1. Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama, Ikoma, Nara 630‐0101, Japan
2. Physics Department, Graduate School of Science, Tohoku University, Sendai 980‐8578, Japan
3. Organo‐Optic Research Laboratory, Seiwa‐dai 6‐2‐1, Kita‐ku, Kobe 651‐1121, Japan
4. Institute for Material Research, Tohoku University, Sendai 980‐8577, Japan
5. Present address: Faculty of Applied Chemical Engineering, Chonnam National University, Kwangju 500‐757, Korea