Determination of Refractive Index Contrast and Surface Contraction in Waveguide Channels Using...

$: Determination of Refractive Index Contrast and Surface Contraction in Waveguide Channels Using Multiobjective Genetic Algorithm Applied to Spectroscopic Ellipsometry$
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 19, OCTOBER 1, 2011 2971

Determination of Refractive Index Contrast andSurface Contraction in Waveguide Channels UsingMultiobjective Genetic Algorithm Applied to

Spectroscopic EllipsometryVasco R. Fernandes, Carlos Miguel Santos Vicente, Edison Pecoraro, Dmitry Karpinsky,

Andrei L. Kholkin, Senior Member, IEEE, Naoya Wada, Member, IEEE,Paulo S. André, Member, IEEE, Member, OSA, and Rute A. S. Ferreira

Abstract—The application of spectroscopic ellipsometry for thecharacterization of UV-patterned channel waveguides to obtainthe refractive index contrast and surface deformation profileis presented. Thin films were prepared with organic–inorganicdi-ureasils hybrids modified with zirconium tetra-propoxidedeposited in silica on silicon substrates. The channel waveguideswere produced by direct writing using UV laser radiation. The re-fractive index contrast and the surface ablation induced by the UVoptical signal were estimated by ellipsometry beingand 30.5 nm, respectively. The deepness of the surface ablationdue to the UV exposition was also estimated by atomic forcemicroscopy measurements that pointed out a value ofnm, concordant with the ellipsometric calculations. The near-fieldintensity technique was used as a support for contextualizingthe proposed ellipsometry method for the characterization ofrefractive index profiles. The estimated refractive index contrast

is in a good agreement with the refractive indexcontrast derived from ellipsometry.

Index Terms—Ellipsometry, genetic algorithms (GAs), opticalwaveguides, organic–inorganic hybrid materials, refractive index.

Manuscript received April 20, 2011; revised June 24, 2011, July 26, 2011;accepted July 30, 2011. Date of publication August 08, 2011; date of current ver-sion September 14, 2011. This work was supported by Fundação para a Ciênciae a Tecnologia, FEDER, COMPETE under Grant PTDC/CTM/72093/2006,Grant PTDC/CTM/101324/2008, Grant SFRH/BD/41943/2007, and COSTAction MP0702.V. R. Fernandes is with the Department of Physics, CICECO and Instituto de

Telecomunicações, University of Aveiro, 3810-193 Aveiro, Portugal.C. M. S. Vicente is with the Department of Physics, CICECO and Instituto

de Telecomunicações, University of Aveiro, Aveiro 3810-193, Portugal (e-mail:[email protected]).E. Pecoraro is with the is with the Instituto de Telecomunicações, University

of Aveiro, 3810-193 Aveiro, Portugal.D. Karpinsky is with the Department of Ceramics and Glass Engineering

and CICECO, University of Aveiro, Aveiro 3810-193, Portugal (e-mail:[email protected]).A. L. Kholkin is with the Department of Ceramics and Glass Engineering

and CICECO, University of Aveiro, Aveiro 3810-193, Portugal (e-mail:[email protected]).N. Wada is with Photonic Network Group, National Institute of Infor-

mation and Communications Technology, Tokyo 184-8795, Japan (e-mail:[email protected]).P. S. André is with the Instituto de Telecomunicações and Depart-

ment of Physics, University of Aveiro, Aveiro 3810-193, Portugal (e-mail:[email protected]).R. A. S. Ferreira is with the Department of Physics, CICECO, University of

Aveiro, Aveiro 3810-193, Portugal (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JLT.2011.2163924

I. INTRODUCTION

T HE data traffic increase within existing telecommuni-cations networks and the need for more energy friendly

solutions are driving the introduction of fiber optics telecom-munication systems into the access networks [1], [2]. In thiscontext, the use of integrated optical (IO) devices in the de-velopment of passive optical networks can be viewed as aviable solution to reduce the cost of implementing this type oftechnology into the access layer, due to the low unit cost ofeach device, which results from mass manufacturing [3]. Theproduction of micro-optics elements is a growing field of re-search, where direct photofabrication techniques rather than theconventional lithography or etching processes (that are morecomplex, expensive, and time consuming) are used. In parallel,the development of photosensitive and self-developed materialsat low cost is also a research target. Photosensitive materialsundergo a refractive index and/or volume permanent variationunder exposure to an optical signal. Among the materialswith potential to be used as low-cost IO substrates, emphasisis given to siliceous-based sol-gel derived organic–inorganichybrids that present themselves as candidates to competewith traditional materials such as semiconductors and silicondioxide [4]. In particular, di-ureasil methacrylic acid (McOH)modified zirconium tetrapropoxide, Zr(OPr ) , classified asdi-ureasil-zirconiumoxo-cluster hybrid (di-ureasils-ZrOMc)has been used in the last few years as IO substrates [4]–[9].Di-ureasils-Zr-OMc can be processed as thin and transparentfilms with controlled refractive index being UV-patternedwithout the need of photo-initiators.In the aforementioned context, the necessity of thin film pro-

cessing and the subsequent characterization of refractive indexand thickness variation between the exposed and nonexposedregions require rapid and accurate characterization methods.Ellipsometry, a well-established optical characterization tech-nique with high thickness resolution, is a nondestructivemethod and permits fast acquisition measurements and multi-sample analysis [10]. Moreover, the use of ellipsometry for themeasurement of refractive index contrast and surface thicknessvariation in channel waveguides is attractive because it doesnot require a guided signal. Other advantage is the ability to

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2972 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 19, OCTOBER 1, 2011

determine the absolute refractive index value, instead of the re-fractive index contrast. Techniques that require a guided signal,such as the near-field intensity method, are used to determinethe refractive index profile inside a channel waveguide. Thedetermination of the absolute value of the refractive index maysometimes be difficult due to small signal-to-noise ratio in theregion of the channel waveguide used for this determination[11]–[15]. The requirement of a guided signal is a severe draw-back as it needs monomode propagation that is not attainableand/or desired in all the cases. Moreover, we should note thatlaser direct write is also used to pattern nonguiding opticalstructures (e.g., gratings and lenses).This paper reports the applicability of spectroscopic ellip-

sometry to the characterization of the refractive index andsurface contraction in UV written channel waveguides. Theellipsometry data are analyzed using multiobjective optimiza-tion in order to find the best final solution. Recently, we havedemonstrated that the use of multiobjective optimization canattenuate problems arising from the inverse nature of theellipsometric data analysis, such us reducing limitations asso-ciated with the nonuniqueness of the solutions, and avoidinglocal minima, thus improving the results confidence [5]. Tra-ditionally, gradient-based minimization algorithms, such asLevenberg–Marquardt, are used as optimization tools in ellipso-metric problems. However, the performance of gradient-basedalgorithms depends on the complexity of the search space, thenumber of variables, and a good starting point, being possiblethat the optimization would be trapped in a local minimum. Theuse of a composite approach, usually designated as hybrid opti-mization, with global optimization (GO), for example, geneticalgorithms (GAs), combined with local optimization (LO) isadvantageous in complex problems due to the improved searchpower and the better capacity to avoid local minima [16], [17].Global optimization is used to find the vicinity of the globalminimum rather than the minimum itself. The remaining searchis performed by the LO method. The simultaneous use of thetwo methods combines the convergence and search capabilityof GO and the high accuracy of the LO. The determinationof refractive index contrast and surface contraction in thewaveguide channels will be validated, using the near-field- andatomic force microscopy (AFM) measurements, respectively.

II. EXPERIMENT

A. Planar Waveguides Preparation

The reagents O, O’-Bis(2-aminopropyl) polypropyleneglycol-block-polyethylene glycol-block-polypropylene glycol(Fluka), commercially known as Jeffamine-ED600, averagemolecular weight 600 g mol , 3-isocyanatepropyltriethoxysi-lane (Aldrich 95%), ethyl alcohol absolute P.A. (Carlo Erba),tetrahydrofuran P.A. (stabilized—Riedel-de Haeün), HCl (ACSReagent 37%—Sigma-Aldrich), zirconium tetrapropoxide,Zr(OPr ) (Aldrich, 70 wt.% in 1-propanol), methacrylic acid(McOH, Aldrich 99%), and 1-butanol (Sigma %) wereused as received. The di-ureasil host, termed as d-U(600),contains 8.5 (OCH CH ) polymer chains with both endsgrafted to a siliceous network by means of urea linkages wasdoped with 40% mol of Zr(OPr ) with the Zr(OPr ) :McOH

molar ratio of 1:1, as detailed described elsewhere [5], [6]. Thehybrids were processed as thin films deposited on oxidizedsilicon wafers (silica thickness of m). To enablethe wafers to be spun, they were held by suction on a chuck,placed on the axis of the spin coater (MIKASA, 1H-DX2)accelerated at 500 r min for 15 s and 1000 r min for 30s. To enable light coupling for the near-field measurements, athicker film was produced by changing the deposition condi-tions to 1000 r min for 60 s (SPIN150-NPP, APT) and thesolution viscosity through Zr(OPr ) :McOH molar ratio to 1:2.All the films were dried at 50 C for 12 h for complete solventremoval. The films will be hereafter termed as dUZ40.

B. Direct UV Laser Writing

Direct UV laser writing was used to pattern channel waveg-uides with different full-width at half-maximum (FWHM)values in the dUZ40 films. The system is composed by apulsed UV excimer laser (Coherent Bragg-Star IndustrialV2.0) operating at 248 nm, pulsed with a frequency of 900 Hz.The laser beam was focused through an objective lens (Thor-labs-LMU-15X-248). The film was moved by a PC-controlledtranslation stage (Newport-XPS and Newport-MFA-CC) inorder to create the waveguide region. The channel FWHM wascontrolled by tuning the laser energy, translation velocity of thefilm, and the distance between the objective lens and the film.One channel with FWHM around 2 m was patterned at thelens focusing point (8.5 mm), with an energy of Jper pulse and a velocity of 0.6 mm s , and a wider channelwith FWHM around 20 m was patterned by moving away2.79 mm the lens from its focusing point, with an energy of

J per pulse and a velocity of 2.5 mm s .

C. Characterization Techniques

The scanning electron microscopy (SEM) images were ob-tained with a field emission type microscope (Hitachi SU-70)operating at 4.0 kV in a secondary electron emission mode. Toreveal the substrate and the film cross section, the samples werecleaved. The films were fixed on the SEM sample holder with aconductive adhesive (Nisshin EMI), and the sample surface wascleaned with an air flux before the coating process on a carbonvacuum evaporator (Emitech K950X). The conductive coatingwas performed with pure graphite rods with an evaporation timeof 30 s.The AFM measurements were performed using a commer-

cial NTEGRA Aura setup (NT-MDT) in tapping mode usingscans areas from 2 m 2 m to 6 m 6 m. The scan ve-locity was always kept at 0.5 Hz (12 m s ). Silicon can-tilevers (Nanosensor PPP-NCHR) with the spring constant of42 N m and tip radius less than 10 nm were used. The ex-perimentally measured data were subjected to an average filterusing an integration area of 10 10 pixels.The spectroscopic ellipsometry measurements were per-

formed using an Horiba Scientific AutoSE spectroscopicellipsometer, with a total of 218 points in the wavelengthinterval 500–850 nm, an incidence angle of 70 , an acquisitiontime per point of 400 ms, and an average of 20 measurementsper point, using the minimum micro spot size of 25 m 60m. Two measurements were performed in the nonexposed

FERNANDES et al.: DETERMINATION OF REFRACTIVE INDEX CONTRAST AND SURFACE CONTRACTION 2973

Fig. 1. Schematic representation of (a) ellipsometric measurement spots for thedUZ40, where A and B represent the nonexposed and exposed channel wave-guide regions, respectively, and of (b) the thickness inhomogeneity parameterswithin the channel region.

region (marked with A in Fig. 1) and in the exposed area withinthe channel waveguide (marked with B in Fig. 1).Ellipsometry is based on the measurement of the change of

the light polarization state upon reflection on a sample. The fun-damental equation of ellipsometry

(1)

relates the signal phase shift, , with being thephase angle, and the change in the relative amplitude of theparallel and perpendicular components of the reflectedelectric field, , where and are the com-plex Fresnel reflection coefficients. For a multilayered sample,the overall Fresnel reflection coefficients are readily calculatedusing a matricial method [13]. Modern phase-modulated ellip-someters measure the parameter values designated as and ,being and calculated using the appropriate formulation [18]

(2)

The material refractive index was calculated using the Lorentzmodel, which expresses the relative complex dielectric constantas a function of the frequency (eV) described by

(3)

where (eV), (eV) are the high frequency relative di-electric constant, the static relative dielectric constant, the oscil-lator resonant frequency, and the damping factor, respectively.The goodness of the fit was evaluated by the (unbiased) esti-mator, mean squared error (MSE)

(4)

where is the number of experimental data points, isthe number of the model variable parameters, and the super-scripts exp and mod stand for the measured and simulated data,respectively.The mode field distribution was acquired using a xyz po-

sitioning system (Thorlabs, Nanomax-TS) and a laser beam

profile analyzer (Newport LPB-1) with a microscope objective( 10, Newport, M-10X), using an integration time of 10 s.The analyzer xy position experimental error is 0.15 m. Themeasurements were made with a laser operating at 635 nm(LPS-635-FC, Thorlabs). The measured near-field light inten-sity distribution inside the channel waveguide was modeledusing two different approaches. 1) Near-field direct method,which applies the direct inversion of the model field equation,from which it is possible to obtain an approximation of the pro-file [14], [15]; and 2) iterative finite-difference method, usingan inverse procedure, in which an index profile is assumed forthe waveguide and in an iterative process the calculated electricfield is compared with the experimental measurement [12], asbriefly described in the following.

D. Near-Field Direct Method [14], [15]

The electric field within the waveguide channel canbe described by

(5)

where is the wavenumber, is the refractiveindex in the waveguide region (B in Fig. 1), is thepropagation constant in the medium, and is the effectivemode index. Considering , whereis the refractive index of the nonexposed region (A in Fig. 1) and

is the channel refractive index contrast, the refractiveindex distribution in the waveguide can be approximated to thefollowing equation:

(6)

The measured near-field intensity can be used to obtain the re-fractive index contrast, since the intensity is directly propor-tional to the square of the electric field, .

E. Iterative Finite-Difference Method [12]

In the iterative finite-difference method, an initial refractiveindex profile is considered for the waveguide, being insertedinto the propagation equation (5). The mode field equation canbe solved as an eigenvalue problem to calculate the electricfield resulting from the considered profile. The calculated elec-tric field is then compared with the measurement one and theerror between them is computed. In the iterative optimizationprocess, the parameters (typically, the waveguide width andrefractive index contrast) that describe the refractive indexprofile are changed so that the error between the measuredand the calculated electric fields is minimized, using theLevenberg–Marquardt algorithm.

III. EXPERIMENTAL RESULTS

A. Spectroscopic Ellipsometry

Fig. 2(a) shows the cross-sectional SEM image of the dUZ40film, revealing that the di-ureasil layer prepared by spin coatinghas a smooth, crack- and pinhole-free dense microstructure withminimal number of defects with an average thickness ofnm.


Fig. 2. (a) SEM photo of the cross section and (b) structure model for the non-exposed and UV radiation exposed channel waveguide regions of the dUZ40.

Based on this layered structure, the model structure for thenonexposed and UV-exposed waveguide channel regions is pre-sented in Fig. 2(b), being composed of a dUZ40 layer withthickness and refractive index values of and

, respectively, on top of a silica on silicon sub-strate.We consider that the refractive index in the channel wave-guide is spatially uniform and wavelength dependent

, where is a complex value repre-senting the refractive index increment with respect to the non-exposed region due to the UV irradiation. Within the exposedregion, a surface depression induced by the UV exposition is ex-pected [7]. The substrate layer is represented in the same fashionas considered for the nonexposed region.From the substrate manufacturer, an approximate value of the

silica layer thickness is known, m. How-ever, the error in this value is significant in the context of el-lipsometric analysis. Therefore, the thickness of the silica layeris considered as an adjustable variable, which takes the samevalue in the model of the nonexposed and exposed regions. Forthe refractive index of silicon and silica, reference values wereused [18].Thickness inhomogeneity is used to model the surface in the

nonexposed and exposed regions. In particular, an additional pa-rameter per measurement, which expresses the extent ofthe inhomogeneity [19], is added to the average thickness value,being the thickness defined within the interval .The physical meaning of the inhomogeneity values in the non-exposed region can be ascribed to surface roughness due to thepresence of ZrOMc oxo-clusters, as we will detail in the fol-lowing. Under UV exposition, these clusters are exhausted andthe surface roughness inside the channel can be neglected [7].In this context, we propose that the inhomogeneity parameterinside the channel stems from the fact that the channel area wasmodeled by ellipsometry as a flat region, which has, in fact, aGaussian shape arising from the UV laser profile. Therefore, thethickness parameters refer to an effective thickness value witha transition region of . Fig. 1(b) presents the interpretationof these parameters within the channel.The total number of fitting parameters in the numerical model

is 11. Table I presents the fitting parameters and the respectivelower and upper bounds limit values used in the optimizationalgorithms, which were defined based on previous knowledge.During the optimization procedure, the most sensitive parame-ters are the ones related with the refractive change due toUV exposure, when compared with the ones related with thick-ness variation. The weighting of the refractive index and thick-

TABLE ILOWER AND UPPER BOUND LIMITS OF THE FITTING PARAMETERS

USED IN THE OPTIMIZATION ALGORITHMS

ness-related parameters in the fit error function is the same, withsensitivity values of 0.0001 and 0.1 nm, respectively.Fig. 3(a) and (b) show the experimental ellipsometric spectra

measured within the nonexposed and in the channel waveguideregion, respectively. Recently, we have introduced, for thefirst time, the use of hybrid multiobjective optimization inspectroscopic ellipsometry data analysis [5]. For the implemen-tation of the hybrid optimization, a single objective commonGA (CGA) was compared to a multiobjective GA (MOGA),showing a higher success rate in the task of finding the bestfinal solution for MOGA. Furthermore, we demonstrated thatthe hybrid MOGA is potentially faster than the CGA and alsothat its efficiency can be improved when the right momentto switch methods is properly chosen [5]. This MOGA wasalso applied to fit the data in Fig. 3. The optimization processbegins with the use of MOGA to allow an effective search inthe space of possible solutions, and approach the vicinity of theglobal minimum. In this stage of the optimization process, thereare two objective functions, one for each measurement. Thealgorithm used was the NSGA-II described in [20]. After 500generations, the best solution found so far by the MOGA wastransferred to the Levenberg–Marquardt algorithm as a startingpoint. The process continues as a single objective problem inthe traditional configuration, and the error function is the sumof the errors for each model. The fitted curve to parametersand and the respective regular residual plots are also shownin Fig. 3.Table II lists the fitting parameter values obtained using the

previously described structural model. From (3), the attainedrefractive index values for the nonexposed region were deter-mined, leading to for a wavelength of 635nm. The value is in a good agreement with that previously re-ported for analogous di-ureasil planar waveguides [5], [9]. Thethickness of the film in the nonexposed region is nmwith an inhomogeneity value of nm. For the channelregion, the refractive index value is for a wave-length of 635 nm, corresponding to a refractive index contrast of

. The average effective thickness in thechannel region was nmwith an inhomogeneity valueof nm. From these values and considering the pre-vious definitions of the effective thickness and inhomogeneityin the exposed region of a film, a maximum contraction valueof 30.5 nm in the channel region can be estimated, as it will befurther demonstrated in the following.The potential effect of the presence of surface roughness and

deformation in the ellipsometric spectra is depolarization of thereflected light, due to the scattering of the incident light by these


Fig. 3. Ellipsometric parameters (open circles) and (open triangles) mea-sured in the (a) nonexposed and in the (b) channel regions. The solid lines cor-respond to the data best fit. (c) and (d) Respective residual plots [between themeasurement and the fitting data, for the same wavelengths presented in (a)and (b)] and the MSE values for a better judgment of the fit quality

. Experimental (open circles) and calculated (solid lines) depo-larization spectra in the (e) nonexposed and in the (f) channel regions.

surface features, which originates light reflected in more thanone polarization states [21]. The polarization degree can bedefined by

(7)

TABLE IIFITTED PARAMETERS (THICKNESSES, LORENTZ MODEL, ANDREFRACTIVE INDEX CONTRAST) FOR THE TWO REGIONS OF

THE dUZ40 OBTAINED USING MOGA/LO

Fig. 3(e) and (f) compare the measured and calculated depolar-ization spectra for the nonexposed region and for the channelarea, revealing a very small depolarization effect with a polar-ization degree very close to the unit, pointing out the pooreffect of the surface features in the ellipsometric analysis.In order to validate the usage of spectroscopic ellipsometry to

estimate the refractive index absolute values and surface con-traction undergone by the materials surface due to the usageof UV-patterning processes, mode field distribution and AFMmeasurements were performed to independently estimate the re-fractive index contrast and surface depression, respectively.

B. Mode Field Profile

Fig. 4(a) shows the intensity profile obtained at the output of adUZ40 channel. The corresponding refractive index profile wascalculated using the near-field direct method and the iterativefinite method with for a wavelength of 635 nm. Themaximum refractive index contrast was , as shownin Fig. 4(b).For the iterative finite-difference method, the inverse proce-

dure was carried out considering the refractive index profile de-scribed by a single Gaussian function. The peak intensity pro-file, marked by the superimposed solid line in Fig. 4(a), wasselected to apply the inverse procedure. For the optimizationprocess based on the comparison of the measured and calculatedintensity profiles, the Levenberg–Marquardt algorithm was uti-lized. Fig. 4(c) presents the selected measured intensity andthe fitted intensity profiles arising from the Gaussian refractiveindex distribution. The optimal values for the Gaussian param-eters yield a waveguide FWHM of 2.536 m and a refractivecontrast of , with . Fig. 4(d)shows the refractive index profile estimated by the iterative sim-plex finite method inverse procedures.The refractive index contrast determined using both the near-

field direct method and the iterative finite-difference method isvery close to that estimated by ellipsometry, validating, there-fore, the proposed ellipsometric-based approach.

C. AFM

Fig. 5(a) presents a 3-D AFM surface image of the dUZ40film.


Fig. 4. (a) Mode field distribution excited at 635 nm in the waveguide area.(b) Refractive index profile obtained by direct inversion of the measured inten-sity. (c) Intensity profile along the solid line in (a) and fitted Gaussian function(dashed line) using the iterative simplex finite method. (d) Refractive index pro-file estimated by the iterative simplex finite method.

Fig. 5. (a) AFM image in 3-D view and (b) average cross-sectional profile ob-tained from AFM measurements (black solid line, yellow shadow) and schemeof the ellipsometry profile (dotted lines) The blue and pink shadows repre-sent the inhomogeneity values estimated for the nonexposed region and for theUV-patterned channel, respectively.

The nonexposed region presents acceptable homogeneitywith an average RMS roughness value of nm. Thepatterned region exposed to UV radiation shows a surface de-pression with a Gaussian shape arising from the beam spectraldistribution. The respective cross section of the topographyimage is plotted in Fig. 5(b), revealing that the depression depthis approximately 31.0 nm and that the corresponding width isabout 25 m at the surface level. Fig. 5(b) also compares thesurface depression estimated by ellipsometry with that madeby AFM.The interval range measured by ellipsometry corresponds to

the thickness inhomogeneity, i.e., the thicknesses distributionat the measurement spot is in line with the model for thick-ness inhomogeneity presented in Fig. 1(b). The good agree-ment between the surface RMS roughness values in the non-exposed region ( nm) and the surface contraction inthe channel region (30.5 nm) estimated through AFM data withthe ellipsometry parameter and surface depres-sion (30.5 nm), respectively, validated the application of spec-troscopic ellipsometry to the characterization of surface topog-raphy. The results of both methods are consistent with the pro-posed model of the UV written channel region.

IV. CONCLUSION

The use of spectroscopic ellipsometry to characterize therefractive index contrast and thickness variation in channelwaveguides patterned using direct UV laser writing was de-scribed. The near-field intensity and iterative finite-differencemethods were utilized to illustrate the differences and applica-bility conditions of the proposed method.Ellipsometry measurements enable a determination of the re-

fractive index contrast of , very similar tothe value estimated by the near-field technique fora single-mode waveguide. To enable a comparison term for thedepression of the patterned channel, AFM measurements wereperformed. The value obtained by ellipsometry for the channelmaximum depth was 30.5 nm, in a good agreement with the oneobtained by AFM (31.0 nm).The main advantages of the spectroscopic ellipsometry pre-

sented method for the characterization of the refractive indexcontrast are no need for the guided light, the ability to estimatethe refractive index values, rather than a simple contrast deter-mination. Regarding the surface depression of the channel, thegood agreement observed between the ellipsometry determinedvalue and the AFM suggests that a reasonable estimation can beobtained from pure optical means.

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Vasco R. Fernandes received the B.Sc. and M.Sc. degrees in engineeringphysics from the University of Aveiro, Aveiro, Portugal, in 2008 and 2010,respectively. He is currently working toward the Ph.D. degree in physics fromthe Norwegian University of Science and Technology, Trondheim, Norway.He was involved in the area of integrated optics and ellipsometry as a Re-

searcher at CICECO and Instituto de Telecomunicações Associated Laborato-ries, University of Aveiro.

Carlos Miguel Santos Vicente received the Master degree in physical engi-neering from the University of Aveiro, Aveiro, Portugal, in 2007. He is currentlyworking toward the Ph.D. degree in physics at CICECO and Instituto de Teleco-municações, University of Aveiro, on the subject “Organic/inorganic hybrids forlow cost integrated optics applications,” (FCT Grant SFRH/BD/41943/2007).He is also participating on the project, “Organic-inorganic hybrids with en-

hanced light-emitting properties for the new generation of optical communica-tions,” (FCT, PTDC/CTM/101324/2008). His research interests include func-tional organic–inorganic hybrid materials, optical communications, integratedoptic devices, and microfabrication.

Edison Pecoraro received the B.Sc. and M.Sc. degrees in chemistry fromthe Institute of Chemistry, São Paulo State University (UNESP), Araraquara,Brazil, and the Ph.D. degree in material science and physics from the Instituteof Physics, Universidade de São Paulo, Brazil.In 2000, he became a Postdoctoral Researcher in a North American company

called Intelligent Optical Systems, Torrance, CA, where he investigated the in-fluence of gravity on silica sol-gel networks. In 2001, he joined the PhotonicMaterials Group, Institute of Chemistry, UNESP, as a Researcher for devel-oping optical fibers for broadband infrared optical amplifiers, where he startedhis first project with bacterial cellulose composites, as a Researcher supportedby Bionext (a biotech Brazilian company) in 2005. In 2006, he was with theDepartment of Physics, Universidade de Aveiro, where he was involved in theresearch of hybrid materials. From 2008 to April 2011, he was an AssistantResearcher at the Instituto de Telecomunicações where he was looking for im-provement in the fields of applications of hybrid materials such as optical andbiomedical sensors, waveguides, dye lasers, hybrid aerogels, bacterial cellulose,etc. Since May 2001, he has been a Professor of Inorganic Chemistry at the In-stitute of Chemistry, UNESP, Brazil, where he is involved in the developmentof materials for optical and bio applications.

Dmitry Karpinsky received the Ph.D. degree in solid state physics from Sci-entific-Practical Materials Research Centre (SSPA), National Academy of Sci-ences (NAS), Minsk, Belarus, in 2007.During his Postdoctoral Fellowship at SSPA, NAS, he was involved in the

study of novel magnetic and ferroelectric materials. In 2008, he became a Post-doctoral Researcher at the University of Aveiro, Aveiro, Portugal. His currentresearch interests include developing advanced magnetoelectric materials andtheir characterization using nanocharacterization techniques, magnetometry,and diffraction methods.

Andrei L. Kholkin (SM’04) received the B.Sc. and M.Sc. degrees in physicsfrom the Saint Petersburg State University, Saint Petersburg, Russia, and thePh.D. degree from the A. F. Ioffe Physical Technical Institute, Saint Petersburg,Russia.In consequent years, he held research positions in Swiss Federal Institute of

Technology, Lausanne, Switzerland and Rutgers University, NJ. He is currentlya Research Coordinator and the Head of the Functional Imaging and Nanochar-acterization Laboratory of CICECO, University of Aveiro, Aveiro, Portugal. Heis developing multifunctional materials (including ferroelectrics and multifer-roics) and scanning probe microscopy techniques. He is the coauthor of morethan 340 technical papers in this area including numerous reviews and bookchapters. He is currently the Coordinator of two European projects.Dr. Kholkin serves as an Associate Editor-in-Chief for the IEEE

TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCYCONTROL (TUFFC). He is a member of the Ferroelectric Committee of theIEEE and was the recipient of the “Excellency” award from the PortugueseFoundation for Science and Technology. He has been a Technical Committeemember of several international conferences. He was the Guest Editor of thespecial issues in TUFFC, Journal of Applied Physics, and Materials ResearchSociety Bulletin. He is a member of the Materials Research Society and thePortuguese Materials Society.

Naoya Wada (M’97) received the B.E., M.E., and Dr. Eng. degrees in elec-tronics from Hokkaido University, Sapporo, Japan, in 1991, 1993, and 1996,respectively.In 1996, he joined the Communications Research Laboratory, Ministry of

Posts and Telecommunications, Tokyo, Japan. He is currently a Project Leaderand Research Manager at Photonic Network Group, National Institute of In-formation and Communications Technology, Tokyo, Japan. He is the author orcoauthor of more than 70 papers published in refereed journals and more than200 papers published in refereed international conferences. His research inter-ests include photonic networks and optical communication technologies, suchas optical packet switching network and optical code-division multiple accesssystem.Dr. Wada is a member of the IEEE Comsoc, the IEEE Lasers and Electro-Op-

tics Society, the Institute of Electronics, Information and Communication Engi-neers (IEICE), the Japan Society of Applied Physics, and the Optical Society ofJapan. He was the recipient of the 1999 Young Engineer Award from the IEICEof Japan, and the 2005 Young Researcher Award from the Ministry of Educa-tion, Culture, Sports, Science, and Technology.


Paulo S. André (S’98–M’03) was born in Luanda, Angola, in April 1971. Hereceived the degree in physics engineering and the Ph.D. degree in physicsboth from the Universidade de Aveiro, Aveiro, Portugal, in 1996 and 2002,respectively.In 2002, he joined the Instituto de Telecomunicações, Universidade de

Aveiro, as an Assistant Researcher. He is currently an Invited AssistantProfessor at the University of Aveiro where he is teaching lecturing courseson optoelectronics, quantum optics, quantum technologies, and functionalmaterials and devices. He is the author or coauthor of more than 80 paperspublished in refereed journals and more than 300 papers published in refereedinternational conferences. His current research interests include the study andsimulation of optoelectronics components, fiber Bragg gratings, integratedoptics, transparent performance monitoring, Raman amplification, multiwave-length optical communications systems, and passive optical networks.Dr. André is a member of the Optical Society of America, the Portuguese

Physics Society, and the Portuguese Materials Society.

Rute A. S. Ferreirawas born in 1974. She received her Ph.D. degree in physicsfrom the University of Aveiro, Aveiro, Portugal, in 2002, studying on photo-luminescence of organic–inorganic hybrids.Since 2003, she has been with the University of Aveiro as an Assistant Re-

searcher at CICECO Associated Laboratory and as an Assistant Professor (parttime) in the Department of Physics. She is the coauthor of more than 154 SCI pa-pers and 4 book chapters, with ca. 2800 citations (h-index 26) and of 200 studiespresented at international conferences. In the last decade, much of her work wasfocused on the photoluminescent features of pristine and lanthanide-doped or-ganic–inorganic hybrid materials for photonic applications (lighting and inte-grated optics). Her research interest also includes ellipsometry as a characteri-zation tool of the refractive index and thickness of thin films and waveguidesand optimization of the inverse algorithms used to calculate those parametersfrom the ellipsometric raw data.

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