1. Department of Materials Science and Engineering; National Taiwan University; Taipei 10617 Taiwan
2. Center for Condensed Matter Sciences; National Taiwan University; Taipei 10617 Taiwan
3. Graduate Institute of Photonics and Optoelectronics; National Taiwan University; Taipei 10617 Taiwan
4. Department of Electrical Engineering; National Taiwan University; Taipei 10617 Taiwan
5. Institute of Atomic and Molecular Sciences; Academia Sinica; Taipei 10617 Taiwan