1. Department of Applied Mathematics and Computer Science, Technical University of Denmark; Kongens Lyngby 2800 Denmark
2. CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-Nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; Beijing 100083 P. R. China
3. School of Nanoscience and Technology, University of Chinese Academy of Sciences; Beijing 100049 P. R. China
4. Department of Photonics Engineering, Technical University of Denmark; Kongens Lyngby DK-2800 Denmark