Zweiquantenabsorption bei freien Exzitonen in Zinkoxid

The two photon absorption spectrum at the intrinsic absorption edge of ZnO is measured with various directions and modes of polarization of the incident light beams. Several peaks are observed, corresponding to the 2P _0,±1-exciton states of the 3 seriesA, B, C (which occur on account of the threefold upper valence band). The anisotropy of the product of reduced effective mass times dielectric constant determined from the line positions is compatible with theoretical predictions. The lack ofS-exciton states in the two quantum absorption spectrum of ZnO is discussed in detail.