1. Graduate Institute of Photonics and Optoelectronics, National Taiwan University, No.1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan
2. Research Center for Applied Sciences, Academia Sinica, No. 128, Sec. 2, Academia Rd., Taipei 11529, Taiwan
3. Department of Physics, National Taiwan University, No.1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan