1. Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan
2. Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
3. Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 300, Taiwan
4. Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 106, Taiwan
5. Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan
6. Department of Physics, University of Massachusetts, Boston, Massachusetts 02125, United States