IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 8, AUGUST 2004 1837 Low-Loss Electrooptic BaTiO Thin Film Waveguide Modulator Pingsheng Tang, D. J. Towner, Anthony L. Meier, and B. W. Wessels Abstract—A strip-loaded electrooptic waveguide modulator based on an epitaxial BaTiO thin film was fabricated and char- acterized for the first time. The strip-loaded waveguide structure greatly improves waveguide propagation and polarization-de- pendent loss performance. A propagation loss of 1.1 dB/cm and polarization dependent loss of 0.1 dB/cm were measured. The electrooptic waveguide modulator exhibited a half-wave voltage-interaction length product of 4.5 V cm at a wavelength of 1542 nm. The measured effective electrooptic coefficient of the as-grown BaTiO waveguide modulator was 38 pm/V. The exper- imental results indicate that a strip-loaded thin film waveguide modulator is suitable for photonic applications. Index Terms—BaTiO , electrooptic modulator, loss, thin film, waveguide. I. INTRODUCTION E LECTROOPTIC waveguide modulators are essential for high-speed optical communication systems and ultrafast information processing applications. Ferroelectric materials are very attractive for electrooptic modulators due to their large electrooptic coefficients, excellent optical transparency, and thermal stability [1], [2]. The ferroelectric oxide LiNbO has been widely utilized for low-loss low-drive voltage high-speed modulators. Low-loss optical waveguides can be produced by locally modifying the composition or stoichiometry in the LiNbO substrate through diffusion or ion-exchange processes [3]. Other ferroelectric materials with much higher electrooptic coefficients could lead to improved electrooptic modulators with lower drive voltages. Of particular interest is the ferro- electric BaTiO , which has electrooptic coefficients as high as 1300 pm/V in the bulk [2]. Furthermore, BaTiO thin films can be integrated with silicon using a thin film buffer layer of MgO [4]. Thin film BaTiO electrooptic modulators have been demonstrated by several groups [5]–[7]. Ridge waveguide modulators that could be modulated out to 20 GHz have been reported, but the waveguides have relatively large propaga- tion loss (3–4 dB/cm) and high polarization-dependent loss (5 dB/cm) [6]. This resulted from surface and sidewall rough- ness induced during etching of the BaTiO ridge waveguide. To decrease waveguide loss and lower polarization-dependent Manuscript received February 23, 2004; revised April 4, 2004. This work was supported by the Air Force under Contract AFRL-33615-02-C-5053, and by the National Science Foundation (NSF) under Contracts ECS-0123469 and DMR-0076977. The authors are with the Department of Materials Science and Engineering and Materials Research Center, Northwestern University, Evanston, IL 60208 USA (e-mail: b-wessels@northwestern.edu). Digital Object Identifier 10.1109/LPT.2004.831255 Fig. 1. Schematic diagram of the electrooptic waveguide modulator structure. losses a strip-loaded waveguide structure was recently devel- oped [8]. The waveguides are formed from a BaTiO film on a MgO substrate with Si N as the loading strip. These strip-loaded waveguides show propagation losses of less than 0.9 dB/cm and a polarization-dependent loss of 0.1 dB/cm. The BaTiO –MgO serves as a composite dielectric medium that lowers the effective microwave index [9] enabling velocity match between the microwave and optical waves needed for high-speed traveling wave operation. In this letter, a low-loss Si N strip-loaded BaTiO thin film waveguide electrooptic modulator is reported. A half-wave voltage-interaction length product of 4.5 V cm at 1542 nm was achieved. Standard reactive ion etching techniques can be utilized to obtain the Si N strip without deteriorating the BaTiO surface. Use of the strip-loaded waveguide structure should enable fabrica- tion of a wide variety of electrooptic devices using standard Si–SiO –Si N technology [10]. II. FABRICATION The modulator was fabricated from epitaxial BaTiO on MgO. The BaTiO thin film was epitaxially deposited on a MgO(100) substrate using a low-pressure metal–organic chem- ical vapor deposition (MOCVD) process [11]. The BaTiO thickness was controlled by MOCVD growth rate and time. A root mean square surface roughness of 7.5 nm for the BaTiO film was measured by atomic force microscopy analysis. A schematic of the strip-loaded waveguide modulator is shown in Fig. 1. The BaTiO film has a much higher refractive index than the Si N and the MgO substrate. The refractive indexes are 2.30, 2.01, and 1.70 at 1550 nm, respectively. In order to keep the waveguide single mode, the Si N strip is designed to be 4 m wide and 125 nm thick. The BaTiO thin film is 560 nm thick. The selected dimensions also provide polarization-insen- sitive waveguide operation. The 125-nm-thick Si N layer was deposited on the BaTiO thin film using a plasma-enhanced chemical vapor deposition process. The waveguide region 1041-1135/04$20.00 © 2004 IEEE