1. Naval Research Laboratory, Washington, DC, 20375-5320, Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, I-56127, Pisa, Italy, and polyLab, CNR-INFM, Largo B. Pontecorvo 3, I-56127, Pisa, Italy, and Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan