APPLIED PHYSICS LETTERS 91, 071909 共2007兲
Low-loss optical Er3+-activated glass-ceramics planar waveguides fabricated by bottom-up approach Y. Jestin,a兲 C. Armellini, A. Chiasera, A. Chiappini, and M. Ferrari CNR-IFN, Istituto di Fotonica e Nanotecnologie, CSMFO group, via Sommarive 14, 38050 Povo, Trento, Italy
E. Moser Dipartimento di Fisica, Università di Trento, CSMFO group, via Sommarive 14, 38050 Povo, Trento, Italy
R. Retoux Laboratoire CRISMAT, UMR 6508, ENSICAEN, 6 Bld. Maréchal Juin, 14050 Caen, France
G. C. Righini CNR, Department of Materials and Devices, via dei Taurini 19, 00185 Roma, Italy
共Received 17 May 2007; accepted 24 July 2007; published online 14 August 2007兲 A low-loss optical erbium activated silica-hafnia glass-ceramic planar waveguide was fabricated using a bottom-up approach. Hafnia nanoparticles were first prepared by colloidal route and then mixed in a silica-hafnia:Er3+ sol. The resulting sol was deposited on SiO2 substrate. Analysis of the photoluminescence has demonstrated that erbium ions are embedded in a crystalline phase. A lifetime enhancement of the 4I13/2 metastable level was observed. Losses measurements at 1542 nm highlight a very low attenuation coefficient 共0.3 dB/ cm兲 making this nanostructured material suitable for single band waveguide amplifier in the C band of telecommunications. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2771537兴 The development of materials with advanced properties is critical to the implementation of optical networks. In optical fiber technology, erbium doped fiber amplifiers are now used in long distance communication fiber links.1 They use the optical transition of erbium ions at a wavelength of 1.55 m for signal amplification. Using the same concept of erbium doping, planar waveguide amplifiers are now being developed.2 At first sight, it may seem straightforward to translate the concept of a fiber amplifier to that of an integrated planar waveguide. However, in scaling down the device dimensions from the long fiber length 共typically 40 m兲 to the small device dimensions of an optoelectronic integrated circuit, the erbium concentration has to be increased to achieve the same optical gain. As it turns out, physical processes that were unimportant in fiber technology become important in planar amplifiers. As an example, high rare earth concentration in glasses leads to the formation of chemical clusters and interaction of clusters which reduce the efficiency of the amplifier due to energy transfer coming from radiative and nonradiative processes.3–6 Thus, glass-ceramic materials may be a valid alternative method to control chemical parameters of the rare earth, and thus may avoid undesirable effect such as clustering. Erbium containing nanocrystallites in a glassy matrix leads to important advantages: 共i兲 a higher optical cross sections of the erbium transitions, 共ii兲 a minimum phonon energies which strongly reduce nonradiative multiphonon relaxation, and 共iii兲 combination of the mechanical and optical properties of the glass with a crystal-like environment for the rare earth ions, where their higher cross sections can be exploited in order to fabricate more compact devices. From a structural point of view, the crystallites will have to be small enough to avoid scattering losses 共a few nanoma兲
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eters兲. In fact, the production of rare earth activated nanocrystallites in a glass matrix permits one to obtain nanocomponent with high fluorescence efficiency,7 thus demonstrating the great importance of this class of materials in photonic. Recently, we have shown that SiO2 – HfO2 : Er3+ glassceramic planar waveguides prepared by sol-gel route present valuable optical, spectroscopic, and structural features for applications in the telecommunications field.8,9 In this letter, we propose an erbium doped silica-hafnia glass-ceramic composite prepared by using a bottom-up approach with low losses for optical planar waveguide amplifier applications. Among the many fabrication techniques, the sol-gel process, which is based on wet chemistry process, is a very interesting one. This low cost technique allows the deposition of high optical quality films of numerous compositions. In this work, erbium doped glass-ceramics silicahafnia thin films were prepared by the sol-gel technique after incorporation of nanocrystals in the sol, permitting in this case to have a better control on the design of the glass ceramic with respect to a top-down method, as previously reported in a recent paper.8 Nanocomposite and glassy films were studied and compared from an optical and spectroscopic point of view. SiO2 – HfO2 : Er3+ glass-ceramic planar waveguides were realized by following the described protocol: 共1兲 Preparation of a colloidal suspension of HfO2 nanoparticles, starting from a HfOCl2 solution in ethanol and using a reflux technique;10 共2兲 Separation of HfO2 nanoparticles from the colloidal suspension; 共3兲 Preparation of a solution of TEOS, alcohol, de-ionized water, and hydrochloridric acid prehydrolized for 1 h at 65 ° C, in which the hafnia precursor HfOCl2 has been added in order to obtain a final solution with a molar ratio Si/ Hf= 80/ 20; 共4兲 At this solution, Er共NO3兲3 · 5H2O with a molar concentration Er/ 共Si+ Hf兲 = 1,
0003-6951/2007/91共7兲/071909/3/$23.00 91, 071909-1 © 2007 American Institute of Physics Downloaded 15 Aug 2007 to 193.205.194.2. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 3. Decay curves of the luminescence from the 4I13/2 → 4I15/2 metastable state of Er3+ ions for W1 共glass-ceramic兲 and W2 共amorphous兲 planar waveguides upon 514.5 nm excitation. FIG. 1. HRTEM image of the glass-ceramic waveguide W1 showing HfO2 nanocrystals dispersed in the amorphous matrix.
and hafnia nanoparticles has been added in order to have 2.5 mol % of nanoparticles in the solution. Nanocomposite planar waveguides were produced by dip coating the final solution on SiO2 substrate and were stabilized by a thermal treatment in air for 22 h. A high-resolution transmission electron microscopy 共HRTEM兲 image of the produced waveguide is shown in Fig. 1. Nanocrystals of about 3 – 4 nm in size are visible and homogeneously dispersed in the amorphous matrix. The energy dispersive spectroscopy analysis has confirmed that the nanocrystals are composed by hafnium oxide. Figure 2 compares the 4I13/2 → 4I15/2 photoluminescence 共PL兲 spectrum of the nanocomposite waveguide 共W1兲 activated by Er3+ ions and the PL spectrum of a silicahafnia:Er3+ waveguide without nanocrystals 共W2兲. The TE0 mode waveguiding were excited with the 514 nm line of an Ar+ ion laser. The luminescence was dispersed by a 320 mm single-grating monochromator with a resolution of 1 nm. The light was detected using an InGaAs photodiode and standard lock-in technique. Decay curves were obtained by chopping the exciting beam with a mechanical chopper and recording the signal by a digital oscilloscope. All the measurements were performed at room temperature. The details about the experimental setup were reported in a previous paper.11 The modification of the emission spectrum of W1 is attributed to the presence of hafnia nanocrystals. The ordering of the local environment limits the inhomogeneous broadening typical of glassy structural environments and, therefore, the full width at half maximum become smaller from 45 to 27 nm for W2 and W1, respectively. We can consider that the thermal treatment which does not damage the
surface of the film, promotes the migration of erbium ions toward hafnia nanocrystals.12 The lifetime of the metastable level 4I13/2 has been measured at 1532 nm and is shown in Fig. 3. The decay curves present a single exponential behavior and show that the lifetime of the emission increases for the sample W1 containing nanocrystals, from 4.5 ms for the sample W2 to 5.6 ms for the W1. It is well known that a crystalline environment around the rare earth induce a shortening in the phonon energies, as seen, for example, in oxyfluoride glass ceramics.13 For W2, the rare earth is placed in a glassy local environment of SiO2 – HfO2 with a cutoff energy around 1000 cm−1, whereas for W1 glass ceramic, the rare earth is surrounded by a hafnia nanocrystal with a cutoff frequency of about 700 cm−1.14 Indeed, the phonon energy of the surrounding environment of the rare earth ions is proportional to the nonradiation contribution of the 4I13/2 → 4I15/2 transition. So the main effect of the presence of nanocrystals is the reduction of the nonradiative process thus inducing a lengthening of the lifetime of the metastable level 4I13/2. The waveguide W1 presents excellent propagation properties at 1.5 m with losses around 0.3 dB/ cm, which are very close to those measured for the amorphous waveguide W2 that are slightly lower of 0.3 dB/ cm. We remark that the presence of nanocrystals in the glass-ceramic waveguide W1 induces negligible losses with respect to the glassy waveguide W2. It thus makes the waveguide W1 a suitable component for low losses amplifier in the C band of telecommunications.15 It is important to note how the bottom-up approach permits one to improve the optical properties of the glass-ceramic waveguide with respect to the ones obtained by the top-down approach and reported in Ref. 8 The topdown method, requiring a high thermal treatment at 1000 ° C to grow nanocrystals in the matrix, induces optical losses of 1 dB/ cm at 1542 nm, caused by a degradation of the waveguide surface. In summary, we have defined a fabrication protocol by sol-gel route of rare earth activated glass-ceramic planar waveguides. The waveguides have been realized using a bottom-up technique. Optical measurements have evidenced that glass ceramics containing nanocrystals of about 3 nm can show a low attenuation coefficient of 0.3 dB/ cm at 1542 nm. Then, we have demonstrated how the crystalline phase can enhance the spectroscopic properties of erbium ions. Such characteristics make this glass ceramics interesting for the development of erbium doped waveguides amplifiers.
FIG. 2. Room temperature luminescence spectra of the 4I13/2 → 4I15/2 transition of Er3+ ions for W1 共glass-ceramic兲 and W2 共amorphous兲 planar waveguides, obtained by exciting the TE0 mode at 514.5 nm. Downloaded 15 Aug 2007 to 193.205.194.2. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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The authors acknowledge the financial support of PAT 共2004–2006兲 FAPVU, CNR-CNRS 2005–2007, and PAT FaStFAL 2007–2009. 1
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Appl. Phys. Lett. 91, 071909 共2007兲 Boulard, A. Chiappini, A. Chiasera, G. Dalba, C. Duverger, M. Ferrari, C. E. Goyes Lopez, M. Mattarelli, M. Montagna, E. Moser, G. Nunzi Conti, S. Pelli, G. C. Righini, and F. Rocca, Proc. SPIE 6183, 438 共2006兲. 9 M. Ferrari, C. Armellini, S. Berneschi, M. Brenci, A. Chiappini, A. Chiasera, Y. Jestin, M. Mattarelli, M. Montagna, E. Moser, G. Nunzi Conti, S. Pelli, G. C. Righini, and C. Tosello, Proc. SPIE 6183, 181 共2006兲. 10 R. R. Gonçalves, G. Caturan, L. Zampedri, M. Ferrari, M. Montagna, A. Chiasera, G. C. Righini, S. Pelli, S. J. L. Ribeiro, and Y. Messaddeq, Appl. Phys. Lett. 81, 28 共2002兲. 11 Y. Jestin, C. Armellini, A. Chiappini, A. Chiasera, M. Ferrari, C. Goyes, M. Montagna, E. Moser, G. Nunzi Conti, S. Pelli, R. Retoux, G. C. Righini, and G. Speranza, J. Non-Cryst. Solids 353, 494 共2007兲. 12 W. C. Liu, D. Wu, A. D. Li, H. Q. Ling, Y. F. Tang, and N. B. Ming, Appl. Surf. Sci. 191, 181 共2002兲. 13 M. Mortier, Philos. Mag. B 82, 745 共2002兲. 14 D. A. Neumayer and E. Cartier, J. Appl. Phys. 90, 1801 共2001兲. 15 P. G. Kik and A. Polman, MRS Bull. 23, 48 共1998兲.
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