1. Center for Nanophotonics, FOM Institute for Atomic and Molecular Physics AMOLF, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands, Philips Research Laboratories, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands, Materials Innovation Institute, 2628CD Delft, The Netherlands, and Institute for Molecules and Materials, Solid State Chemistry Department, Radboud University Nijmegen, Toernooiveld 1, 6525ED Nijmegen, The Netherlands