Lighting applications require directional and polarization control of the emitted light, which is currently achieved by bulky optical components such as lenses, parabolic mirrors, and polarizers. Ideally, this control would be achieved without any external optics, but at the nanoscale, during the generation of light. Semiconductor nanowires are promising candidates for lighting devices due to their efficient light outcoupling and synthesis flexibility. In this work, we demonstrate a precise control of both the directionality and the polarization of the nanowire array emission by changing the nanowire diameter. We change the angular emission pattern from a large-angle doughnut shape to a narrow-angle beaming along the nanowire axis. In addition, we tune the polarization from unpolarized to either p- or s-polarized. Both the far-field emission pattern and its polarization are controlled by the number and type of guided or leaky modes supported by the nanowire, which are determined by the nanowire diameter.
We present a theoretical framework that allows us to investigate the scattering of terahertz surface plasmon polaritons ͑SPP's͒ by arrays of subwavelength grooves and ridges on semiconductors. The formulation is based on the reduced Rayleigh equation resulting upon imposing an impedance boundary condition. Guided by approximate estimations of the broadening with temperature of the first gap in the SPP dispersion relation in the case of indium antimonide samples with rectangular grooves, numerical calculations are carried out to determine the spectral dependence of all the SPP scattering channels ͑reflection, transmission, and radiation͒ in the immediate vicinity of that gap. The thermally induced switching of the SPP reflection and transmission nearby the lower SPP band edge is investigated as a function of groove parameters ͑size and number͒; near-field intensity maps are also presented. We thus shed light on the SPP scattering and switching physical mechanisms, thereby providing the most suitable experimental configurations.
We measure the polarization-resolved angular emission distribution from thin (diameter ∼110 nm) and thick (diameter ∼180 nm) GaAs nanowires with cathodoluminescence polarimetry. The nanowires, which are horizontally resting on a thin carbon film, are excited by a 5 keV electron beam and emit bandgap luminescence at a central wavelength of 870 nm. The emission can couple to different waveguide modes that propagate along the wire. These waveguide modes are dependent on the wire diameter and determine the directionality and polarization of the emission. Although each measured nanowire can support different modes, the polarized emission is dominated by the 1 TM01 waveguide mode in all cases, independently of wire diameter. When exciting the nanowires away from the center, close to the end facets, the thin and thick wires exhibit an opposite directional emission. The emission from thin nanowires is dominated by a leaky TM01 mode that leads to emission in the opposite direction to the excitation position. For the thick wires, however, the TM01 mode is guided but also lossy due to absorption in the substrate. In that case, the wires emit towards the same direction as the excitation position. We show that the measurements agree well with both a simple 1D current model and numerical simulations. The high spatial resolution of angular and polarization resolved cathodoluminescence spectroscopy provides detailed insight into the nanoscale emission and propagation of light in semiconductor nanowires.
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