Near atomically smooth, void-free, centimeter-long amorphous silicon waveguides are produced by high pressure chemical fluid deposition in the pores of a microstructured optical fiber template (right inset). Semiconductor waveguides fabricated by conventional deposition/fabrication approaches have a surface roughness that is a constraining factor in most optoelectronic devices, but these waveguides conform to the extraordinarily geometrically perfect optical fiber pores (DIC image, bottom inset).