1. Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, 106 Taiwan, ROC, Department of Electrical Engineering, National Taiwan University, Taipei, 106 Taiwan, ROC, Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, 300 Taiwan, ROC, and Department of Materials Science and Engineering, National Taiwan University, Taipei, 106 Taiwan, ROC