Nano Focus: Tailored nanowire with embedded quantum dot yields bright single-photon source
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s Tailored nanowire with embedded quantum dot yields bright single-photon source
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ingle-photon sources with near-unity light-extraction efficiencies (η) are ideally required for processes relying on the transfer of quantum information between two remote stationary quantum bits, such as quantum cryptography or the development of a quantum computer. Moving toward this goal, M.E. Reimer and co-workers, from the Delft University of Technology and M.A. Verheijen and E.P.A.M. Bakkers from the Eindhoven University of Technology, The Netherlands, have recently succeeded in developing a highly efficient method of collecting single photons. As reported in the March 13 issue of Nature Communications (DOI: 10.1038/ncomms1746), the researchers have fabricated a highefficiency single-photon source based on a nanowire with controlled shape, which they suggest will pave the way toward a single-photon source with near-unity light-collection efficiency. The researchers designed a tapered
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MRS BULLETIN
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VOLUME 37 • MAY 2012
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conducting materials in resonant metamaterials, although geometrical factors are important when comparing samples of different thicknesses. The researchers said that although charge-neutral graphene displays minimal resistivity in the mid-infrared and visible band, it is not a good candidate for metamaterial applications because ζ and ξ for graphene are several orders of magnitude larger than those for gold. Similarly, silver performs better as a conducting material at terahertz frequencies than high-Tc superconductors because it has significantly smaller ζ and ξ values. Transparent conducting metal oxides, such as indium tin oxide and aluminum tin oxide, have also recently been proposed for use in metamaterials, but the researchers found that their microwave resistivities are two orders of magnitude larger than the optical resistivity of silver. For plasmonics, the researchers use
the ratio of the propagation length to the surface plasmon wavelength as the measure of loss performance. The larger kinetic inductance of biased graphene makes it a better candidate for plasmonic applications than charge-neutral graphene. However, the researchers also showed that the propagation length is, at best, on the order of a few SPP wavelengths, which is too short for most plasmonic applications. The researchers also discussed why alkali–noble intermetallics, such as KAu and LiAg, are not good candidates for use in metamaterials, although they said, “These examples show, nevertheless, the possibility of band engineering to tune the resistivity of alloys,” and that, “it is worth continuing the research effort to develop better conducting materials, because of the considerable improvement such materials would bring.” Steven Trohalaki
nanowire as a waveguide where a single InAs0.25P 0.75 quantum dot was precisely positioned on its axis such that it can collect photons as they are generated. This was achieved by growing InP nanowires using low pressure metal– organic vapor-phase epitaxy (MOVPE) methods, and then synt
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