1. State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, School of Electronics and Information TechnologySun Yat‐sen University Guangzhou 510275 China
2. Institute for Integrative NanosciencesLeibniz IFW Dresden Helmholtzstrasse 20 01069 Dresden Germany
3. Department of Electrical and Computer EngineeringCarnegie Mellon University Pittsburgh PA 15213 USA
4. Physical Measurement LaboratoryNational Institute of Standards and Technology Gaithersburg MD 20899 USA