[1] Department of Microelectronics, Phys. Electron. Photonics, M.C.C., Chalmers University of Technology, Fysikgränd 3, S-412 96 Göteborg, Sweden
[2] National Lab. for Infrared Physics, Shanghai Inst. of Technical Physics, Chinese Academy of Sciences, 500 Yutian Road, 200083 Shanghai, China
[3] Inst. for Microstructural Sciences, National Research Council, Ottawa, Ont. K1A 0R6, Canada