For my parents,

I would like to start by thanking my advisor, Prof. Lúıs Rodrigues, not only for this opportunity, but also for his guidance and advices during the elaboration of the thesis. I also want to thank Paolo Romano and Nuno Carvalho, for all the help and support they provided throughout the development of this work. Finally, I wish to thank my family and friends for their endless support, encouragement and patience