1. a Institute of Atomic Physics, Solid State Quantum Electronics Laboratory , PO Box MG-36, Bucharest-Magurele , Romania , 76900
2. b Technical University of Denmark, Physics Department , Building 309, Lyngby , Denmark , 2800
3. c University of Bucharest, Faculty of Physics , PO Box MG-11, Bucharest-Magurele , Romania , 76900