E., Montagna M., Righini G.C., Pelli S., Nunzi Conti G., Ribeiro S.J.L., Messaddeq Y., Minotti A., Foglietti V., and Portales H., Proc. SPIE, Vol.4990, 111-120(2003). [10] Almeida R.M., Morais P.J., and Marques A.C., Phil. Mag. B., Vol.82, 707-19(2002). [11] Orignac X., Barbier D., Du X.M., Almeida R.M., McCarty O., and Yeatman E., Opt. Mater., Vol.12, 1-18(1999). [12] Almeida R.M., Marques A.C., Pelli S., Righini G.C., Chiasera A., Mattarelli M., Montagna M., Tosello C., Gonçalves R.R., Portales H., Chaussedent S., Ferrari M., and Zampedri L., Phil. Mag., Vol.84, 1659-66(2004). [13] Montagna M., Moser E., Visintainer F., Ferrari M., Zampedri L., Martucci A., Guglielmi M., and Ivanda M., J. Sol-Gel Sci. Technol., Vol.26, 241-4(2003). [14] Gonçalves R.R., Carturan G., Zampedri L., Ferrari M., Montagna M., Chiasera A., Righini G.C., Pelli S., Ribeiro S.J.L., and Messaddeq Y., Appl. Phys. Lett., Vol.81, 28-30(2002). [15] Gonçalves R.R., Carturan G., Zampedri L., Ferrari M., Chiasera A., Montagna M., Righini G.C., Pelli S., Ribeiro S.J.L., and Messaddeq Y., J. Non-Cryst. Solids, Vol.322, 306-10(2003). [16] Gonçalves R.R., Carturan G., Montagna M., Ferrari M., Zampedri L., Pelli S., Righini G.C., Ribeiro S.J.L., and Messaddeq Y., Opt. Mat., Vol.25, 131-39(2004). [17] Lohmeyer M., “Wave-matching method for mode analysis of dielectric waveguides”, http://www.physik.uni-osnabrueck.de/theophys/, 1999. [18] Layne C.B., Lowdermilk W.H., and Weber M.J., Phys. Rev. B., Vol.16, 10-20(1977). [19] Joubert M.F., Opt. Mat., Vol.11, 181-203(1999).
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