Garnet/SOI Magneto-Optical Devices Fabricated by Direct Wafer Bonding
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of the sample surface increased from a value between –5.6 and –3 to a pKa of –1.2. The pore volume decreased after surface modification from 0.143 cm3/g to 0.083 cm3/g, indicating that the sulfonic groups covered the pore surface of the sample. The conductivity of the samples was measured at 95% and 100% relative humidity at temperatures ranging from 40°C to 120°C. The maximum value of conductivity was 4.2 × 10–2 S/cm at 120°C and 100% relative humidity. Under these conditions and after being in contact with methanol, the sample did not suffer any degradation. The research team reports that these results show the good mechanical and thermal stability of the material, making it a good candidate for electrolyte membranes in the direct methanol fuel cell. This ionic conductive material overcomes the limitations in operating temperature and water swelling of currently used proton conductive membranes such as Nafion. The researchers said that the thermal stability of their material originates from the thermal stability of porous glass and the covalent bond formed between the organic molecules and the glass surface. “By this process, we can obtain welldesigned organic–inorganic nano hybrid materials,” Yazawa said. MARIA CORTALEZZI
Garnet/SOI Magneto-Optical Devices Fabricated by Direct Wafer Bonding Silicon-on-insulator (SOI) is an attractive substrate for dense integrated optical circuits because of its high index contrast, which allows small, high-performance components such as waveguides, bends, splitters, and modulators. In addition, SOI substrates allow seamless integration with electronics for on-chip optical devices. Current integration schemes for garnet– semiconductor devices are insufficient due to the difficulty of alignment with active devices on the semiconductor substrate or small nonreciprocal phase shifts, requiring long device lengths. In the May 1 issue of Optics Letters (p. 941), R.L. Espinola of Columbia University, H. Dötsch of the University of Osnabrück, and their colleagues have reported an integrated magneto-optical (MO) device design consisting of a garnet film directly bonded to SOI waveguides. Theoretical predictions indicate their design can achieve a threefold enhancement of the nonreciprocal phase shift over previous designs. Optical isolators or circulators are important components for photonic integrated circuits. To date, neither has been developed in SOI or any integrated mateMRS BULLETIN/JUNE 2004
rials platform. Optical isolator research generally focuses on the MO effect in a magnetic garnet film. A promising concept depends on the nonreciprocal phase shift—the difference between the forward and backward propagation constants. To measure the nonreciprocal phase shift, the researchers fabricated thin Si waveguides onto an SOI wafer with e-beam lithography, thermal evaporation, and reactive ion etching. Bismuthlutetium-neodymium-substituted iron garnet (BiLuNd-IG) was deposited epitaxially on gadolinium gallium garnet, and then bonded to Si waveguides by using a direct wafer-bonding
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