1. Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan
2. Department of Physics, National Taiwan University, Taipei 10617, Taiwan
3. Graduate Institute of Materials Science and Engineering and Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
4. Graduate Institute of Electronics Engineering and Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan