1. Department of Applied Physics, KTH Royal Institute of Technology, SE-10691Stockholm, Sweden
2. Department of Theoretical Chemistry and Biology, KTH Royal Institute of Technology, SE-10691Stockholm, Sweden
3. Wallenberg Wood Science Center, Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, SE-10691Stockholm, Sweden
4. Department of Physics and Astronomy, Uppsala University, UppsalaSE-75120, Sweden