1. Graduate Institute of Electro‐Optical Engineering & Department of Electrical Engineering, National Taiwan University, Taipei, 106‐17 Taiwan
2. Centre for Optoelectronics, Department of Electrical & Computer Engineering, National University of Singapore, Singapore 119260
3. Current address: Paul‐Drude‐Institut für Festkörperelektronik, 10117 Berlin, Germany
4. Department of Physics, Fisk University, Nashville, TN 37208, USA