1. Department of Electrophysics, National Chiao-Tung University, Hsinchu 300, Taiwan; ICORP, Japan Science and Technology Agency, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, 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; and Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0971, Japan