1. Contribution from the NTT Basic Research Laboratories, 3-1 Wakamiya, Morinosato, Atsugi, Kanagawa 243-0198, Japan, Ritsumeikan University, Research Organization of Science and Engineering, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan, and CREST-JST (Japan Science and Technology Corporation), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan