1. Mid‐Infrared Photonics Group Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK
2. George Green Institute for Electromagnetics Research Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK
3. Nottingham Nanoscience and Nanotechnology Centre Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK
4. Wolfson Centre for Advanced Materials Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK
5. College of Physics and Electronic Science Changsha University of Science & Technology Changsha 410114 China