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... du Docteur A. Schweiter, F-33608 Pessac, France. 3University of Plymouth, B333, Portland Square, Drake Circus, Plymouth, Devon PL4 8AA, England, UK.
April 1, 2013 / Vol. 38, No. 7 / OPTICS LETTERS

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Guided-wave electro-optic characterization of BaTiO3 thin films using the prism coupling technique Floriane Leroy,1 Anthony Rousseau,2 Sandrine Payan,2 Elhadj Dogheche,1,* David Jenkins,3 Didier Decoster,1 and Mario Maglione2 1

2

Institut Electronique Microélectronique et Nanotechnologies (IEMN), UMR CNRS 8520, Avenue Poincaré, P.O. Box 60069, F-59652 Villeneuve d’Ascq Cedex, France

Institut de Chimie et de la Matiére Condensée Bordeaux (ICMCB), CNRS, Avenue du Docteur A. Schweiter, F-33608 Pessac, France 3

University of Plymouth, B333, Portland Square, Drake Circus, Plymouth, Devon PL4 8AA, England, UK *Corresponding author: elhadj.dogheche@univ‑valenciennes.fr Received January 21, 2013; revised February 23, 2013; accepted February 23, 2013; posted February 25, 2013 (Doc. ID 183947); published March 20, 2013

Ferroelectric BaTiO3 (BTO) thin films are grown by RF sputtering onto an indium tin oxide bottom electrode on a MgO single-crystal substrate. We have studied here the optical properties by the prism coupling technique. We report the ordinary and extraordinary refractive indices of the films, the film thickness, and the optical losses that are obtained on the planar waveguides: n0  2.224  0.001 and ne  2.219  0.001 at 1539 nm. Furthermore, in order to demonstrate the active property of the BTO films, we have investigated the electro-optic (EO) properties by using the change of the resonant coupling angle (variation of fundamental TE0 guided mode) when the transverse electric field is applied. The latter is induced by the refractive index variation (Δn) caused by the EO effect when a static electric field is applied transversely to the film. The EO coefficient obtain is about 18 pm∕V for TE mode and 23 pm∕V for TM modes at 1539 nm. This value illustrates the suitability of the BTO material thin film with a polycrystalline structure for applications such as modulations, switching, and interconnections. © 2013 Optical Society of America OCIS codes: 160.2100, 310.6860, 310.2790.

Complex ferroelectric oxide materials are extensively studied for use in the field of integrated optics because of their desirable optical properties and high electro-optic (EO) coefficients. Bulk single crystals are commonly used [1]. The development of guided-wave applications has concentrated on the fabrication of materials in thin film performing the same function as bulk, taking the advantage of geometrical flexibility and possible integration with semiconductor circuits [2]. Different ferroelectric thin films have been investigated for applications in optical waveguide devices, and the studies focused on the optimization of the growth process and the comprehensive study of the relationship between microstructures and optical properties [3–7]. For EO characterization, a widely used technique is based on transmission geometry, such as Sénarmont’s method and transmission ellipsometry, to monitor the birefringence change in the materials [8]. The limitation of these techniques is the inability to separate the electric-field-induced refractive index change along different optical axes of the materials, and thus the individual EO tensors coefficients. In this article, we report the investigation of the optical and the electro-optical properties of BaTiO3 (BTO) thin films using the prism coupling technique. Films were deposited by RF magnetron sputtering on indium tin oxide (ITO)/MgO substrates. Both TE and TM guided modes (m-lines) were excited, from which we inferred the thickness and the refractive indices of the planar waveguide. For electro-optic measurements, we used the angular shift of the guided modes induced by the refractive index variation when an electric field is applied. BaTiO3 thin films were grown on ITO-coated single-crystal MgO substrates by RF magnetron sputtering from ceramic targets. The experimental setup for the sputtering deposition has been optimized to assure the composition transfer between the target and the film giving stoichiometry 0146-9592/13/071037-03$15.00/0

(Table 1). The ITO electrode has a polycrystalline structure with a resistivity of 2.10−4 Ωcm and an optical transmittance of 90% in the visible and 60% at 1539 nm. In order to crystallize the material, it is necessary to densify the layer with a post-deposition annealing treatment. This process is very important to eliminate the porosities that are the main cause of scattering losses. The crystal structure has been analyzed by x-ray diffraction in the θ∕2θ configuration with a monochromatic Cu K α wavelength. The morphology of the film was examined using atomic force microscopy (AFM), as given in Fig. 1(a). The grain size is laterally measured from AFM in the range from 50 to 100 nm with an rms roughness of 4 nm. The film is polycrystalline and transparent, and the layer is dense, with a uniform thickness of 1.2 μm observed by SEM [Fig. 1(b)]. Optical indices and electro-optic coefficients of BTO thin films are determined using the same prism coupling method [9], with various lasers sources (450, 532, 633, 975, and 1539 nm) for TE and TM polarizations. For the EO investigation, we have used the following procedure: first, the refractive indices nTE and nTM of the film (with an accuracy of 0.001) and its thickness d are measured through a rutile (TiO2 ) prism without applying any electric field E. This can be considered as a reference. Considering the BTO sample sandwiched between the top and the bottom electrodes of ITO (deposited by magnetron sputtering), we have applied a static electric field across the film (Fig. 2). TE and TM mode spectra indicate a good light confinement within the waveguide (Fig. 3). The inset of Fig. 3 shows a typical guided optical mode spectrum: the observed intensity minima correspond to the TE mode coupled into the structure. Up to five modes (TE0 −TE4 ) could be excited in our sample. The angular width of the modes guided within the BTO layer are very narrow; this © 2013 Optical Society of America

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OPTICS LETTERS / Vol. 38, No. 7 / April 1, 2013

Table 1. Deposition Conditions of ITO Electrodes and BTO Thin Film Material −2

RF power (W cm ) Pressure (Pa) Oxygen content (%) Substrate temperature (°C)

ITO

BTO

1.5 0.5 0.5 50

2.5 5 1 650

Fig. 3. Dispersion of the refractive index ina BTO optical waveguide structure deposited on ITO/MgO substrate (for TE and TM polarizations).

where

Fig. 1. (Color online) (a) AFM and (b) SEM images of 1.2 μm BTO film deposited on ITO/MgO (001).

is an indication of the microstructure quality. Indeed, optical losses are generally related to the half-width of guided-wave modes. For our sample, using the fiber technique, optical losses are estimated to be less than 5 dB∕cm. From the angular position of the guided modes, we computed the corresponding effective indices. These values serve to calculate the thin-film refractive indices and thickness. The dispersion of TE and TM refractive indices is plotted in Fig. 3. We found no  2.224  0.001 and ne  2.219  0.001 at 1539 nm, which is quite close to the value reported in the literature [2]. The thickness value was determined to be 1.24 μm, which is in good agreement with the profilometer measurements. For an electric field applied to the optical axis of BTO thin film, the EO coefficients r 13 and r 33 are determined using TE polarization light (ordinary excitation) and TM polarization (extraordinary excitation), respectively. Since the effective indices N m of the guided modes of the structure strongly depend on the refractive index n of the guiding region, a variation in the refractive index, Δn, induces a shift of the synchronous angle given the following relation: Δα 

dα dα dN m Δn; Δn  dn dN m dn

1 Δn  n3 r c E; 2

(2)

α is the external incident angle, E is the amplitude of the applied electric field, and r c is the EO coefficient. In our study, we have considered the case of TE0 guided modes, where the electric field E is parallel to the surface of the sample; the variation Δd of the film thickness induced by the piezoelectric effect can be neglected. For TM guided modes, the electric field E is perpendicular to the film surface. In Fig. 4, we plot the angular shifts of the TE0 guided mode subject to an external electric field. Using the TE0 and TM0 guided modes, the EO response of BTO thin films is investigated by applying the static electric field [10,11] and we plotted the curve Δn versus E from −328 to 328 kV∕cm. Finally, the recorded data allow us to draw the linear variation of Δn versus the applied voltage, as shown in Fig. 5. From this analysis and using Eqs. (1), (2), we found r 13  18  0.33 pm∕V for TE0 mode and r 33  23  0.35 pm∕V for TM0 mode at 1539 nm. These values compare favorably with that reported by Tang et al. [12] (38 pm∕V), measured using a waveguide modulator configuration. Recent work from Wang et al. [13] using the same prism coupling technique with a metal-coated prism has shown extra large values for the electro-optic coefficients of Ba0.7 Sr0.3 TiO3 ; however these coefficients are overestimated based on the previous values

(1)

Fig. 2. Schematic cross section of the EO measurement setup using the prism coupling technique.

Fig. 4. Effective index variation in BTO obtained by applying a DC voltage.

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fundamental material study reveals the potential of BTO films to be used in integrated optical devices for optical modulations. The authors are grateful to D. Troadec (IEMN, Lille) for focused ion beam and SEM images.

Fig. 5. Variation of Δn as a function of the applied electric field.

published and those from our experiments. These values are actually lower compared to the EO properties of bulk BTO reference materials, with the constants in the range 1000–1500 pm∕V at 1539 nm [2]. The objective is to optimize the material quality in order for the thin-film EO properties to tend toward the bulk properties. This study shows the opportunity to develop such a thin-film ferroelectric material in comparison with the well-known LiNbO3 single crystal and clearly shows the suitability of BTO materials for use in future active optical devices [14–16]. In conclusion, polycrystalline thin films of BTO have been grown by RF sputtering on an ITO-electrode-coated MgO substrate. We have investigated the guided-wave optical properties of the films in a planar waveguide configuration. Using the prism coupling technique, the linear and nonlinear refractive indices are determined from the significant angular shifts of fundamental guided modes TE0 and TM0 as a function of the electric field. The measured EO coefficient r 13 is 18 pm∕V for TE, and r 33 is 23 pm∕V for TM at 1539 nm wavelength. This

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