62 research outputs found
Radiative heat transfer between metallic gratings using adaptive spatial resolution
We calculate the radiative heat transfer between two identical metallic
one-dimensional lamellar gratings. To this aim we present and exploit a
modification to the widely-used Fourier modal method, known as adaptive spatial
resolution, based on a stretch of the coordinate associated to the periodicity
of the grating. We first show that this technique dramatically improves the
rate of convergence when calculating the heat flux, allowing to explore smaller
separations. We then present a study of heat flux as a function of the grating
height, highlighting a remarkable amplification of the exchanged energy,
ascribed to the appearance of spoof-plasmon modes, whose behavior is also
spectrally investigated. Differently from previous works, our method allows us
to explore a range of grating heights extending over several orders of
magnitude. By comparing our results to recent studies we find a consistent
quantitative disagreement with some previously obtained results going up to
50\%. In some cases, this disagreement is explained in terms of an incorrect
connection between the reflection operators of the two gratings.Comment: 10 pages, 6 figures. Some typos corrected with respect to the
previous versio
Graphene-based amplification and tuning of near-field radiative heat transfer between dissimilar polar materials
The radiative heat transfer between two dielectrics can be strongly enhanced
in the near field in the presence of surface phonon-polariton resonances.
Nevertheless, the spectral mismatch between the surface modes supported by two
dissimilar materials is responsible for a dramatic reduction of the radiative
heat flux they exchange. In the present paper we study how the presence of a
graphene sheet, deposited on the material supporting the surface wave of lowest
frequency, allows to widely tune the radiative heat transfer, producing an
amplification factor going up to one order of magnitude. By analyzing the
Landauer energy transmission coefficients we demonstrate that this
amplification results from the interplay between the delocalized plasmon
supported by graphene and the surface polaritons of the two dielectrics. We
finally show that the effect we highlight is robust with respect to the
frequency mismatch, paving the way to an active tuning and amplification of
near-field radiative heat transfer in different configurations.Comment: 8 pages, 8 figure
Inverse design method for periodic and aperiodic metasurfaces based on the adjoint-method: metalens with random-like distributed nano-rods
The classical adjoint-based topology optimization (TO) method, based on the
use of a random continuous dielectric function as an adjoint variable
distribution, is known to be one of the most efficient optimization methods
that enable the design of optical devices with outstanding performances.
However, the strategy for selecting the optimal solution requires a very fine
pixelation of the permittivity function of the profile under optimization.
Typically, at least 28 pixels are needed while optimizing a one wavelength wide
1D metagrating. This makes it very difficult to extend TO methods to
large-scale optimization problems. In this paper, we introduce a new concept of
adjoint-based topology optimization that enables fast and efficient geometry
based design of both periodic and aperiodic metasurfaces. The structures are
built from nano-rods whose widths and positions are to be adjusted. Our new
approach requires a very low number of design parameters, thus leading to a
drastic reduction in the computational time: about an order of magnitude.
Hence, this concept makes it possible to address the optimization of
large-scale structures in record time. As a proof-of-concept we apply this
method to the design of (i) a periodic metagrating, optimized to have a
specific response into a particular direction, and (ii) a dielectric metalens
(aperiodic metasurface), enabling a high energy focusing into a well-defined
focal spot.Comment: 14 pages , 9 figure
Casimir-Lifshitz force out of thermal equilibrium between dielectric gratings
We calculate the Casimir-Lifshitz pressure in a system consisting of two
different 1D dielectric lamellar gratings having two different temperatures and
immersed in an environment having a third temperature. The calculation of the
pressure is based on the knowledge of the scattering operators, deduced using
the Fourier Modal Method. The behavior of the pressure is characterized in
detail as a function of the three temperatures of the system as well as the
geometrical parameters of the two gratings. We show that the interplay between
non-equilibrium effects and geometrical periodicity offers a rich scenario for
the manipulation of the force. In particular, we find regimes where the force
can be strongly reduced for large ranges of temperatures. Moreover, a repulsive
pressure can be obtained, whose features can be tuned by controlling the
degrees of freedom of the system. Remarkably, the transition distance between
attraction and repulsion can be decreased with respect to the case of two
slabs, implying an experimental interest for the observation of repulsion.Comment: 13 pages, 11 figures, accepted on Phys. Rev.
Casimir interaction between a sphere and a grating
We derive the explicit expression for the Casimir energy between a sphere and
a 1D grating, in terms of the sphere and grating reflection matrices, and valid
for arbitrary materials, sphere radius, and grating geometric parameters. We
then numerically calculate the Casimir energy between a metallic (gold) sphere
and a dielectric (fused silica) lamellar grating at room temperature, and
explore its dependence on the sphere radius, grating-sphere separation, and
lateral displacement. We quantitatively investigate the geometrical dependence
of the interaction, which is sensitive to the grating height and filling
factor, and show how the sphere can be used as a local sensor of the Casimir
force geometric features. To this purpose we mostly concentrate on separations
and sphere radii of the same order of the grating parameters (here of the order
of one micrometer). We also investigate the lateral component of the Casimir
force, resulting from the absence of translational invariance. We compare our
results with those obtained within the proximity force approximation (PFA).
When applied to the sphere only, PFA overestimates the strength of the
attractive interaction, and we find that the discrepancy is larger in the
sphere-grating than in the sphere-plane geometry. On the other hand, when PFA
is applied to both sphere and grating, it provides a better estimate of the
exact results, simply because the effect of a single grating is underestimated,
thus leading to a partial compensation of errors.Comment: 16 pages, 7 figure
Near-field heat transfer between graphene/hBN multilayers
We study the radiative heat transfer between multilayer structures made by a
periodic repetition of a graphene sheet and a hexagonal boron nitride (hBN)
slab. Surface plasmons in a monolayer graphene can couple with a hyperbolic
phonon polaritons in a single hBN film to form hybrid polaritons that can
assist photon tunneling. For periodic multilayer graphene/hBN structures, the
stacked metallic/dielectric array can give rise to a further effective
hyperbolic behavior, in addition to the intrinsic natural hyperbolic behavior
of hBN. The effective hyperbolicity can enable more hyperbolic polaritons that
enhance the photon tunneling and hence the near-field heat transfer. However,
the hybrid polaritons on the surface, i.e. surface plasmon-phonon polaritons,
dominate the near-field heat transfer between multilayer structures when the
topmost layer is graphene. The effective hyperbolic regions can be well
predicted by the effective medium theory (EMT), thought EMT fails to capture
the hybrid surface polaritons and results in a heat transfer rate much lower
compared to the exact calculation. The chemical potential of the graphene
sheets can be tuned through electrical gating and results in an additional
modulation of the heat transfer. We found that the near-field heat transfer
between multilayer structure does not increase monotonously with the number of
layer in the stack, which provides a way to control the heat transfer rate by
the number of graphene layers in the multilayer structure. The results may
benefit the applications of near-field energy harvesting and radiative cooling
based on hybrid polaritons in two-dimensional materials.Comment: 10 pages, 11 figure
MODAL METHOD BASED ON SUBSECTIONAL GEGEN- BAUER POLYNOMIAL EXPANSION FOR LAMELLAR GRATINGS: WEIGHTING FUNCTION, CONVERGENCE AND STABILITY
International audienceThe Modal Method by Gegenbauer polynomials Expan- sion (MMGE) has been recently introduced for lamellar gratings by Edee [8]. This method shows a promising potential of outstanding convergence but still suffers from instabilities when the number of polynomials is increased. In this work, we identify the origin of these instabilities and propose a way to remove them
Enhanced transmission beyond the cut-off through sub-Lambda Annular Aperture Arrays
A cascaded structure of annular aperture arrays perforated in silver films is shown to act as a high quality Fabry-PĂ©rot interferometer (quality factor up to 200). The transmission of a single nanostructured layer exhibits a cut-off wavelength beyond which there is no transmission. It is demonstrated, here, that the double structure permits to overcome this cut-off. It is also found that transmission is enhanced by a factor of 150 for certain wavelengths. This kind of cascaded nanostructured metallic layers offers many promising applications as well as for optical wavelengths than for THz-waves because this effect still exists for perfect metals. It opens up the path for the conception of a new generation of integrated components based on metallo-dielectric structures that can be easily tailored as tunable devices
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