Chapter 8 Influence of External Fields M.R. Wagner and A. Hoffmann Abstract The application of external fields provides a powerful tool to investigate a large variety of properties of excitons and exciton related processes. Within this chapter, we focus on the fundamental effects of static magnetic and strain fields on the optical properties of excitons in ZnO. The description is complemented by relevant examples. A general review of this topic can be found for constant fields in [Cho, Excitons, Topics in Current Physics, vol. 14 (Springer, Heidelberg, 1979)] and H¨ onerlage et al. [Phys. Rep. 124:161, 1985] and for modulation techniques in [Cardona, Modulation Spectroscopy (Academic, New York, 1969); Seraphin, Modu- lation Spectroscopy (North Holland, Amsterdam, 1973); Goldsmith, NATO Science Series II, Frontiers of Optical Spectroscopy, vol. 168 (Springer Netherlands, 2005)]. Not much has been published on the influence of static electric fields on excitons. A few references are given at the end of Sect. 8.2. 8.1 Excitons in Magnetic Fields The investigation of optical transitions of free and bound excitons in magnetic fields enables the determination of binding energies, electron/hole g-values, impurity type (donor or acceptor), and charge state of excitons and defect centers as well as the symmetry ordering of the valence bands. Exposed to a magnetic field B electrons and holes perform a cyclotron motion which leads to a cyclotron energy E c D„! c D„.eB=/; (8.1) where ! c is the cyclotron frequency and  is the reduced mass of the exciton. Depending on the binding energy of the free exciton and the strength of the magnetic A. Hoffmann Institut f¨ ur Festk¨ orperphysik der Technischen Universit¨ at, Berlin, Germany e-mail: hoffmann@physik.tuberlin.de M.R. Wagner Institut f¨ ur Festk¨ orperphysik der Technischen Universit¨ at, 10623 Berlin, Germany e-mail: markus.wagner@physik.tu-berlin.de 201