1. School of Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom
2. Fraunhofer Institute for Photonic Microsystems IPMS, Maria-Reiche-Str. 2, 01109 Dresden, Germany
3. Faculty of Physics and Technology, Donetsk National University, 24 Universitetskaya Street, Donetsk 83001, Ukraine
4. HGST, San Jose Research Center, 3403 Yerba Buena Road, San Jose, California 95135, USA