1. School of Physics, Beijing Institute of Technology, Beijing 100081, P. R. China
2. Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan
3. Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and Advanced Materials and Bioengineering Research (AMBER), School Chemistry, Trinity College, Dublin, Ireland
4. Elementary Educational College, Beijing key Laboratory for Nano-Photonics and Nano-Structure, Capital Normal University, Beijing 100048, P. R. China
5. Department of Electronics Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 807, Taiwan
6. Department of Physics, National Taiwan University, Taipei 106, Taiwan