Low optical loss planar waveguides prepared in an organic–inorganic hybrid system S. J. L. Ribeiro, Y. Messaddeq, R. R. Gonçalves, M. Ferrari, M. Montagna, and M. A. Aegerter Citation: Applied Physics Letters 77, 3502 (2000); doi: 10.1063/1.1329159 View online: http://dx.doi.org/10.1063/1.1329159 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/77/22?ver=pdfcov Published by the AIP Publishing This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 186.217.234.225 On: Tue, 14 Jan 2014 12:32:50 http://scitation.aip.org/content/aip/journal/apl?ver=pdfcov http://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/2079205716/x01/AIP-PT/APL_ArticleDL_1213/aipToCAlerts_Large.png/5532386d4f314a53757a6b4144615953?x http://scitation.aip.org/search?value1=S.+J.+L.+Ribeiro&option1=author http://scitation.aip.org/search?value1=Y.+Messaddeq&option1=author http://scitation.aip.org/search?value1=R.+R.+Gon�alves&option1=author http://scitation.aip.org/search?value1=M.+Ferrari&option1=author http://scitation.aip.org/search?value1=M.+Montagna&option1=author http://scitation.aip.org/search?value1=M.+A.+Aegerter&option1=author http://scitation.aip.org/content/aip/journal/apl?ver=pdfcov http://dx.doi.org/10.1063/1.1329159 http://scitation.aip.org/content/aip/journal/apl/77/22?ver=pdfcov http://scitation.aip.org/content/aip?ver=pdfcov APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 22 27 NOVEMBER 2000 This a Low optical loss planar waveguides prepared in an organic–inorganic hybrid system S. J. L. Ribeiro, Y. Messaddeq, and R. R. Gonçalves Institute of Chemistry-UNESP, P.O. Box 355, 14801-970 Araraquara, SP, Brazil M. Ferrari CNR-CeFSA, Centro Fisica Stati Aggregati, via Sommarive 14, 38050 Povo-Trento, Italy M. Montagna Dipartimento di Fisica and INFM, Universita` di Trento, via Sommarive 14, 38050 Povo-Trento, Italy M. A. Aegerter Institut für Neue Materialien, GmbH, Saarbrucken, Germany ~Received 12 June 2000; accepted for publication 29 September 2000! HfO2-~3-glycidoxipropil!trimethoxisilane~GPTS! planar waveguides were prepared by a sol–gel route. A stable sol of Hafnia nanocrystals was prepared and characterized by photon correlation spectroscopy and high resolution transmission electron microscopy. The suspension was incorporated in GPTS host and the resulting sol was deposited on borosilicate substrates by the spin coating technique. Optical properties such as refractive index, thickness, number of propagating modes, and attenuation coefficient were measured at 632.8, 543.5, and 1550 nm by the prism coupling technique as a function of the HfO2 content. © 2000 American Institute of Physics. @S0003-6951~00!03348-9# lop fo s so ze a o e t ti r la in c an us ith g , ee ys o T n e al ig ar- ed s os- b- of Cl Sol–gel materials have been widely used in the deve ment of optical materials in the planar waveguide format use in integrated optics~IO!.1–5 One of the important reason for that relies on the particle size control of the precur material. In general, it is possible to produce particle si sufficiently small to keep Rayleigh scattering losses to acceptable low level even for important volume fraction nanoparticles.3,6 In fact, in multicomponent materials th mixing of the precursors on a molecular scale in order avoid phase separation is one of the intrinsic characteris of the sol–gel methodology. However, if the phase sepa tion is controlled, solid state properties of the nanoparticu phase could lead to special properties. In this way, load silica based hosts with different nanosized materials, one tailor refractive index, increase scratch resistance, or enh optical properties.3,7,8 In particular, the ~3- glycidoxipropil!trimethoxisilane (GPTS) – HfO2 binary sys- tem could be of significant technological importance beca it offers the possibility of producing planar waveguides w a controlled refractive index depending on the HfO2/GPTS molar ratio. Hafnium dioxide is transparent over the ran from 300 nm to 10mm and exhibits high refractive index about 1.95 at 1000 nm.9 Moreover, densification of GPTS based films can be achieved at low temperatures while k ing optical quality.10 This letter reports on the preparation of hafnia nanocr tals with controlled sizes and the optical characterization the hybrid planar waveguides obtained by loading GP with the nanocrystals. Hafnia nanocrystals were prepared from a suspensio hafnium oxichloride in ethanol. The mixture was kept und reflux for 2 h resulting in completely transparent colloid suspensions. Hafnia nanocrystals could be identified by h resolution transmission electron microscopy~HRTEM! 3500003-6951/2000/77(22)/3502/3/$17.00 rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue - r r s n f o cs a- te g an ce e e p- - f S of r h analysis. Figure 1 shows the HRTEM image where the rows show typical nanocrystals that could be easily identifi by the planes spaced by 3.17 Å (hkl5111 for monoclinic HfO2–PCPDF file 34-0104!. Figure 2 shows the particle size distribution obtained from photon correlation spectr copy analysis. An effective mean diameter of 4.0 nm is o tained for the hafnia sol with a narrow size distribution about 3 nm. GPTS was diluted in buthanol~volume ratio 1:0.5!. A prehydrolysis treatment was performed by addition of H FIG. 1. HRTEM image obtained for the HfO2 sol. The arrows indicate some typical hafnia nanocrystals. 2 © 2000 American Institute of Physics ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: , 14 Jan 2014 12:32:50 m cr tio 0 e de - a a n r- g tio w ig ro e a ex ib ur TE ov a n a b- T g A th nic rp- lly op- ous er- g at- ght a ith the for ve- and fi- 3503Appl. Phys. Lett., Vol. 77, No. 22, 27 November 2000 Ribeiro et al. This a ub to IP: 0.1 M. The molar ratio GPTS:H2O was 1:0.5. The resulting solution was submitted to vigorous stirring overnight at roo temperature before loading with the HfO2 nanocrystals. Three waveguide samples, hereafter identified by the a nyms W1, W2, and W3, were prepared with molar ra HfO2:GPTS510:90, 30:70, and 50:50, respectively. Thin films were deposited on 5035032 mm borosili- cate glass substrates by spin coating at 2000 rpm for 3 The films were treated by ultraviolet~UV! exposition with a Beltron UV lamp emitting in the range 280–320 nm and th heated at 130 °C for 1 h in order to achieve densification. The refractive index and the thickness of the wavegui were measured for both transverse electric~TE! and trans- verse magnetic~TM! polarization by anm-line apparatus ~Metricon mod. 2010! based on the prism coupling tech nique. We used a gadolinium gallium garnet prism with refractive index of 1.9644 at 632.8 nm. The apparatus w equipped with Si and Ge detectors to collect the visible a near infrared~NIR!, respectively. Two He–Ne lasers, ope ating at 632.8 and 543.5 nm and one diode laser operatin 1550 nm were employed. The resolution in the determina of the angles synchronous to the propagation modes 0.0075°. In order to measure propagation losses the l intensity scattered out of the waveguide plane, which is p portional to the guided intensity, was recorded by a fib probe scanning down the length of the propagating stre The losses were evaluated by fitting the intensity to an ponential decay function, assuming a homogeneous distr tion of the scattering centers in the waveguide. The meas ments were performed by exciting the transverse electric0 mode of the waveguide with the three lasers cited hereab The refractive index of the borosilicate substrate w 1.4732, 1.4703, and 1.4560 at 543.5, 632.8, and 1550 respectively for both TE and TM polarization. The optic parameters of the HfO2–GPTS planar waveguides so o tained by modal measurements, are reported in Table I. waveguides support many propagating modes dependin their thickness, refractive index, and exciting wavelength. expected, the refractive index of the film increases with nominal HfO2 content. The observed variation is of the sam FIG. 2. Particle size distribution obtained for the HfO2 sol by photon cor- relation spectroscopy. rticle is copyrighted as indicated in the article. Reuse of AIP content is s 186.217.234.225 On: Tu e o- s. n s s d at n as ht - r k. - u- e- e. s m, l he on s e e magnitude from that observed in related organic–inorga hosts containing modified zirconia nanoparticles.11 The total loss of a planar waveguide consists of abso tion and scattering contributions, with the latter being usua predominant at the wavelengths of interest in integrated tics. The scattering optical loss measured for an amorph waveguide is the sum of two contributions: volume scatt ing, due to local fluctuation in the refractive index resultin from density and compositional variation, and surface sc tering due to surface roughness. The HfO2–GPTS planar waveguides exhibit reasonably low loss for the wavelen of interest in IO. In particular, the W1 waveguide exhibits single propagation mode in the third telecom window w an attenuation coefficient of 0.4 dB/cm. Figure 3 shows absorption spectrum, after baseline subtraction, obtained a bulk sample with the same composition of the W1 wa guide. Some absorption bands assigned to combination TABLE I. Optical parameters measured at 543.5, 632.8, and 1550 nm~TE polarization! for the HfO2–GPTS planar waveguides. Attenuation coef cient is measured by exciting the TE0 mode. Sample W1 W2 W3 HfO2 /GPTS ~molar ratio! 10:90 30:70 50:50 Thickness~60.1 mm! 3.3 3.4 4.5 Refractive index at 543.5 nm~60.0005! 1.5007 1.5150 1.5302 Refractive index at 632.8 nm~60.0005! 1.4960 1.5111 1.5252 Refractive index at 1550 nm~60.001! 1.481 1.498 1.516 Number of modes at 543.5 nm 4 5 7 Number of modes at 632.8 nm 3 4 6 Number of modes at 1550 nm 1 2 3 Attenuation coefficient at 632.8 nm~60.1 dB/cm! 1.7 2.6 2.6 Attenuation coefficient at 1550 nm~60.1 dB/cm! 0.4 1.2 1.2 FIG. 3. Absorption spectrum of the HfO2–GPTS system (HfO2:GPTS 510:90). ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded , 14 Jan 2014 12:32:50 th m m d id b e re ys in 1 2 . es in de i g. g e ro k- th n p n le n t es- ec- is ith PTS es. that li- P, y t. cot, n- S. . S. cte . . 3504 Appl. Phys. Lett., Vol. 77, No. 22, 27 November 2000 Ribeiro et al. This a overtones of the fundamental vibrational modes of organic–inorganic system can be observed.8 The attenuation coefficient at the wave number corresponding to 1550 n indicated by the arrow in the figure, is about 0.18 dB/c The scattering losses contribution could then be estimate be about the 50% of the total losses for the W1 wavegu The increase of the losses with the hafnia content could assigned to the increase in both volume fraction and siz the nanocrystals. In fact, the scattering loss due to the p ence of particles is proportional to the number of nanocr tals and to the sixth power of the particle size, neglect multiple scattering.6,12 Figure 4 shows the refractive index profile of the W waveguide reconstructed from the effective indices at 63 nm by an inverse Wentzel–Kramers–Brillouin method13 The very small difference of the refractive index profil obtained for TE and TM modes indicates that the birefr gence in this waveguide is negligible. The other wavegui exhibit a similar behavior. The hafnia loaded hybrid host gives satisfying results the preparation of the waveguides. In fact, as shown in Fi and Table I, the planar waveguides support many propa tion modes in the visible region and one propagation mod the NIR region. Furthermore, they exhibit a single step p file with an uniform refractive index throughout the thic ness. Figure 5 shows the squared electric field profiles of TE0 mode of the W1 waveguide, calculated at 632.8 a 1550 nm by using the parameters obtained by them-line measurements. The modeling indicates that the optical rameters of the W1 waveguide, i.e., refractive index a thickness, appear appropriate for application in the third te communication window. In fact, the ratio of integrated inte sity, i.e., the square of the electric field, in the waveguide FIG. 4. Refractive index profiles of the W1 planar waveguide reconstru from modal measurements at 633 nm for:~a! the TE and~b! TM polariza- tion. The effective indices of the TE~d! and TM ~l! modes are reported rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue e , . to e. e of s- - g .8 - s n 4 a- in - e d a- d - - o the total intensity, which includes also the squared evan cent fields, is 0.99 and 0.90 at 632.8 and 1550 nm, resp tively. This means that an efficient injection at 1550 nm possible for the produced waveguide. In conclusion, stable sols of hafnia nanocrystals w mean size of 4 nm have been prepared and loaded in G in order to prepare good optical quality planar waveguid The optical parameters measured at 1550 nm, indicates the monomode waveguide W1 produced by a HfO2/GPTS 51:9 molar ratio is the more suitable for passive IO app cation. This research was partially supported by FAPES CNPq, CAPES-DAAD~Brazilian Agencies!, and a MURST- Cofin99 project ~Italy!. M.F. acknowledges the grant b FAPESP. 1X. Orignac, D. Barbier, X. M. Du, and R. M. Almeida, Appl. Phys. Let 69, 895 ~1996!. 2M. Benatsou, B. Capoen, M. Bouazaoui, W. Tchana, and J. P. Vil Appl. Phys. Lett.71, 428 ~1997!. 3C. Strohho¨fer, J. Fick, H. C. Vasconcelos, and R. M. Almeida, J. No Cryst. Solids226, 182 ~1998!. 4C. Duverger, M. Ferrari, C. Mazzoleni, M. Montagna, G. Pucker, and Turrell, J. Non-Cryst. Solids245, 129 ~1999!. 5X. Orignac, D. Barbier, X. M. Du, R. M. Almeida, O. McCarty, and E Yeatman, Opt. Mater.12, 1 ~1999!. 6R. M. Almeida, P. J. Morais, and H. C. Vasconcelos, Proc. SPIE3136, 296 ~1997!. 7H. K. Schmidt, J. Sol-Gel Sci. Technol.8, 557 ~1997!. 8Y. Chen, L. Jin, and Y. Xie, J. Sol-Gel Sci. Technol.13, 735 ~1998!. 9J. D. T. Kruschwitz and W. T. Paulewicz, Appl. Opt.36, 2157~1997!. 10P. Innocenzi, G. Brusatin, M. Guglielmi, and R. Bertani, Chem. Mater.11, 1672 ~1999!. 11X. M. Du, T. Touam, L. Degachi, J. L. Guilbault, M. P. Andrews, and I. Najafi, Opt. Eng.~Bellingham! 37, 1101~1998!. 12H. C. van de Hulst, inLight Scattering by Small Particles~Dover, New York, 1981!, p. 70. 13K. S. Chiang, J. Lightwave Technol.LT3 , 385 ~1985!. d FIG. 5. Calculated squared electric field profiles of the TE0 mode at 1550 nm ~a! and 632.8 nm~b! across the layered structure–cladding of airC, waveguideW, and the borosilicate substrateS of the W1 planar waveguide ject to the terms at: http://scitation.aip.org/termsconditions. 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