1. Dept. of Photonics, Ritsumeikan Univ., 1‐1‐1 Noji‐Higashi, Kusatsu, Shiga 525‐8577, Japan
2. Core Technology Development Center, Core Technology & Network Company, Sony Corporation,4‐14‐1 Asahi‐cho, Atsugi, Kanagawa 243‐0014, Japan
3. Center for Promotion of the COE Program, Ritsumeikan Univ., 1‐1‐1 Noji‐Higashi, Kusatsu,Shiga 525‐8577, Japan