1. Université Clermont Auvergne, CNRS, Clermont Auvergne INP, Institut Pascal, Clermont-Ferrand F-63000, France
2. Department of Engineering Physics, McMaster University, Hamilton L8S4L7, Ontario, Canada
3. IBM Research Europe, Saumerstrasse 4, 8803 Rüschlikon, Switzerland
4. IES, Univ. Montpellier, CNRS, Montpellier F-34000, France