Anomalous birefringence of light in free-standing samples of porous silicon

The birefringence of light in freely suspended samples of porous silicon is observed and investigated. The effect is interpreted as "shape birefringen ce," i.e., the effect caused by the structure of a material consisting of a nisotropic formations with sizes less than the wavelength of the light and with a predominant orientation. It is checked experimentally that the sampl es do not possess optical activity or optical anisotropy in the plane of th e porous-silicon film. It is determined that the effect is observed for pol arization of incident light that rules out the possibility of observing bir efringence in a uniform optical medium, and it is not observed in the conve ntional experimental geometry. Qualitative explanations are given for the a nomalous character of the observed defect. (C) 2000 MAIK "Nauka /Interperio dica".