1. JSPS Research Fellow, 8 Ichibancho, Chiyoda-ku, Tokyo 102-8472, Japan, Department of Applied Physics and Chemistry and Institute for Laser Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan, Department of Electrophysics, National Chiao-Tung University, Hsinchu 300, Taiwan, ICORP, JST, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan, and Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0971, Japan