1. Department of Applied Physics and Chemistry and Institute of Laser Science, The University of Telecommunications, Choufu-gaoka 1-5-1, Chofu, Tokyo, 182-8585 Japan, Institute of Laser Engineering, Osaka University, Yamadakami 2-6, Suita, 565-0871, Osaka, Japan, and Department of Electrophysics, National Chiao Tung University, 1001 Ta Hsueh Rd. Hsinchu 300, Taiwan