Via electrochemical etching of silicon, followed by materials conversion, 3D gradient refractive index micro-optics were formed.  Elements including flat lenses, Bragg mirrors, polarization sensitive optical splitters and structures with nearly arbitrary refractive index distributions were formed with a particular focus on micro-optics important for solar energy harvesting.  The conversion from silicon to silica and titania enabled the optics to operate in the visible with minimal loss, something particularly important for solar energy harvesting applications.