1. Department of Physics, Santa Clara University, 500 El Camino Real, Santa Clara, California 95053-0315, USA
2. Institute of Applied Physics, Academy of Sciences of Moldova, Academiei str. 5, MD 2028 Chisinau, Moldova
3. Departments of Electrical Engineering and Computer Science, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, Wisconsin 53201, USA