1. Graduate
Institute of Electro-Optical Engineering and Department of Electronic
Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan R.O.C.
2. Scientific
Research Division, National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan R. O. C.
3. Department
of Physics, Tamkang University, Tamsui 251, Taiwan R.O.C.