**Acknowledgements**

Author would like to thank his colleagues from FEMTO-ST, Besançon, Ms. Nathalie Chol‐ ley, Engineer, Dr. Abdelhamid Hmima and Dr. Yanne K. Chembo, CNRS senior researcher, for fruitful discussions. Author also acknowledges the French National Research Agency (ANR) grant "ANR 2010 BLAN 0312 02".
