1. Institute of Photonics and Nanotechnology, Vilnius University, Sauletekio av. 3, LT-10257 Vilnius, Lithuania
2. Center for Physical Sciences and Technology, Sauletekio av. 3 LT-10257 Vilnius, Lithuania
3. Institute of Technical Physics, Faculty of Materials Science and Applied Chemistry, Riga Technical University, P. Valdena 3/7, Riga LV-1048, Latvia
4. Center for Condensed Matter Sciences and Graduate Institute of Electronic Engineering, National Taiwan University, Roosevelt Road No. 1, Section 4, Taipei 10617, Taiwan