1. Department of Physics, National Taiwan University, Taipei 10617, Taiwan
2. Department of Engineering, University of Massachusetts Boston, Boston, Massachusetts 02125, USA
3. Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan
4. Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan
5. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan