The present invention is notably directed to an electro-optical device. The latter comprises a layer structure with: a silicon substrate; a buried oxide layer over the silicon substrate; a tapered silicon waveguide core over the buried oxide layer, the silicon waveguide core cladded by a first cladding structure; a bonding layer over the first cladding structure; and a stack of III-V semiconductor gain materials on the bonding layer, the stack of III-V semiconductor gain materials cladded by a second cladding structure. The layer structure is configured to optically couple radiation between the stack of III-V semiconductor gain materials and the tapered silicon waveguide core. The first cladding structure comprises a material having: a refractive index that is larger than 1.54 for said radiation; and a bandgap, which, in energy units, is larger than an average energy of said radiation.