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Graphene nanoribbons (GNRs) are nanostructures considered to be promising building blocks for the realization of graphene-based devices. The optical properties of GNRs are hard to determine due to their nanoscopic dimensions. Reflectance Anisotropy Spectroscopy/Reflectance Difference Spectroscopy (RAS/RDS) is a powerful optical tool to characterize highly anisotropic structures. RAS/RDS has shown to be very useful to measure the optical response of materials including semiconductor heterostructures. The technique is non-destructive and can be used in air or in vacuum conditions. Considering the highly anisotropic geometry of the GNRs, the RAS/RDS becomes a quite convenient technique to characterize the optical properties of GNRs and in general to study the dependence on the thickness of the optical properties of graphene. The GNRs used in the present work were synthesized on 6H-SiC stepped substrates and annealed in air to obtain quasifree-standing bilayer graphene (widths: 240 nm, 210, and 120 nm). For this system, the isolation of the optical signal coming from the GNRs in the RAS spectra is not an easy task due to the fact that both GNRs and the 6H-SiC stepped substrate are highly anisotropic. To study and characterize the GNRs, we present and discuss an experimental approach to isolate the RAS signal coming from the GNRs. We also have performed nano-RAS measurements by using a near-field scanning optical microscopy technique (nanometric resolution) that supports our method. We show that RAS and nano-RAS are powerful complementary optical probes that can be used to characterize GNRs and also properties such as the visual transparency of one-, two-, or few-layer thick graphene. [ABSTRACT FROM AUTHOR]

We report reflectance (R) and reflectance difference spectroscopy (RDS) spectra of wurtzite heterostructures grown on Si(111) and Si(110) substrates in the energy range from 2.0 to 3.5 eV. Due to the threefold symmetry of the Si(111) surface, the heterostructures grown on this surface will relax isotropically through the formation of misfit dislocations, preferably at the AlN/Si interface, and no in-plane anisotropies are expected. In fact, only a small in-plane anisotropy of reflectance is observed, due to the initial residual off-cut of the silicon substrate that leads to wurtzite layers with a c-axis slightly tilted with respect to the surface. In contrast, for the Si(110) substrate, strong differences in lattice parameters appear between silicon and GaN/AlN depending on the considered in-plane direction, leading to a large in-plane anisotropy. By using a multiple reflection model for R and the in-plane anisotropies described, we developed a model to describe the RDS spectra in the vicinity and below the fundamental gap of GaN. [ABSTRACT FROM AUTHOR]

We have used transmission anisotropy (TA) and photoreflectance-difference (PRD) spectroscopies as tools for the characterization of heterostructures consisting of 100 Å GaxIn1-xAsySb1-y quantum wells with 200 Å AlxGa1-xAsySb1-y barriers. Exctions associated to two allowed and two forbidden transitions of the confined levels in the quantum wells (QWs) are detected with PRD at 15 K. Using TA spectroscopy they all can be clearly resolved at room temperature. The signal of the anisotropy is proved experimentally to stem exclusively from the QWs and the surrounding interfaces. We show TA spectroscopy to be a powerful tool for interface characterization in heterostructures with energy gap below that of the substrate. [ABSTRACT FROM AUTHOR]