We examine the polarization memory effect for porous Si excited by lin
early polarized light. The various observations for the red-luminescin
g, slow band are discussed in the general framework of particle shape
asymmetry. We show that because of the intrinsically nonlinear lumines
cence response, measurement parameters influence the polarization resp
onse. The preparation of porous Si with photoassisted etching is found
to control the polarization retention parameter rho. Using linearly p
olarized light during etching produces in-plane asymmetries. We find a
substantial rho-anisotropy linked to crystal symmetry planes and axes
as a consequence of anisotropic etching. The effects are discussed wi
th reference to current models of the light emission mechanism.