290 research outputs found
All-optical tuning of EIT-like dielectric metasurfaces by means of chalcogenide phase change materials
Electromagnetically induced transparency (EIT) is a pump-induced narrowband
transparency window within an absorption lin
e of the probe beam sp
ectrum in an atomic
system. In this paper we propose a way to bring together the all-dielectric metamaterials to
have EIT-like effects and to optically tune the
response by hybridizing them with a layer of a
phase change material. We propose a design of the metamaterial based on Si nanoresonators
that can support an EIT-like resonant response. On the top of the resonators we consider a
thin layer of a chalcogenide phase change material, which we will use to tune the optical
response. Our choice is Ge
2
Sb
2
Te
5
(GST), since it has two stable phases at room temperature,
namely amorphous and crystalline, between which it can be switched quickly, nonvolatively
and reversibly, sustaining a large number of
switching cycles. They differ in optical
properties, while still having moderately low losses in telecom range. Since such dielectric
resonators do not have non-radiative losses of metals around 1550nm, they can lead to a high-
Q factor of the EIT-like response in this range
. Firstly, we optimize the starting structure so
that it gives an EIT-like response at 1550 nm wh
en the GST layer is in the amorphous state.
Our starting design uses glass as a substrate, but we also consider implementation in SOI
technology. If we then switch the thin layer of
GST to its crystalline phase, which has higher
losses, the EIT-like response is red shifted, pr
oviding around 10:1 contrast at 1550nm. This
reversible tuning can be done with an ns visible pulsed laser. We discuss the results of the
simulation of the dielectric metasurface for differe
nt configurations and
the tuning possibilit
Photoacoustic technique for the characterization of plasmonic properties of 2D periodic arrays of gold nanoholes
We apply photo-acoustic (PA) technique to examine plasmonic properties of 2D
periodic arrays of nanoholes etched in gold/chromium layer upon a glass substrate
Chiral near-field manipulation in Au-GaAs hybrid hexagonal nanowires
We demonstrate the control of enhanced chiral field distribution at the surface of hybrid metallo-dielectric nanostructures composed of self-assembled vertical hexagonal GaAs-based nanowires having three of the six sidewalls covered with Au. We show that weakly-guided modes of vertical GaAs nanowires can generate regions of high optical chirality that are further enhanced by the break of the symmetry introduced by the gold layer. Changing the angle of incidence of a linearly polarized plane wave it is possible to tailor and optimize the maps of the optical chirality in proximity of the gold plated walls. The low cost feasibility of the sample combined to the simple control by using linearly polarized light and the easy positioning of chiral molecules by functionalization of the gold plates make our proposed scheme very promising for enhanced enantioselective spectroscopy applications
Optimization of thermochromic VO2 based structures with tunable thermal emissivity
In this paper, we design and simulate VO2/metal multilayers to obtain a large tunability of the thermal emissivity of infrared (IR) filters in the typical mid wave IR window of many infrared cameras. The multilayer structure is optimized to realise a low emissivity filter at high temperatures useful for military purposes. The values of tunability found for VO2/metal multilayers are larger than the value for a single thick layer of VO2. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4739489
Light scattering from a rough metal surface: theory and experiment
There is still great interest in the determination of microtopographic properties of rough metallic surfaces from light scattering measurements. According to Beckmann–Kirchhoff theory a clear relationship is established between the in-plane angular scattered light intensity and the statistical properties of the surface. We discuss one way to invert this relationship, and we introduce a new iterative procedure to retrieve the height autocorrelation function even for a very rough metallic surface (rms surface roughness of the same order of the optical wavelength). The procedure is eventually applied to the experimental data of a known metallic surface for validation
Detection of second-order nonlinear optical magnetization by mapping normalized Stokes parameters
A measurable magnetic (nonlocal) contribution to the second harmonic generation (SHG) of nonmagnetic materials is an intriguing issue related to chiral materials, such as biomolecules. Here we report the detection of an intensity-dependent optically induced magnetization of a chiral bacteriorhodopsin film under femtosecond pulse excitation (830 nm) and far from the material's resonance. The analysis of the pump intensity-dependent noncollinear SHG signal, by means of the polarization map of normalized Stokes parameters, allows one to improve the detection of the nonlinear optical magnetization M (2 omega) contribution to the SHG signal. (c) 2013 Optical Society of Americ
Self-Phase-Matched Second-Harmonic and White-Light Generation in a Biaxial Zinc Tungstate Single Crystal
Second-order nonlinear optical materials are used to generate new frequencies by exploiting second-harmonic generation (SHG), a phenomenon where a nonlinear material generates light at double the optical frequency of the input beam. Maximum SHG is achieved when the pump and the generated waves are in phase, for example through birefringence in uniaxial crystals. However, applying these materials usually requires a complicated cutting procedure to yield a crystal with a particular orientation. Here we demonstrate the first example of phase matching under the normal incidence of SHG in a biaxial monoclinic single crystal of zinc tungstate. The crystal was grown by the micro-pulling-down method with the (102) plane perpendicular to the growth direction. Additionally, at the same time white light was generated as a result of stimulated Raman scattering and multiphoton luminescence induced by higher-order effects such as three-photon luminescence enhanced by cascaded third-harmonic generation. The annealed crystal offers SHG intensities approximately four times larger than the as grown one; optimized growth and annealing conditions may lead to much higher SHG intensities
Method for thermal diffusivity measurements based on photothermal deflection
The thermal diffusivity measurement through pulsed photodeflection in a modified collinear configuration is presented and discussed; comparison between theory and experiment is also shown
Midinfrared thermal emission properties of finite arrays of gold dipole nanoantennas
We studied the far-field thermal emission properties of finite arrays of resonant gold dipole nanoantennas at
equilibrium temperature. We numerically investigated the transition from the super-Planckian emission of the
single resonant antenna to the sub-Planckian emission inherent to infinite periodic arrays. Increasing the number
of unit cells of the array, the overall size of the system increases, and the relative emissivity quickly converges to
values lower than the unity. Nevertheless, if the separation between nanoantennas in the array is small compared
to the wavelength, the near-field interaction makes the emission of each unit cell multipolar. This opens the doors
for additional tailoring of the emitted power and directionality of thermal radiation
A graphical environment to express the semantics of control systems
We present the concept of a unified graphical environment for expressing the
semantics of control systems. The graphical control system design environment
in Simulink already allows engineers to insert a variety of assertions aimed
the verification and validation of the control software. We propose extensions
to a Simulink-like environment's annotation capabilities to include formal
control system stability, performance properties and their proofs. We provide a
conceptual description of a tool, that takes in a Simulink-like diagram of the
control system as the input, and generates a graphically annotated control
system diagram as the output. The annotations can either be inserted by the
user or generated automatically by a third party control analysis software such
as IQC or -tool. We finally describe how the graphical
representation of the system and its properties can be translated to annotated
programs in a programming language used in verification and validation such as
Lustre or C
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