77 research outputs found
Freestanding metasurfaces for optical frequencies
We present freestanding metasurfaces operating at optical frequencies with a
total thickness of only 40nm. The metasurfaces are fabricated by focused
ion beam milling of nanovoids in a carbon film followed by thermal evaporation
of gold and plasma ashing of the carbon film. As a first example, we
demonstrate a metasurface lens based on resonant V-shaped nanovoids with a
focal length of 1mm. The second example is a metasurface phase-plate
consisting of appropriately oriented rectangular nanovoids that transforms a
Gaussian input beam into a Laguerre-Gaussian mode
The inverse eigenvalue problem for a Hermitian reflexive matrix and the optimization problem
The inverse eigenvalue problem and the associated optimal approximation problem for Hermitian reflexive matrices with respect to a normal {k+1}-potent matrix are considered. First, we study the existence of the solutions of the associated inverse eigenvalue problem and present an explicit form for them. Then, when such a solution exists, an expression for the solution to the corresponding optimal approximation problem is obtained
Evidence for Kosterlitz-Thouless type orientational ordering of CFBr monolayers physisorbed on graphite
Monolayers of the halomethane CFBr adsorbed on graphite have been
investigated by x-ray diffraction. The layers crystallize in a commensurate
triangular lattice. On cooling they approach a three-sublattice
antiferroelectric pattern of the in-plane components of the dipole moments. The
ordering is not consistent with a conventional phase transition, but points to
Kosterlitz-Thouless behavior. It is argued that the transition is described by
a 6-state clock model on a triangular lattice with antiferromagnetic nearest
neighbor interactions which is studied with Monte-Carlo simulations. A
finite-size scaling analysis shows that the ordering transition is indeed in
the KT universality class.Comment: 4 pages, 5 figure
A 160-Gb/s OTDM demultiplexer based on parametric wavelength exchange
Parametric wavelength exchange (PWE) has been demonstrated as a versatile device in providing different functionalities. In this paper, we will concentrate, numerically and experimentally, on one of these functionalities, namely, all-optical time demultiplexing of 160-Gb/s return-to-zero (RZ) signals based on a pulsed-pump PWE in a 400 m highly nonlinear dispersion-shifted fiber. Experimental results show power penalties < 2.7 dB at bit-error rate of 10-9 for all demultiplexed 10-Gb/s RZ signals. We also derive theoretical expressions for the conversion/residual efficiencies and investigate the impact of pump pulse width and phase mismatch on these efficiencies. Furthermore, the impacts of pulsed-pump wavelength and power level on the characteristics of the switching window are investigated numerically. As a result, the demultiplexer can be easily upgraded to an add-drop multiplexer because of the complete exchange nature of PWE, which is justified by the surviving channels' waveform performance. © 2009 IEEE.published_or_final_versio
Critical behavior of the frustrated antiferromagnetic six-state clock model on a triangular lattice
We study the anti-ferromagnetic six-state clock model with nearest neighbor
interactions on a triangular lattice with extensive Monte-Carlo simulations. We
find clear indications of two phase transitions at two different temperatures:
Below a chirality order sets in and by a thorough finite size scaling
analysis of the specific heat and the chirality correlation length we show that
this transition is in the Ising universality class (with a non-vanishing
chirality order parameter below ). At the spin-spin
correlation length as well as the spin susceptibility diverges according to a
Kosterlitz-Thouless (KT) form and spin correlations decay algebraically below
. We compare our results to recent x-ray diffraction experiments on the
orientational ordering of CFBr monolayers physisorbed on graphite. We argue
that the six-state clock model describes the universal feature of the phase
transition in the experimental system and that the orientational ordering
belongs to the KT universality class.Comment: 8 pages, 9 figure
Recent advances in organic synthesis using light-mediated n-heterocyclic carbene catalysis
The combination of photocatalysis with other ground state catalytic systems have attracted much attention recently due to the enormous synthetic potential offered by a dual activation mode. The use of N-heterocyclic carbene (NHC) as organocatalysts emerged as an important synthetic tool. Its ability to harness umpolung reactivity by the formation of the Breslow intermediate has been employed in the synthesis of thousands of biologically important compounds. However, the available coupling partners are relatively restricted, and its combination with other catalytic systems might improve its synthetic versatility. Thus, merging photoredox and N-heterocyclic carbene (NHC) catalysis has emerged recently as a powerful strategy to develop new transformations and give access to a whole new branch of synthetic possibilities. This review compiles the NHC catalyzed methods mediated by light, either in the presence or absence of an external photocatalyst, that have been described so far, and aims to give an accurate overview of the potential of this activation modeL.M. acknowledges the Autonomous Community of Madrid (CAM)
for the financial support (PEJD-2019-PRE/AMB-16640 and SI1/PJI/
2019-00237) and for an “Atracción de Talento Investigador”
contract (2017-T2/AMB-5037
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