1. Department of Electronics and Nanoengineering, Aalto University, Espoo FI-02150, Finland
2. QTF Centre of Excellence, Department of Applied Physics, Aalto University, Espoo FI-00076, Finland
3. Key Laboratory of In-Fiber Integrated Optics of Ministry of Education, College of Physics and Optoelectronic Engineering, Harbin Engineering University, Harbin 150001, China