Various three-dimensional devices that can be formed within the bulk of a semiconductor by photo-controlled selective etching are described herein. With more particularity, semiconductor devices that incorporate three-dimensional electrical vias, waveguides, or fluidic channels that are disposed within a semiconductor are described herein. In an exemplary embodiment, a three-dimensional interposer chip includes an electrical via, a waveguide, and a fluidic channel, wherein the via, the waveguide, and the fluidic channel are disposed within the body of a semiconductor element rather than being deposited on a surface. The three-dimensional interposer is usable to make electrical, optical, or fluidic connections between two or more devices.