Time-resolved spectroscopy of low-dimensional semiconductor structures

This dissertation is a survey of ultrafast time-resolved optical measurements conducted on a variety of low-dimensional semiconductor systems to further the understanding of the dynamic behavior in the following systems: ZnMnTe/ZnSe quantum dots, ZnTe/ZnMnSe quantum dots, InGaAs quantum wells, CdMnSe colloidal quantum dots, multi-shell CdSe/CdMnS/CdS colloidal nanoplatelets, and graphene and graphene-related solutions and films.;Using time-resolved photoluminescence to study epitaxially-grown ZnTe and ZnMnTe quantum dots in corresponding ZnMnSe and ZnSe matrices, the location dependence of manganese ions in respect to magnetic polaron formation is shown. The structure with manganese ions located in the matrix exhibited magnetic polaron behavior consistent with previous literature, whereas the structure with the magnetic ions located within the quantum dots exhibited unconventional magnetic polaron properties. These properties, including temperature and magnetic field insensitivity, were explained through the use of a model that predicted an increased internal magnetic field due to a decreased effective volume of the magnetic polaron and a higher effective temperature due to laser heating.;Magneto-time-resolved photoluminescence measurements on a system of colloidal CdMnSe quantum dots show that the magnetic polaron properties differ significantly from the epitaxially grown quantum dots. First the timescales at which the magnetic polaron forms and the polarization saturates are different by more than an order of magnitude, and second, the magnetic polaron energy exhibited step-like behavior as the strength of the externally applied magnetic field is increased. The field dependent MP formation energy that is observed experimentally is explained as due to the breaking of the antiferromagnetic coupling of Mn dimers within the QDs. This model is further verified by the observation of quantized behavior in the Zeeman energy splitting.;Through the use of magneto-photoluminescence measurements of InGaAs quantum wells, the observation of optical Aharonov-Bohm oscillations were identified in the photoluminescence emission intensity. This effect, generally observed in type-II systems, was unexpected in a quantum well structure, however the recombinations dynamics were identified through the use of a lifetime comparison with samples that did not contain indium and cross-sectional scanning tunneling microscopy that show indium rich islands within the quantum well layer. Analysis of the oscillations yielded a value of 17.3 nm for the exciton radius for sample 1 and 14.8 nm for sample 2.;Magneto-photoluminescence is used to probe the behavior of a series of coreshell- shell CdSe/CdMnS/CdS nanoplatelets. This study shows that the magnetic character of the nanoplatelets is directly related to the wave function overlap of the regions containing manganese ions. The atomically-precise synthesis of these nanoplatelets allows for the placement of magnetic ions in specific monolayers of these two-dimensional heterostructures and the optical characterization identifies how this placement affects the magneto-optical properties. A surprising result is that in some magnetic samples with limited carrier wave function overlap of the regions containing manganese ions, the magneto-optical properties more closely resemble those of non-magnetic samples.;Using time-resolved differential transmission, the carrier lifetime in several graphene related structures is determined. In these measurements, the changes in transmission are too small to measure using conventional techniques, therefore a background-free technique is required to obtain the necessary sensitivity. These measurement yielded carrier lifetimes ranging from 0.4 ps to 12 ps depending on the composition of the structure and the form it was measured as i.e., in solution or film.