93 research outputs found
Tilted subwavelength gratings: controlling anisotropy in metamaterial nanophotonic waveguides
Subwavelength grating (SWG) structures are an essential tool
in silicon photonics, enabling the synthesis of metamaterials
with a controllable refractive index. Here we propose, for the
first time to the best of our knowledge, tilting the grating elements
to gain control over the anisotropy of the metamaterial.
Rigorous finite difference time domain simulations
demonstrate that a 45° tilt results in an effective index variation
on the fundamental TE mode of 0.23 refractive index
units, whereas the change in the TM mode is 20 times smaller.
Our simulation predictions are corroborated by experimental
results. We furthermore propose an accurate theoretical
model for designing tilted SWG structures based on rotated
uniaxial crystals that is functional over a wide wavelength
range and for both the fundamental and higher order modes.
The proposed control over anisotropy opens promising venues
in polarization management devices and transformation
optics in silicon photonics.Universidad de Málaga (UMA); Ministerio de
EconomÃa y Competitividad (MINECO) (IJCI-2016-30484,
TEC2015-71127-C2-R, TEC2016-80718-R); Ministerio de
Educación, Cultura y Deporte (MECD) (FPU16/06762);
European Regional Development Fund (ERDF); Comunidad
de Madrid (SINFOTON-CM S2013/MIT-2790); European
Association of National Metrology Institutes (EURAMET)
(H2020-MSCA-RISE-2015:SENSIBLE, JRP-i22 14IND13
Photind)
Backscattering reduction in a twisted water wave channel
We study theoretically and experimentally how to reduce the backscattering of
water waves in a channel with multiple turns. We show that it is not only
possible to cancel backscattering but also to achieve a remarkable transmission
in such geometries. In order to avoid the reflection that naturally arises at
each turn of the waveguide, an anisotropic metamaterial made of closely-spaced
thin vertical plates is used. The efficiency of the metamaterial arrangement
depends only slightly on the frequency of the incident wave, as long as its
wavelength is much larger than the periodicity of the array. This phenomenon is
not restricted only to water wave channels but also applies to any type of
waves with Neumann boundary conditions
Fundamental constraints on broadband passive acoustic treatments in unidimensional scattering problems
[EN] In a passive lossy acoustical system, sum rules derived from passivity explicitly relate the broadband response to the spatial dimension of the system, which provide important design criteria as well. In this work, the theory of Herglotz function is applied systematically to derive sum rules for unidimensional scattering problems relying on passive acoustic treatments which are generally made of rigid, motionless and subwavelength structures saturated by air. The rigid-boundary reflection, soft-boundary reflection and transmission problems are analysed. The derived sum rules are applied for guiding the designs of passive absorbers and mufflers: the required minimum space is directly predicted from the target (i.e. the desired absorption or transmission-loss spectra) without any specific design. Besides, it is possible to break this type of sum rules and fundamental constraints in particular cases. This property, if well used, could result in ultra-compact absorbers working at long wavelength up to infinity.This work is supported by Valeo company and the ANR-RGC METARoom project (grant nos. ANR-18-CE08-0021 and RGC A-HKUST601/18).Meng, Y.; Romero-GarcÃa, V.; Gabard, G.; Groby, J.; Bricault, C.; Goudé, S.; Sheng, P. (2022). Fundamental constraints on broadband passive acoustic treatments in unidimensional scattering problems. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences (Online). 478(2265):1-22. https://doi.org/10.1098/rspa.2022.0287122478226
Functional analytic methods for discrete approximations of subwavelength resonator systems
We survey functional analytic methods for studying subwavelength resonator
systems. In particular, rigorous discrete approximations of Helmholtz
scattering problems are derived in an asymptotic subwavelength regime. This is
achieved by re-framing the Helmholtz equation as a non-linear eigenvalue
problem in terms of integral operators. In the subwavelength limit, resonant
states are described by the eigenstates of the generalised capacitance matrix,
which appears by perturbing the elements of the kernel of the limiting
operator. Using this formulation, we are able to describe subwavelength
resonance and related phenomena. In particular, we demonstrate large-scale
effective parameters with exotic values. We also show that these systems can
exhibit localised and guided waves on very small length scales. Using the
concept of topologically protected edge modes, such localisation can be made
robust against structural imperfections
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