[1] City Univ Hong Kong, Dept Phys & Mat Sci, 83 Tat Chee Ave, Kowloon Tong, Hong Kong, Peoples R China
[2] City Univ Hong Kong, Ctr Funct Photon, 83 Tat Chee Ave, Kowloon Tong, Hong Kong, Peoples R China
[3] Ludwig Maximilians Univ Munchen, Dept Phys, Amalienstr 54, D-80799 Munich, Germany
[4] Ludwig Maximilians Univ Munchen, CeNS, Amalienstr 54, D-80799 Munich, Germany