1. Department of Materials Science and Engineering City University of Hong Kong Kowloon Hong Kong 999077 China
2. Department of Electrical Engineering City University of Hong Kong Kowloon Hong Kong 999077 China
3. Department of Electrical Engineering The Hong Kong Polytechnic University Hung Hom Hong Kong 999077 China
4. Department of Physics National Taiwan University Taipei 10617 Taiwan
5. Engineering Research Center of Optical Instrument and Systems Ministry of Education and Shanghai Key Lab of Modern Optical System University of Shanghai for Science and Technology No. 516 Jun Gong Road Shanghai 200093 P. R. China