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 birefringence,” i.e., the effect caused by the structure of a material consisting of anisotropic formations with sizes less than the wavelength of the light and with a predominant orientation. It is checked experimentally that the samples do not possess optical activity or optical anisotropy in the plane of the porous-silicon film. It is determined that the effect is observed for polarization of incident light that rules out the possibility of observing birefringence in a uniform optical medium, and it is not observed in the conventional experimental geometry. Qualitative explanations are given for the anomalous character of the observed defect.