1. Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon999077, Hong Kong
2. Photonics Research Institute, The Hong Kong Polytechnic University, Hung Hom, Kowloon999077, Hong Kong
3. Department of Energy and Process Engineering, Norwegian University of Science and Technology, Høgskoleringen 1, 7491Trondheim, Norway