1. Korea Photonics Technology Institute, Gwangju 500-779, South Korea
2. Department of Electrical and Computer Engineering, University of Illinois, Urbana, Illinois 61801, United States
3. Korea Institute of Science and Technology, Seoul 136-791, South Korea
4. School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science & Technology (UNIST), Ulsan 689-798, South Korea
5. KIST-UNIST Ulsan Center for Convergent Materials (KUUC), Ulsan 689-798, South Korea