1. Tianjin Key Laboratory of Advanced Electrical Engineering and Energy TechnologySchool of Electrical Engineering and AutomationTianjin Polytechnic University Tianjin 300387 P. R. China
2. Engineering Research Center of High Power Solid State Lighting Application System of Ministry of EducationInternational Research Center for PhotonicsTianjin Polytechnic University Tianjin 300387 P. R. China
3. Institut für PhysikOtto-von-Guericke-Universität Magdeburg Magdeburg 39106 Germany
4. Optoelectronics Research CentreUniversity of Southampton Southampton SO17 1BJ UK