The UV-induced positive and negative refractive index changes in GeO2-SiO2 films L. T. Zhang, J. Wang, W. F. Xie, Y. Hou, J. Zheng, W. Zheng and Y. S. Zhang Eur. Phys. J. Appl. Phys., 32 2 (2005) 105-108 Published online: 26 October 2005 DOI: 10.1051/epjap:2005076