1. Department of Electrical and Electronic Engineering; The University of Hong Kong; Pok Fu Lam Road Hong Kong
2. Department of Physics; The Hong Kong University of Science & Technology; Clear Water Bay Kowloon Hong Kong
3. Laboratory for Photonics and Interfaces; Institute of Chemical Sciences and Engineering; School of Basic Sciences; Ecole Polytechnique Fédérale de Lausanne; CH-1015 Lausanne Switzerland