1. ETH Zurich Department of Physics Institute for Quantum Electronics Optical Nanomaterial Group Auguste-Piccard Hof 1 CH-8093 Zurich Switzerland
2. Aix Marseille Univ CNRS Ecole Centrale Marseille Institut Fresnel 13397 Marseille France
3. ETH Zurich Department of Mechanical and Process Engineering Institute of Process Engineering Particle Technology Laboratory Sonneggstrasse 3 CH‐8092 Zurich Switzerland