An electro-optic effect, in which a refractive index changes because of interaction between an electric field and a substance, provides a high-speed characteristic, a power-saving characteristic due to voltage drive, and structural simplicity, and therefore is applied to an optical modulator.
An optical modulator using LiNbO3 is structured by forming a Mach-Zender waveguide on a monocrystalline LiNbO3 substrate by a Ti diffusion method, and then combining electrodes therewith. This optical modulator changes a refractive index of the waveguide by applying a voltage, to thereby control ON/OFF of an optical signal. However, such an optical modulator is expensive because the monocrystalline substrate is necessary. Further, this optical modulator has a small electro-optic effect of LiNbO3 and has a planar electrode structure. Hence, a long waveguide is necessary, which greatly increases an element size to the scale of cm.
Pb1-xLax(ZryTi1-y)O3 (PLZT), which is transparent ceramic, has an electro-optic coefficient larger by approximately two digits than that of monocrystalline LiNbO3 used for the current optical modulator. Therefore, the use of PLZT raises expectations for a small-sized optical element, and higher cost efficiency, higher power efficiency, and higher speed due to the downsizing, and hence formation of a thin film of PLZT by a sol-gel method has been studied thus far (Non-patent Document 1 and Non-patent Document 2).
As an innovative technology expected in the future, a silicon photonic device that enables optic and electronic integration on one chip has been studied. When this technology is realized, an LSI such as a CPU and a memory, and an active optical element such as an optical switch may be formed on the same substrate, which leads to a higher-speed performance of the LSI. In addition, an inexpensive optical device may be attained because an LSI manufacturing technology may be applied to a manufacturing process for the optical communication device.
However, silicon is an indirect transition semiconductor, which makes it difficult to form a light-emitting element such as a laser device directly on a silicon substrate. Therefore, it is important to form an optical modulator for converting an electric signal into an optical signal on the silicon substrate. It is demanded that the optical modulator for LSI optical interconnection be driven at low voltage under LSI operating conditions, have high power efficiency, and be small.
As an optical modulator satisfying the demand, a ring resonator type structured by using silicon has been studied (Non-patent Document 3). The silicon-ring resonator changes a refractive index by injecting a carrier into a waveguide layer of light, and changes a resonant wavelength, to thereby perform modulation operation.    Non-patent Document 1: K. D. Preston and G. H. Haertling: Appl. Phys. Lett. 60 (1992) 2831.    Non-patent Document 2: K. Nashimoto, K. Haga, M. Watanabe, S. Nakamura and E. Osakabe: Appl. Phys. Lett. 75 (1999) 1054.    Non-patent Document 3: A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, M. Paniccia: Opt. Exp., 15 (2007) 660.    Non-patent Document 4: Yasuo Kokubun, Shigeru Yoneda, and Shinnosuke Matsuura: IEICE Trans. Electron. Vol. E81-C, No. 8 (1998) 1187.    Non-patent Document 5: H. Tanobe, Y. Kondo, H. Yasaka and Y. Yoshikuni: IEEE Photo. Technol. Lett. Vol. 8, No. 11, (1996) 1489.    Non-patent Document 6: Y. Inoue, A. Kaneko, F. Hanawa, H. Takahashi, K. Hattori and S. Sumida: Electron. Lett. Vol. 33, No. 23, (1997) 1945.