In modern epitaxial growth techniques, the control of the electronic and optical properties of a semiconductor heterostructure along the growth direction is achieved easily through layer-by-layer deposition of materials with different chemical composition and thickness.[1, 2] On the contrary, the control of those material properties in the growth plane, which is required to fabricate 1D or 0D nanostructures, is not easy to attain. First, so called “top-down” methods were employed to achieve a lateral modulation of the material bandgap through chemical removal of parts of the specimen defined by lithographic processes. In this way, highly uniform quantum dots, wires, and rings have been prepared and studied starting from quantum-well heterostructures.[3] However, the optical quality of those nanostructures was poor because of the air-exposed sidewalls produced by chemical attack. On the other hand, three-dimensional nanometer-sized aggregates with high optical efficiencies may form spontaneously by self-assembly in highly strained heterostructures (“bottom-up” methods).[4, 5] The lack of control in the spatial arrangement of these self-assembled structures hampers severely, however, their full exploitation in truly 0D devices.[6] These drawbacks limit the freedom of modulating the in-plane optical properties of a heterostructure, a feature highly desirable, for example, in designing optical circuits, where materials with different bandgaps and refractive indexes are combined on the same chip. As a matter of fact, monolithic integration of passive and active optical elements (as planar optical waveguides, couplers, and lasers) requires skilful and time …