C. P. Huang,1,2,* X. G. Yin,3 Y. Zhang,2 S. B. Wang,1 Y. Y. Zhu,4 H. Liu,4 and C. T. Chan1,† 1Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China 2Department of Applied Physics, Nanjing University of Technology, Nanjing 210009, China 3State Key Laboratory of Millimeter Waves, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China 4National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China (Received 29 January 2012; revised manuscript received 5 May 2012; published 6 June 2012)