Characterization of planar waveguides fabricated by multiple Sol-Gel dip-coatings

F. Rey-Garcíaa,b, C. Gómez-Reinoa, M.T. Flores-Arias*a, G.F. De La Fuenteb, W. Assenmacherc, W.

Maderc, S. Berneschid,e, S. Pellie, G. Nunzi Contie and G.C. Righinie

aUnidad Asociada de Óptica & Microóptica GRIN (CSIC-ICMA), Departamento de Física Aplicada, Escola de Óptica e

Optometría, Universidade de Santiago de Compostela, Campus Sur s/n, E-15782 Santiago de Compostela, Spain; bLaboratorio de Aplicaciones del Láser, Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de

Zaragoza, María de Luna 3, E-50018 Zaragoza, Spain; cInstitut für Anorganische Chemie, Universität Bonn, Römerstraße 164, D-53117 Bonn, Deutschland;dMuseo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Piazza del Viminale 2, I-00184, Roma, Italy; eIstituto di

Fisica Applicata “Nello Carrara”, IFAC-CNR, via Madonna del Piano 10, I-50019 Sesto Fiorentino, Italy.

Corresponding author: *maite.flores@usc.es

ABSTRACT

Planar step-index waveguides of SiO2:TiO2 and ZrO2:CeO2 in multilayer structures were prepared onto commercial glass substrates using a sol-gel technique combined with dip-coating. These coatings were previously optically characterized by Ellipsometry. These glassy coatings were structural characterized by Transmission Electron Microscopy (TEM), Energy Dispersive X-ray analysis and Confocal Microscopy. Thicknesses of 1050 nm and 500 nm and refractive indices of 1.64 and 2.07 for SiO2:TiO2 (70:30) and ZrO2:CeO2 (70:30) waveguides were obtained, respectively, by the analysis of the guided TE and TM modes observed by Dark m-line Spectroscopy. Losses of 1.32 dB/cm and 0.86 dB/cm were respectively measured by a method based on scattered light.

Keywords: Planar waveguides, sol-gel technology, transmission electron microscopy, dark m-line Spectroscopy.

1. INTRODUCTION The preparation of planar waveguides has attracted considerable attention because of their potential use and applications in photonics devices in the field of communications and data transmission. Traditional methods as ion-exchange and chemical or physical vapor deposition have been extensively used to develop this kind of materials [1-2]. Recently, laser deposition and direct writing techniques have been also employed to obtain planar waveguides on glass substrates of different compositions [3-5]. These techniques have high economical cost and the compositions used raise the process complexity.

An economical method developed in the last years is based on sol-gel technology. This technology is widely used in different fields, from materials science to biological applications for the development of bisosensors [6], bulk materials [7], superconductors [8], fuel cells [9], ceramic composites [10] etc. A wide range of compositions is used for this purpose. In particular, this technology permits us to obtain coatings with controlled thickness and refractive index. The sol-gel process is suitable to produce both thin and thick films by single and multideposition techniques. Several papers dealing with planar waveguides fabricated with this method have been reported in literature [12-16].

The microstructure of sol-gel derived coatings, thus their optical properties, is affected by different parameters involved in their synthesis. These include starting materials, and thermal processing conditions, among others [17-18]. The optimization and control of all these parameters should allow the production of efficient sol-gel derived planar waveguides. Following this concept, one important advantage provided for the sol-gel technology is the lower surface roughness obtained for the coatings. This fact allows to drastically reduce propagation losses.

22nd Congress of the International Commission for Optics: Light for the Development of the World,edited by Ramón Rodríguez-Vera, Rufino Díaz-Uribe, Proc. of SPIE Vol. 8011, 80110R

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Only few papers have employed the transmission electron microscopy to study the structural characteristics between sol-gel films and substrates and the process involved in the union between them; most of the literature reports the study of thin films, of their interfaces and of their crystalline structures [19-20].

The aim of this work is to prepare planar waveguides by sol-gel technology. The sol-gel film structural characteristics, the interface structure and the processes involved in the union between materials are studied by electron microscopy and optical confocal microscopy. Waveguide thickness and index are evaluated by Dark m-line Spectroscopy method. Surface roughness is evaluated by confocal microscopy.

2. EXPERIMENTAL Hybrid silica-titania (SiTi sol) and inorganic zirconia-ceria (ZrCe sol) sols were prepared by acid catalysis following different routes [21-23]. SiTi sol was prepared by complexing tetraisopropoxytitanate (TISP) with acetic acid and adding methyl-triethoxysilane (MTES) in a proportion of 70:30 and acidified water in a dropwise manner, to complete synthesis reactions. On the other hand, the inorganic ZrCe sol was prepared by mixing tetraisopropoxyzirconate (TPZ) with glacial acetic acid (AcOH) as complexing agent to control the hydrolisis and condensation reactions; cerium nitrate (Ce(NO3)36H2O) was used as cerium precursor, adding it in a dropwise manner the ethanolic solution.

Coatings were deposited onto commercial soda-lime glass substrates (microscope slides) by dip-coating at the withdrawal rate of 25 cm/min. First, the substrates were cleaned in an ultrasonic bath with absolute ethanol for 15 min. Multilayer coatings were prepared in steps, using the corresponding sol and intermediate heat treatments at 450 ºC during 15 minutes, followed by a final treatment at 450ºC during 1 hour. Substrates were dip-coated with five-layers for SiTi sol and with four-layers for ZrCe sol.

Thickness (e) and refractive index (n) were previously measured by profilometry (Talystep, Taylor-Hobson, UK) using a variable angle spectroscopic ellipsometer (Woollam M2000U). Values measured are shown in Table I.

Table I: Refractive index (λ = 633 nm) and thickness for the studied coatings measured by ellipsometry.

System Molar composition Refractive index (n) Thickness (nm)

SiTi 70:30 1.59 1230

ZrCe 70:30 2.04 489

Microstructural studies were performed with a FEG/UT-STEM Phillips CM300 field emission gun and ultra twin lens and scanning unit operated at 300kV and equipped with an energy dispersive x-ray detector (HPGe detector, Noran Inc.) and analytical system (energy dispersive X-ray unit, Noran Inc.).

The optical characterization of the TE and TM guided modes was carried out employing the dark m-line spectroscopy. This technique was implemented using the COMPASSO instrument and is based on prism coupling method [23]. The measure of the waveguide propagation losses was obtained by an image caption device, specifically a vidicon camera (Hamamatsu) which collects the light scattered out of the plane of the film.

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3. RESULTS AND DISCUSSION We have prepared two samples to evaluate the quality of planar waveguides obtained via Sol-Gel. These samples are 70:30 SiO2:TiO2 (SiTi-M) and 70:30 ZrO2:CeO2 (ZrCe-M), both in multilayer configuration.

A diffusion process involving alkali and alkaline earth species is observed at the interface area of a SiTi sample in the micrograph of Figure 1, where observation is made under high magnification and complemented with EDX analysis. This Figure suggests that Na, K and Ca ions migrate from the glass substrate towards the sol-gel coating. The substrate area adjacent to the coating (near the substrate-coating interface) has thus become, with respect to the rest of the substrate, enriched with Si. The Ti concentration in the proximity of the interface area is found to be higher than at the external part of the coating, and its presence, is not detected within the substrate. The associated alkali and alkaline-earth metal cation diffusion process is thus held responsible for the excellent chemical matching between the coating and the substrate materials, deduced from the interfaces observed and the EDS spectra given in Fig. 1.

Figure 1: TEM micrography (left side) at the interface of the SiTi sample and the corresponding spectra (right side) of the marked places. The red dot corresponds with the silicon rich zone at the top of the substrate. The blue dot notes the film darker area close to the interface; this area is just a little richer in alkaline cations than the external area of

the film (green dot).

The study of the interface between layers allows checking the homogeneity and continuity between layers, as demonstrated in Figure 2, where a close-up look at the interfaces between layers 3-4-5 is shown. This micrograph exhibit interfaces which appear to be clean, suggesting that coupling between amorphous layers take place at the atomic level, apparently with no significant structural perturbations. No long-range structural order is observed. This result confirms the possibility to obtain a single, homogeneous thick layer from the stacking of successive thin layers.

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Figure 2: TEM micrography of layers 3 to 5 viewed in the polished and thinned cross section of sample SiTi-M. Concrete and well defined interfaces are observed. The layers seem homogeneous between them like a continuous

material; no long-range order, nor amorphous are indentified. The diferent contrast observed are due to the progressive thining of the sample, yielding a difference in thickness but not in composition.

The sol-gel system provide surface with low rates of roughness as we can observe in Fig. 3. The surface roughness is an important factor in the waveguiding process and may strongly affect propagation losses. The values of the roughness, measured by confocal microscopy for SiTi-M and ZrCe-M sample were 5.7 nm and 7.2 nm, respectively.

Figure 3: Confocal microscopy topography of the surface of the ZrCe sample.

The samples were completely optically characterized by dark m-line spectroscopy. This is a technique widely used for the characterization of planar optical waveguides, allowing to test for effective waveguiding, to determine the main waveguide parameters (refractive index and thickness) and to get an estimation of the waveguide losses.

The SiTi-M sample was the first to be characterised. At the wavelength of 635 nm, we found two modes in TE and TM polarization of the light. In the infrared region of the light up to 1550 nm, we could only observe one mode for each polarization. Table II summarizes these results. After the mathematical treatment of the data with a dedicated software (based on the inverse of the WKB method), we calculate the corresponding refractive index and thickness for this SiTi system (Table III). These values are somehow different from those obtained by ellipsometry, however the work developed during years has demonstred that the values obtained by the dark m-line spectroscopy are highly reliable [12, 23].

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Table II: TE and TM mode indices (± 0.0005) for multilayer SiO2:TiO2 (70:30) deposited onto microscope slide substrates.

Wavelenght TE mode TM mode

635 nm N0 1.6206 N0 1.6218

N1 1.5693 N1 1.5604

980 nm N0 1.5864 N0 1.5798

1550 nm N0 1.5381 N0 1.5381

Table III: Refractive index (± 0.001) and thickness calculated from mode indices compared to ellipsometry values for multilayer

SiO2:TiO2 (70:30) deposited onto microscope slide substrates.

SiO2:TiO2 (70:30) Ellipsometry TE TM

multilayer (633 nm) (635 nm) (635 nm)

Refractive index (n) 1.5900 1.640 1.643

Thickness (nm) 1230 1080 1040

Finally on ZrCe-M we observed three guided modes for TE polarization (Fig. 4) and only two modes for TM polarization of the light at a wavelenght of 635 nm.

Figure 4: Results from dark m-line spectroscopy obtained using the COMPASSO system based on prism coupling technique. TE

fundamental modes (N0=2.01081 (blue), rutile prism; N1=1.80779 (yellow) and N2=1.53158 (green), GGG prism) at 635 nm for the multilayer ZrO2:CeO2 (70:30). The knee signal due to coupling to the substrate is around n=1.512 (violet).

The guided modes observed for the ZrCe-M sample are summarized in Table IV. The Table V summarizes the calculated values for refractive index and thickness. Observing this values and observing the difference between the results obtained for each polarization of the light we can conclude that the ZrCe-M is a birefringence material.

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Table IV: TE and TM mode mode indices (± 0.0005 for multilayer ZrO2:CeO2 (70:30).

Wavelenght TE mode TM mode

635 nm N0 2.0108 N0 2.0068

N1 1.8078 N1 1.7635

N2 1.5316 - -

1550 nm N0 1.8826 N0 1.7020

Table V: Refractive index and thickness for multilayer ZrO2:CeO2 (70:30).

ZrO2:CeO2 (70:30) multilayer

Ellipsometry (633 nm)

QB64 TE (635 nm)

QB64 TM (635 nm)

Refractive index (n) 2.040 2.069 2.087

Thickness (nm) 489 520 500

The losses of both multilayers were measured by a method based on the light scattered from the waveguides. Table VI summarizes these values. On the ZrCe-M samples we observed losses which depend on the polarization of the light. The reason for this difference is still under investigation.

Table VI: Losses measured for sol-gel waveguides.

System Polarization Losses (dB/cm)

SiO2:TiO2 (70:30) TE polarization 1.3 ± 0.3 (?)

ZrO2:CeO2 (70:30) TE polarization 0.9± 0.3 (?)

TM polarization 1.5± 0.3 (?)

4. CONCLUSIONS Planar waveguides of SiO2:TiO2 and ZrO2:CeO2 were prepared in multilayer configuration by sol-gel technology. The different composition layers were deposited over microscope slides by dip-coating. In particular, we present the SiO2:TiO2 and the ZrO2:CeO2 multilayers with refractive index values of 1.64 and 2.07 for a wavelength of 633 nm, and thickness around 1050 nm and 500 nm, respectively. A microstructural study was performed by TEM obtaining that, in all cases, the interface between layers is homogeneous and continuous. Optical characterization of the samples was made, measuring the guided modes for TE and TM polarization of the light. The losses were measured obtaining values of 1.3 dB/cm for SiO2:TiO2 and, 0.9 dB/cm and 1.5 dB/cm for ZrO2:CeO2 at TE and TM polarization of the light, respectively.

ACKNOWLEDGEMENTS

The authors acknowledge funding from MICINN (TEC2006-10469, CEN 2007-2014, SURFALUX SOL-00030930 and MAT2010-18519), from DGA (T87: Laboratory of Laser Applications) and XUNTA DE GALICIA/FEDER (INCITE08PXIB206013PR).

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