1. Laser Micro/Nano Fabrication LaboratorySchool of Mechanical EngineeringBeijing Institute of Technology Beijing 100081 P. R. China
2. School of Optics and PhotonicsBeijing Institute of Technology Beijing 100081 P. R. China
3. Beijing Engineering Research Center of Applied Laser TechnologyInstitute of Laser EngineeringBeijing University of Technology Beijing 100124 P. R. China
4. Department of Electrical and Computer EngineeringUniversity of Nebraska‐Lincoln Lincoln NE 68588‐0511 USA