The influence of size and shape for nanocrystallines (NC) of semiconducting materials (like GaN, ZnO, US, ZnS with sizes about 20-30 nm) incorporated into olygoetheracrylate polymer matrices on photoinduced optical properties, particularly the two-photon absorption (TPA), was investigated. We have shown that maximal values of the TPA diagonal tensor component was observed at low temperatures for the GaN NC (about 4.2 K), which corresponds to a NC concentration of 2.6-3.2 wt.% and size of about 24 nm. The shape and value of ellipticity play crucial roles in the observed non-linear optical phenomena. This contradicts the generally adopted model wherein the non-linear optical properties are mainly determined by averaged NC sizes. The observed temperature dependence is explained within a framework of anharmonic electron-phonon interaction in the large-sized nanocrystallites. (c) 2005 Elsevier B.V. All rights reserved.