[1] Nanyang Technol Univ, Sch Math & Phys Sci, Div Phys & Appl Phys, Singapore 637616, Singapore
[2] Natl Univ Singapore, Dept Mat Sci & Engn, Singapore 117576, Singapore
[3] Wuhan Univ, Dept Phys, Key Lab Acoust & Photon Mat & Devices, Minist Educ, Wuhan 430072, Peoples R China