[1] Ecole Polytech, Phys Mat Condensee Lab, F-91128 Palaiseau, France
[2] Univ Glasgow, Optoelect Res Grp, Glasgow G12 8LT, Lanark, Scotland
[3] Ecole Polytech Fed Lausanne, Inst Photon & Elect Quant, CH-1015 Lausanne, Switzerland
[4] Inst Opt, F-91403 Orsay, France