Optical non-linearities and excitation dynamics in II–VI bulk and epitaxial materials



Excitonic processes in the band edge regime determine the optical properties of II–VI bulk and novel epitaxial materials. The interdependences of excitonic systems (i.e. free and bound excitons), high-density systems (e.g. biexcitons) and the transition into the electron-hole plasma state are of high complexity and depend sensitively on sample quality and purity, dopant concentration, excitation condition and density, etc. Resonant excitation spectroscopy and time-resolved analysis of creation and decay processes provide valuable experimental access to clarification of the above-mentioned mutual interactions. In this paper recent results obtained using these methods are surveyed.

In the first part the development of luminescence and resonant excitation of bound exciton systems is treated under various excitation densities, for high excitation levels accompanied by biexciton formation and exciton–exciton collision processes. The specific properties observable when using heteroepitaxial structures instead of conventional bulk samples are discussed.

In the second part the time characteristics of excitonic transitions are evaluated for various impurities, dopants and dopant concentrations, excitation via particular resonant excitation channels, and various excitation densities. Relaxation and conversion channels between excitonic systems are analysed, in particular in strained heteroepitaxial systems which show splitting effects of the bands from which the carriers stem. Time-resolved analysis is demonstrated to be extremely helpful for the analysis of unknown excitonic systems and transitions. Methods of varying the characteristic time constants are discussed, in particular with regard to intentional changes in impurity contents and excitation densities which are interesting for any application.