1. School of Physics, Aristotle University of Thessaloniki 1 , GR-54124 Thessaloniki, Greece
2. Nanostructured Materials and Devices Laboratory, School of Engineering, University of Cyprus 2 , P.O. Box 20537, Nicosia 1678, Cyprus
3. Laboratory of Ultrafast Science, School of Physical Sciences, University of Cyprus 3 , P.O. Box 20537, Nicosia 1678, Cyprus