1. Dipartimento di Fisica, Università della Calabria, Rende 87036, Italy
2. CNR Nanotec, Università della Calabria, Rende 87036, Italy
3. Institute of Applied Physics, Military University of Technology, 2 Kaliskiego Str., Warsaw 00-908, Poland
4. Department of Electrical and Computer Engineering, George Washington University, Washington, D.C. 20052, United States
5. Optoelectronics, Istituto Italiano di Tecnologia, Via Morego 30, Genova 16163, Italy
6. Department of Physics and Materials Science, University of Luxembourg, Luxembourg 1511, Luxembourg
7. Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China