1. Department of Electronic Engineering, Okayama University of Science, Ridai-cho, Okayama 700-0005
2. Department of Electrical and Electronic Engineering, Utsunomiya University, Utsunomiya 321-8585
3. Department of Physical Technologies and New Materials, Frascati Research Center, ENEA, Via E. Fermi 45, Frascati, 00044 Rome, Italy