devant le jury compose ́ de:

group. It is an honour for me to finally have this chance to thank those who made this thesis possible. My deepest gratitude goes to my advisor Mr. Olivier MULLER, who made avail-able his support in a number of ways. Just after my arrival, Olivier provided me with a series of good quality tutorials which helped me quickly integrate into the group and set up a good working manner. During the four years, I was always appreciating the constructive and interesting discussions with him. He constantly encouraged me to develop my own ideas and to have more self-confidence. When I lost myself into certain details, his strong sense of priorities and capacity of struc-turing ideas prevented me many times from sticking into the mud of trivialities. I highly valued his practical writing advices for my manuscript, and his useful com-munication tips for my defence presentation. Especially, I am grateful for his pa-tience and efforts for aiding me in coming out of my shell of shyness. In addition, Olivier spared his precious time to help me with all the administrative procedures, which allows me to concentrate to the research work