1. Crystal Lab of Laser Technology Research Center, 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. Advanced Laser Technology Laboratory of Anhui Province, Hefei 230037, P. R. China
4. Key Laboratory of Photonic Devices and Materials, Hefei 230031, Anhui Province, PR China