ABSTRACTWe first present an analysis of the band line-up in the case of SiGe/Si quantum wells and in the case of SiGe/Si self-assembled islands. The conduction and valence band diagrams are obtained from a 30 band k.p Hamiltonian which allows to describe simultaneously conduction and valence band states. The strain field is obtained from a microscopic valence force field theory. The band edge alignment is strongly dependent on the input parameters for this heterosystem. We determine the average valence band offset from photoluminescence measurements of heterostructures grown on relaxed SiGe buffer layers. A type II band line-up is calculated for all Ge compositions in the case of two-dimensional quantum wells and SiGe/Si self-assembled islands. The 30-band formalism allows the determination of the near-infrared interband recombination energy as a function of the self-assembled island structural parameters. We then present recent results obtained by embedding SiGe/Si self-assembled islands in two-dimensional photonic crystals. The photoluminescence of GeSi islands acts as an internal probe to characterize the optical properties of silicon-based two-dimensional photonic crystals designed for the near-infrared spectral range. Cavities, defect-free photonic crystals operated at the second Bragg order and two-dimensional photonic crystals fabricated on top of one-dimensional Bragg mirrors (2D + 1D) are described. We show that, in the case of 2D +1D structures, we can control the quality factor of optical modes at the second Bragg order by matching the resonance conditions and controlling the thickness of the layers. Photonic crystals with pure Ge layers are finally described.