1. Guangdong Provincial Key Laboratory of Electronic Functional Materials and Devices Huizhou University Huizhou Guangdong 516001 P. R. China
2. Institute of Nanophotonics Jinan University Guangzhou 511443 P. R. China
3. NEST Istituto Nanoscienze‐CNR and Scuola Normale Superiore Piazza S. Silvestro 12 Pisa I‐56127 Italy
4. Dipartimento di Fisica Università di Pisa Largo B. Pontecorvo 3 Pisa I‐56127 Italy