An optical semiconductor device comprises, on a substrate, a fin of diamond-cubic semiconductor material and, at the base of the fin, a slab of that semiconductor material, in a diamond-hexagonal structure, that extends over the full width of the fin, the slab being configured as an optically active material. This semiconductor material can contain silicon. A method for manufacturing the optical semiconductor device comprises annealing the sidewalls of the fin, thereby inducing a stress gradient along the width of the fin.