325,069 research outputs found

    Direct transmission of pictorial information in multimode optical fibers

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    The problem of coherent image transmission through a single multimode optical fiber is discussed. A scheme is presented for recovering the transmitted image after distortions brought about by the fiber modes dispersion. Realization of this scheme by holographic techniques and with lens systems is proposed, and its limitations pointed out. The application of this scheme in canceling out temporal signal dispersion in a multimode fiber transmission line is also discussed briefly

    Resonant Raman scattering of quantum wire in strong magnetic field

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    The resonant Raman scattering of a quantum wire in a strong magnetic field is studied, focused on the effect of long range Coulomb interaction and the spin-charge separation. The energy-momentum dispersions of charge and spin excitation obtained from Raman cross-section show the characteristc cross-over behaviour induced by inter-edge Coulomb interaction. The "SPE" peak near resonance in polarized spectra becomes broad due to the momentum dependence of charge velocity. The broad peak in the depolarized spectra is shown to originate from the disparity between charge and spin excitation velocity.Comment: RevTex file, 6 pages, no figure: To appear in Int. Jour. Mod. Phys.

    Theoretical analysis of spectral gain in a THz quantum cascade laser: prospects for gain at 1 THz

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    In a recent Letter [Appl. Phys. Lett. 82, 1015 (2003)], Williams et al. reported the development of a terahertz quantum cascade laser operating at 3.4 THz or 14.2 meV. We have calculated and analyzed the gain spectra of the quantum cascade structure described in their work, and in addition to gain at the reported lasing energy of ~= 14 meV, we have discovered substantial gain at a much lower energy of around 5 meV or just over 1 THz. This suggests an avenue for the development of a terahertz laser at this lower energy, or of a two-color terahertz laser.Comment: in press APL, tentative publication date 29 Sep 200

    A Possible Origin of Dark Energy

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    We discuss the possibility that the existence of dark energy may be due to the presence of a spin zero field ϕ(x)\phi(x), either elementary or composite. In the presence of other matter field, the transformation ϕ(x)ϕ(x)+\phi(x)\to \phi(x) + constant can generate a negative pressure, like the cosmological constant. In this picture, our universe can be thought as a very large bag, similar to the much smaller MIT bag model for a single nucleon.Comment: 4 pages, no figure, typos correcte

    Embedded heterostructure epitaxy: A technique for two-dimensional thin-film definition

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    Selective multilayer epitaxial growth of GaAs-Ga1–xAlxAs through stripe openings in Al2O3 mask is reported. The technique results in prismatic layers of GaAs and Ga1–xAlxAs "embedded" in each other and leads to controllable uniform structures terminated by crystal faces. The crystal habit (shape) has features which are favorable for fabrication of cw injection lasers, laser arrays, and integrated optics components which require planar definition

    Coherence scale of the two-dimensional Kondo Lattice model

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    A doped hole in the two-dimensional half-filled Kondo lattice model with exchange J and hopping t has momentum (pi,pi) irrespective of the coupling J/t. The quasiparticle residue of the doped hole, Z_{(\pi, \pi)}, tracks the Kondo scale, T_K, of the corresponding single impurity model. Those results stem from high precision quantum Monte Carlo simulations on lattices up to 12 X 12. Accounting for small dopings away from half-filling within a rigid band approximation, this result implies that the effective mass of the charge carriers at the Fermi level tracks 1/T_K or equivalently that the coherence temperature T_{coh} \propto T_K. This results is consistent with the large-N saddle point of the SU(N) symmetric Kondo lattice model.Comment: 4 pages, 4 figure

    Low-threshold room-temperature embedded heterostructure lasers

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    Room-temperature embedded double-heterostructure injection lasers have been fabricated using selective liquid phase epitaxial growth. Threshold current densities as low as 1.5 kA/cm^2 have been achieved in lasers grown through stripe windows opened in epitaxial GaAlAs masks

    Self-Consistent Theory of the Gain Linewidth for Quantum Cascade Lasers

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    The linewidth in intersubband transitions can be significantly reduced below the sum of the lifetime broadening for the involved states, if the scattering environment is similar for both states. This is studied within a nonequilibrium Green function approach here. We find that the effect is of particular relevance for a recent, relatively low doped, THz quantum cascade laser.Comment: 3 pages, figures include

    Topological Superconductivity and Majorana Fermions in Metallic Surface-States

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    Heavy metals, such as Au, Ag, and Pb, often have sharp surface states that are split by strong Rashba spin-orbit coupling. The strong spin-orbit coupling and two-dimensional nature of these surface states make them ideal platforms for realizing topological superconductivity and Majorana fermions. In this paper, we further develop a proposal to realize Majorana fermions at the ends of quasi-one-dimensional metallic wires. We show how superconductivity can be induced on the metallic surface states by a combination of proximity effect, disorder, and interactions. Applying a magnetic field along the wire can drive the wire into a topologically non-trivial state with Majorana end-states. Unlike the case of a perpendicular field, where the chemical potential must be fined tuned near the Rashba-band crossing, the parallel field allows one to realize Majoranas for arbitrarily large chemical potential. We then show that, despite the presence of a large carrier density from the bulk metal, it is still possible to effectively control the chemical potential of the surface states by gating. The simplest version of our proposal, which involves only an Au(111) film deposited on a conventional superconductor, should be readily realizable.Comment: 9 Pages, 6 Figure
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