Paloma Arroyo Huidobro is a Marie Curie Fellow in the Department of Physics at Imperial College of Science, Technology and Medicine, London. Antonio I. Fernández-Domínguez is a Ramón y Cajal Fellow in the Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), at Universidad Autónoma de Madrid, Spain. John B. Pendry works in the Department of Physics at Imperial College of Science, Technology and Medicine, London. Luis Martín-Moreno works in the Instituto de Ciencia de Materiales de Aragón and Departamento de Fisica de la Materia Condensada at Universidad de Zaragoza, Spain. Francisco J. García-Vidal works in the Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC) at Universidad Autónoma de Madrid, Spain.