Optical investigations of oxygen ordering and persistent photo-doping in tetragonal YBCO by Widder, K, Munzel, J, Goppert, M, Luerssen, D, Becker, R, Dinger, A, Geserich, H, Klingshirn, C, Klaser, M, Muller-Vogt, G, Geerk, J, Burlakov, V