REFRACTIVE-INDEX CHANGES DURING PHOTODARKENING IN SEMICONDUCTOR-DOPEDGLASSES

The diffraction efficiency of gratings photoinduced in semiconductor-d oped glasses by interfering pump beams is used to demonstrate that ref ractive-index changes occur during the photodarkening process in such media. The refractive-index modifications are due to the static Kerr e ffect following the creation of a space-charge electric field that is photoinduced in the semiconductor nanocrystallites. This electric fiel d, 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 gr atings. A simple theory based on these ideas is in good agreement with the experimental results. Moreover, the present measured low diffract ion 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 m edia attractive for the purpose of recording permanent holograms.