Excitons Under the Influence of External or Internal Fields

Application of external fields is a well suited strategy to reveal the properties of excitons in semiconductors. It further leads to new phenomena which are interesting for device applications. The external field can be applied temporally constant [79E1, 85H1] or as systematic perturbation in modulation techniques [69C1, 73S1, 04G1]. We will restrict our discussion here to the presentation of general features and of some selected topics and examples. We consider the influence of magnetic, electric and strain fields on the optical properties of excitons including their continuum states and we proceed again from bulk materials to structures of lower quasi-dimensionality. Mostly these fields are applied from outside, but there are also cases of internal fields, e.g., strain fields due to lattice misfit in epitaxy or piezo-electric fields resulting from such strain.