1. Department of Materials Science & Engineering, National Tsing Hua University, No. 101, Sec. 2 Kuang-Fu Road, Hsinchu 30013, Taiwan
2. Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan
3. Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China