Investigation of the optical gap in Ge nanowires.*

We investigate the role of quantum confinement for the optical and electronic properties of Ge nanowires.~ Real space pseudopotentials constructed within density functional theory were used to solve the electronic structure problem.~ We predict the quasi-particle and optical gaps as a function of the diameter up to approximately 3 nm for wires oriented along the (110) and (111) directions.~ We compare our results to previous work on Si wires.~