1. Department of Physics, University of Cordoba, Campus Universitario de Rabanales, E-14071 Córdoba, Spain (J.M.P., J.T., A.G., A.S.); Department of Optics, University of Valladolid, Campus Universitario La Merced, E-47072 Valladolid, Spain (M.A.G.); and Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MD Eindhoven, The Netherlands (J.J.A.M.v.d.M.)