1. Photonics & Optoelectronics Group, Physics Department and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Amalienstrasse 54, 80799 München, Germany, Nanomanufacturing Technologies Division, Centro Ricerche Fiat, Strada Torino 50, 10043 Orbassano, Italy, and Institute of Composite and Biomedical Materials, National Research Council, Piazzale Tecchio, 80125 Napoli, Italy