1. Quantum Engineering Laboratory, Department of Physics, Tamkang University, Tamsui Town, Taipei 251, Taiwan
2. Department of Electronic Engineering, Lee-Ming Institute of Technology, Taishan Township, Taipei 243, Taiwan
3. Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
4. Department of Electrical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan