1. Antennas Group-TERALAB, Universidad Pública de Navarra, Campus Arrosadía, 31006 Pamplona, Spain
2. Optical and Semiconductor Devices Group, Department of Electrical and Electronic Engineering, Imperial College London, London SW7 2BT, United Kingdom
3. Centre for Plasmonics and Metamaterials, Imperial College London, London SW7 2AZ, United Kingdom
4. Centre for Terahertz Science and Engineering, Imperial College London, London SW7 2AZ, United Kingdom
5. Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA