1. School of Science; China University of Geosciences; Beijing 100083 P. R. China
2. Department of Physics; Chinese University of Hong Kong; Hong Kong
3. Beijing Key Laboratory of Optical Detection Technology for Oil and Gas; China University of Petroleum; Beijing 102249 P. R. China
4. School of Materials Science and Technology; China University of Geosciences; Beijing 100083 P. R. China
5. Department of Mechanical and Automation Engineering; Chinese University of Hong Kong; Hong Kong