1. School of Electronic and Optical EngineeringNanjing University of Science and Technology Nanjing Jiangsu 210094 China
2. Holonyak Micro and Nanotechnology LabUniversity of Illinois at Urbana‐Champaign Urbana IL 61801 USA
3. Collaborative Innovation Center of Advanced MicrostructuresNational Laboratory of Solid State MicrostructuresNanjing University Nanjing 210023 China