1 Centre for Optical and Electromagnetic Research, Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics , South China Normal University , 510006 Guangzhou , China.
2 Experimental Biomolecular Physics, Department of Applied Physics , KTH Royal Institute of Technology , SE-106 91 , Stockholm , Sweden.
3 Department of Theoretical Chemistry and Biology , KTH Royal Institute of Technology , Roslagstullsbacken 15 , SE-106 91 Stockholm , Sweden.