11,307 research outputs found
Enhanced Bragg reflections from size-matched heterostructure photonic crystal thin films prepared by the Langmuir-Blodgett method
The Langmuir-Blodgett method was used to engineer photonic crystal thin films of an AB architecture. Structures were studied by transmittance and reflectance spectroscopies. For an AB structure in which the silica particle diameter B is twice that of A, reflectance features associated with the first order Bragg peak for the ``A'' domain are only observed when the structure is probed from the A side of the structure. Furthermore, this feature is enhanced in intensity compared to that for a structure consisting solely of A particles. These findings are attributed to a matching of first and second order Bragg processes
Photonic crystal thin films of GaAs prepared by atomic layer deposition
Photonic crystal thin films were fabricated via the self-assembly of a lattice of silica spheres on silicon (100) substrates. Progressive infilling of the air spaces within the structure with GaAs was achieved using trimethylgallium and arsine under atomic-layer-deposition conditions. Samples with the highest levels of GaAs infill were subsequently inverted using selective etching. Reflectance spectra are interpreted via the Bragg expression and calculated photonic band structure diagrams. For GaAs infilled and inverted samples, the relative positions of the first and second order Bragg reflections are strongly influenced by the wavelength dependent refractive index
Growth and formation of inverse GaP and InP opals
Opals consist of an ordered array of SiO2 spheres. This leads to a modulation of the refractive index and hence photonic stop bands behaviour over the visible/IR range of the electro-magnetic spectrum. The exact position of the stop bands depends on the size of the silica spheres. However, the refractive index contrast between the SiO2 spheres and air spaces is not great enough to open up a full photonic band gap (PBG), only the pseudogap. To increase the contrast the air spaces are filled with a material of high refractive index such as InP or GaP. To further increase the contrast the SiO2 is removed leaving a III-V framework as the inverse opal structure.
By use of MOCVD we have been able to infill opals with InP and GaP to such a level that has supported the inversion of the composite forming a structure of air holes within a III-V lattice. XRD and Raman confirmed the quality of the III-V infill, while the extent of the infill was studied by SEM and reflectance measurements
A class of constant modulus algorithms for uniform linear arrays with a conjugate symmetric constraint
A class of constant modulus algorithms (CMAs) subject to a conjugate symmetric constraint is proposed for blind beamforming based on the uniform linear array structure. The constraint is derived from the beamformer with an optimum output signal-to-interference-plus-noise ratio (SINR). The effect of the additional constraint is equivalent to adding a second step to the original adaptive algorithms. The proposed approach is general and can be applied to both the traditional CMA and its all kinds of variants, such as the linearly constrained CMA (LCCMA) and the least squares CMA (LSCMA) as two examples. With this constraint, the modified CMAs will always generate a weight vector in the desired form for each update and the number of adaptive variables is effectively reduced by half, leading to a much improved overall performance. (C) 2010 Elsevier B.V. All rights reserved
Formal vs self-organised knowledge systems: a network approach
In this work we consider the topological analysis of symbolic formal systems
in the framework of network theory. In particular we analyse the network
extracted by Principia Mathematica of B. Russell and A.N. Whitehead, where the
vertices are the statements and two statements are connected with a directed
link if one statement is used to demonstrate the other one. We compare the
obtained network with other directed acyclic graphs, such as a scientific
citation network and a stochastic model. We also introduce a novel topological
ordering for directed acyclic graphs and we discuss its properties in respect
to the classical one. The main result is the observation that formal systems of
knowledge topologically behave similarly to self-organised systems.Comment: research pape
Scaling ansatz, four zero Yukawa textures and large
We investigate 'Scaling ansatz' in the neutrino sector within the framework
of type I seesaw mechanism with diagonal charged lepton and right handed
Majorana neutrino mass matrices (). We also assume four zero texture of
Dirac neutrino mass matrices () which severely constrain the
phenomenological outcomes of such scheme. Scaling ansatz and the present
neutrino data allow only Six such matrices out of 126 four zero Yukawa
matrices. In this scheme, in order to generate large we break
scaling ansatz in through a perturbation parameter and we also show our
breaking scheme is radiatively stable. We further investigate CP violation and
baryogenesis via leptogenesis in those surviving textures.Comment: 25 pages, 8 figures, Accepted for publication in Phys. Rev.
Search for Boosted Dark Matter at ProtoDUNE
We propose the first experimental test of the inelastic boosted dark matter
hypothesis, capitalizing on the new physics potential with the imminent data
taking of the ProtoDUNE detectors. More specifically, we explore various
experimental signatures at the cosmic frontier, arising in boosted dark matter
scenarios, i.e., relativistic, inelastic scattering of boosted dark matter
often created by the annihilation of its heavier component which usually
comprises of the dominant relic abundance. Although features are unique enough
to isolate signal events from potential backgrounds, vetoing a vast amount of
cosmic background is rather challenging as the detectors are located on the
ground. We argue, with a careful estimate, that such backgrounds nevertheless
can be well under control by performing dedicated analyses after data
acquisition. We then discuss some phenomenological studies which can be
achieved with ProtoDUNE, employing a dark photon scenario as our benchmark
dark-sector model.Comment: Supplemental material include
Wave energy localization by self-focusing in large molecular structures: a damped stochastic discrete nonlinear Schroedinger equation model
Wave self-focusing in molecular systems subject to thermal effects, such as
thin molecular films and long biomolecules, can be modeled by stochastic
versions of the Discrete Self-Trapping equation of Eilbeck, Lomdahl and Scott,
and this can be approximated by continuum limits in the form of stochastic
nonlinear Schroedinger equations.
Previous studies directed at the SNLS approximations have indicated that the
self-focusing of wave energy to highly localized states can be inhibited by
phase noise (modeling thermal effects) and can be restored by phase damping
(modeling heat radiation).
We show that the continuum limit is probably ill-posed in the presence of
spatially uncorrelated noise, at least with little or no damping, so that
discrete models need to be addressed directly. Also, as has been noted by other
authors, omission of damping produces highly unphysical results.
Numerical results are presented for the first time for the discrete models
including the highly nonlinear damping term, and new numerical methods are
introduced for this purpose. Previous conjectures are in general confirmed, and
the damping is shown to strongly stabilize the highly localized states of the
discrete models. It appears that the previously noted inhibition of nonlinear
wave phenomena by noise is an artifact of modeling that includes the effects of
heat, but not of heat loss.Comment: 22 pages, 13 figures, revision of talk at FPU+50 conference in Rouen,
June 200
- …