1. Shaanxi Joint Lab of Graphene, State Key Lab Incubation Base of Photoelectric Technology and Functional Materials, International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics and Photon‐TechnologyNorthwest University Xi'an 710069 P. R. China
2. Guangxi Key Laboratory of Automatic Detecting Technology and InstrumentsGuilin University of Electronic Technology Guilin 541004 P. R. China
3. The Institute of OpticsUniversity of Rochester Rochester NY 14627 USA