1. Department of Optoelectronic and Materials TechnologyNational Taiwan Ocean University Keelung 202 Taiwan
2. Department of PhysicsNational Taiwan University Taipei 106 Taiwan
3. Department of Physics and Center for NanotechnologyChung Yuan Christian University ChungLi 32023 Taiwan
4. Institute of Optoelectronic SciencesNational Taiwan Ocean University Keelung 202 Taiwan