1. Department of Photonics Engineering , Technical University of Denmark , DK-2800 Kongens Lyngby , Denmark
2. Center for Silicon Photonics for Optical Communications , Technical University of Denmark , DK-2800 Kongens Lyngby , Denmark
3. Department of Health Technology , Technical University of Denmark , DK-2800 Kongens Lyngby , Denmark
4. Wuhan National Laboratory for Optoelectronics , Huazhong University of Science and Technology , 430074 Wuhan , China
5. Center for Nanostructured Graphene , Technical University of Denmark , DK-2800 Kongens Lyngby , Denmark