364 research outputs found
Broadband achromatic anomalous mirror in near-IR and visible frequency range
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 with high radiation efficiency
(exceeding 98 ) and small Joule losses (below 10 )
Strain-induced pseudo-magnetic field for novel graphene electronics
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
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
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 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
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
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
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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