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Localized and polarized photoluminescence spectra are observed in single GaAs nanowires with mixed zincblende and wurtzite phases, grown using molecular beam epitaxy. For low excitation intensities, the photoluminescence emission exhibits narrow spectral features predominantly polarized perpendicular to the nanowire axis. For high excitation intensities, the photoluminescence spectra transform into dominant broadened features, which exhibit different peak energies and polarization properties. The strongly polarized emission at high excitation intensities is identified as being due to a spatially direct transition in wurtzite sections of the nanowires. The analysis, including band structure calculations suggests that carriers in the wurtzite sections diffuse into regions where the average low-temperature peak emission energy and crystal field parameter are 1.535 eV and 20meV, respectively. [ABSTRACT FROM AUTHOR]

High resolution photoreflectance (PR) spectroscopy study on a single GaAs/AlGaAs quantum well representing a two-dimensional (2D) system, shows additional distinct spectral features on the high energy side of the first confined heavy-hole and light-hole exciton transitions. The PR experiments involved a special dual detection technique which significantly improved the measurement sensitivity. Photoluminescence excitation spectroscopy data on the sample showed broadened step-like features around these energies. A detailed lineshape analysis, including first principles simulations, was performed to understand the origins of these additional PR spectral features. They are shown to arise primarily from inhomogeneously broadened first excited state transition of the excitons, rather than from a change in the joint density of states at the exciton continuum edge. The analysis suggests that such features are more likely in the case of 2D excitons as compared to 3D excitons in bulk material. Apart from its significance for post-growth characterization, identification of these additional PR features enables direct estimation of the exciton binding energy. [ABSTRACT FROM AUTHOR]

Reflectance spectrum measured using an optical microscope with a large numerical aperture objective lens is shown to get modified. The change is most prominent when there are optical interference related features in the spectrum. This modification is shown to arise primarily due to the wide range of angles of incidence involved in the measurement and a simple formulation is provided to correct for this in simulations. The importance of such analysis is brought out through a reflectance contrast spectroscopy based study for identifying mono-layer and bi-layer graphene and MoS2. [ABSTRACT FROM AUTHOR]