364 research outputs found

    Broadband achromatic anomalous mirror in near-IR and visible frequency range

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    The anomalous achromatic mirror operating in near-IR and visible frequency range was designed using an array of metal-insulator-metal (MIM) resonators. An incident wave interacting with MIM resonator experiences phase shift that is equal to the optical path travelled by the gap plasmon, excited by the wave. The phase gradient along the mirror surface is created through the difference in plasmons optical paths in resonators of different lengths. In the frequency region well below the plasma frequency of the metal, the phase gradient is a linear function of frequency, and thus the mirror operates in achromatic regime, i.e. reflection angle does not depend on the radiation frequency. Using silver-air-silver resonators, we predicted that the mirror can steer normally incident beam to angles as large as 40∘^{\circ} with high radiation efficiency (exceeding 98 %\%) and small Joule losses (below 10 %\%)

    Strain-induced pseudo-magnetic field for novel graphene electronics

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    Particular strain geometry in graphene could leads to a uniform pseudo-magnetic field of order 10T and might open up interesting applications in graphene nano-electronics. Through quantum transport calculations of realistic strained graphene flakes of sizes of 100nm, we examine possible means of exploiting this effect for practical electronics and valleytronics devices. First, we found that elastic backscattering at rough edges leads to the formation of well defined transport gaps of order 100meV under moderate maximum strain of 10%. Second, the application of a real magnetic field induced a separation, in space and energy, of the states arising from different valleys, leading to a way of inducing bulk valley polarization which is insensitive to short range scattering.Comment: 5 pages, 5 figure

    Electronic transport properties of a tilted graphene pn junction

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    Spatial manipulation of current flow in graphene could be achieved through the use of a tilted pn junction. We show through numerical simulation that a pseudo-Hall effect (i.e. non-equilibrium charge and current density accumulating along one of the sides of a graphene ribbon) can be observed under these conditions. The tilt angle and the pn transition length are two key parameters in tuning the strength of this effect. This phenomenon can be explained using classical trajectory via ray analysis, and is therefore relatively robust against disorder. Lastly, we propose and simulate a three terminal device that allows direct experimental access to the proposed effect.Comment: 7 pages, 7 figure

    Adaptive confidence balls

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    Adaptive confidence balls are constructed for individual resolution levels as well as the entire mean vector in a multiresolution framework. Finite sample lower bounds are given for the minimum expected squared radius for confidence balls with a prespecified confidence level. The confidence balls are centered on adaptive estimators based on special local block thresholding rules. The radius is derived from an analysis of the loss of this adaptive estimator. In addition adaptive honest confidence balls are constructed which have guaranteed coverage probability over all of RN\mathbb{R}^N and expected squared radius adapting over a maximum range of Besov bodies.Comment: Published at http://dx.doi.org/10.1214/009053606000000146 in the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org

    Nonparametric estimation over shrinking neighborhoods: Superefficiency and adaptation

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    A theory of superefficiency and adaptation is developed under flexible performance measures which give a multiresolution view of risk and bridge the gap between pointwise and global estimation. This theory provides a useful benchmark for the evaluation of spatially adaptive estimators and shows that the possible degree of superefficiency for minimax rate optimal estimators critically depends on the size of the neighborhood over which the risk is measured. Wavelet procedures are given which adapt rate optimally for given shrinking neighborhoods including the extreme cases of mean squared error at a point and mean integrated squared error over the whole interval. These adaptive procedures are based on a new wavelet block thresholding scheme which combines both the commonly used horizontal blocking of wavelet coefficients (at the same resolution level) and vertical blocking of coefficients (across different resolution levels).Comment: Published at http://dx.doi.org/10.1214/009053604000000832 in the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org

    On adaptive estimation of linear functionals

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    Adaptive estimation of linear functionals over a collection of parameter spaces is considered. A between-class modulus of continuity, a geometric quantity, is shown to be instrumental in characterizing the degree of adaptability over two parameter spaces in the same way that the usual modulus of continuity captures the minimax difficulty of estimation over a single parameter space. A general construction of optimally adaptive estimators based on an ordered modulus of continuity is given. The results are complemented by several illustrative examples.Comment: Published at http://dx.doi.org/10.1214/009053605000000633 in the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org

    Nonquadratic estimators of a quadratic functional

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    Estimation of a quadratic functional over parameter spaces that are not quadratically convex is considered. It is shown, in contrast to the theory for quadratically convex parameter spaces, that optimal quadratic rules are often rate suboptimal. In such cases minimax rate optimal procedures are constructed based on local thresholding. These nonquadratic procedures are sometimes fully efficient even when optimal quadratic rules have slow rates of convergence. Moreover, it is shown that when estimating a quadratic functional nonquadratic procedures may exhibit different elbow phenomena than quadratic procedures.Comment: Published at http://dx.doi.org/10.1214/009053605000000147 in the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org
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