1. Departamento de Ingeniería de Circuitos y Sistemas, EUIT de Telecomunicación, Universidad Politécnica de Madrid, Ctra de Valencia km 7, 28031 Madrid, Spain
2. Nonlinear Optics and OptoElectronics Laboratory (NooEL), Department of Electronic Engineering and INFM-CNISM, University "Roma Tre", Via della Vasca Navale 84, 00146 Rome, Italy
3. Instituto de Optica, C.S.I.C., Serrano 121, 28006 Madrid, Spain