REFRACTIVE-INDEX CHANGES DURING PHOTODARKENING IN SEMICONDUCTOR-DOPED GLASSES

The diffraction efficiency of gratings photoinduced in semiconductor-doped glasses by interfering pump beams is used to demonstrate that refractive-index changes occur during the photodarkening process in such media. The refractive-index modifications are due to the static Kerr effect following the creation of a space-charge electric field that is photoinduced in the semiconductor nanocrystallites. This electric field, which is created by the trapping of one electron in the host glass matrix and one hole in the nanocrystallite, is in fact annihilated if A second electron - hole pair is created in the dot, making the space-charge field and photodarkening effects quite different for writing gratings. A simple theory based on these ideas is in good agreement with the experimental results. Moreover, the present measured low diffraction efficiencies (a few 10(-4)) could probably be greatly increased if a high static electric field were applied to the glass sample during the grating writing process, thus perhaps making semiconductor-doped media attractive for the purpose of recording permanent holograms.