We demonstrate a nonmechanical, on-chip optical beam-steering device using a photonic-crystal waveguide with a doubly periodic structure that repeats the increase and decrease of the hole diameter. We fabricated the device using a complementary metal-oxide-semiconductor process. We obtained a beam-deflection angle of 24° in the longitudinal direction, while maintaining a divergence angle of 0.3°. Four such waveguides were integrated, and one was selected by a Mach-Zehnder optical switch. We obtained lateral beam steering by placing a cylindrical lens above these waveguides. By combining the lateral and longitudinal beam steering, we were able to scan the collimated beam in two dimensions, with 80 × 4 resolution points.

The doubly periodic Si photonic crystal waveguide radiates the guided slow light into free space as an optical beam. The waveguide also functions as a beam steering device, in which the steering angle is changed substantially by a slight variation in the wavelength generated due to the large angular dispersion of the slow light. A similar function is obtained when the wavelength is fixed and the refractive index of the waveguide is changed. In this study, we tested two kinds of integrated heater structures and observed the beam steering using the thermo-optic effect. For a p-i-p doped waveguide, the heating current was made to flow directly across the waveguide and a beam steering range of 21° was obtained with a relatively low heating power and high-speed response of the order of 100 kHz, maintaining a narrow beam divergence of 0.1-0.3° and a 120 resolution points. We also performed a preliminary life test of the device but did not observe any severe degradation in the temperature variation of 80-430 K for the duration up to 20‒40 h. For a TiN heater device, we obtained the comparable beam steering characteristics, but the required heating power increased, and the response speed decreased drastically.

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As a candidate of highly-sensitive hydrogen sensors, Palladium (Pd)-decorated graphene sensors have been investigated. Thanks to high reactivity of Pd nanoparticles to hydrogen and high surface-to-volume ratio of two-dimensionally layered graphene, graphene decorated with Pd nanoparticles can be used as a highly sensitive hydrogen sensor. Although the conductivity of Pd-graphene sensor is known to be greatly changed in hydrogen atmosphere, the mechanisms of conductivity change have not been fully understood. In this work, both carrier concentration and mobility in hydrogen are investigated using Hall effect measurement. It is shown that relative change of carrier concentration is greater when carrier concentration is lower. On the other hand, mobility is not systematically changed as a function of carrier concentration. This mobility behavior is attributed to the competition between greater effective mass and shorter screening length at higher carrier concentration. Based on these observations, we develop strategy for achieving high sensitivity in Pd graphene hydrogen sensor.

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