[1] Politecn Bari, DRIEI, Via Re David 200, I-70125 Bari, Italy
[2] Univ Minnesota, ECE, Minneapolis, MN 55455 USA
[3] Ludwig Maximilians Univ Munchen, Fak Phys, Koniginstr 10, D-80539 Munich, Germany
[4] Imperial Coll London, Dept Phys, London SW7 2AZ, England
[5] Ist Italiano Tecnol, CBN, Via Barsanti 14, I-73010 Arnesano, LE, Italy