Two-photon spectroscopy of point defects

Resume.- Les informations experimentales de la spectroscopie ri deux photons dans les impuretes et les defauts dans les soiides sont ici r6sumCs. Les resultats fes plus importants sont la mesure de la probabilite d'absorption et I'attribution de la symetrie des transitions. Les effets non lineaires sont importants aussi dans d'autres cas cornme la creation des centres colores dans les cristaux d'halogenures alcalins. Abstract.- The experimental results on two-photon spectroscopy in impurities and defects in solids are reviewed. The main results are the measurement of absorption cross sections and in some cases the assignement of the symmetry of the transitions. Nonlinear effects can be of importance also in the process of colour centre formation in alkali halides crystals. I. Introduction.- Two-photon spectroscopy has recently become a powerful tool for the investiga-tion o f the optical properties of solids. Two-photon effects, predicted theoretically by Maria Goeppcrt-Mayer in 1931 [I], have been experimentally verifie