[1] Trinity Coll Dublin, Sch Phys, Semicond Photon Grp, Dublin 2, Ireland
[2] Trinity Coll Dublin, CRANN, Dublin 2, Ireland
[3] Huazhong Univ Sci & Technol, Wuhan Natl Lab Optoelect, Wuhan 430074, Peoples R China
[4] McGill Univ, Dept Elect & Comp Engn, Montreal, PQ H3A 0E9, Canada