1. Beijing Advanced Innovation Center for Biomedical EngineeringSchool of Mechanical Engineering and AutomationBeihang University Beijing 100191 China
2. Department of PhysicsHarbin Institute of Technology Harbin 150001 China
3. Key Laboratory of Terahertz OptoelectronicsMinistry of EducationBeijing Key Laboratory of Metamaterials and DevicesBeijing Advanced Innovation Center for Imaging TechnologyDepartment of PhysicsCapital Normal University Beijing 100048 China