1. Department of Electrical and Electronic Engineering, University of Manchester, M13 9PL Manchester, U.K.
2. State Key Laboratory of Crystal Materials, Shandong Technology Centre of Nanodevices and Integration and School of Microelectronics, Shandong University, 250101 Jinan, China
3. Photon Science Institute, Department of Electrical and Electronic Engineering, University of Manchester, M13 9PL Manchester, U.K.
4. Department of Engineering Sciences, Ångström Laboratory, Uppsala University, 75121 Uppsala, Sweden