1. School of Optics and CREOL, University of Central Florida, 4000 Central Florida Boulevard, Orlando, FL 32826–2700, USA.
2. Physics Department and Solid State Institute, Technion–Israel Institute of Technology, Haifa 32000, Israel.
3. Electrical Engineering Department and Center for Photonics and Opto-Electronic Materials (POEM), Princeton University, Princeton, NJ 08544, USA.