1. Advanced Laser Technology Laboratory of Anhui Province, Anhui Institute of Optics and Fine Mechanics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, P. R. China
2. University of Science and Technology of China, Hefei 230026, P. R. China
3. Anhui Provincial Key Laboratory of Photonics Devices and Materials, Hefei 230031, P. R. China
4. Key Laboratory of Solid-State Laser Technology, North China Research Institute of Electro-Optics, Beijing 100015, P. R. China