19 research outputs found
Force de Casimir dans les dispositifs opto-électromécaniques à base de graphène
No full textNous avons étudié la force de Casimir-Lifshitz (CLF) entre des nanostructures à base de graphène, à l'équilibre et hors équilibre thermique, i.e. entre deux feuilles de graphène posées sur un substrat diélectrique se faisant face dans diverses conditions thermiques. Nous avons également étudié la CLF entre deux réseaux de graphène à l'équilibre thermique. Pour analyser ce problème, nous avons utilisé l'approche de la matrice de diffusion (S-matrix), qui nécessite des méthodes numériques efficaces pour être calculée. Parmi ces méthodes, nous avons adapté la Méthode Modale de Fourier équipée de fonctions de base locales (FMM-LBF : Fourier Modal Method - Local basis Functions) en incidence conique. De plus, nous avons étendu cette méthode pour analyser des structures cylindriques recouvertes de bandes de graphène.En ce qui concerne la CLF entre des réseaux de graphène posés sur des substrats diélectriques, nous avons observé une variation notable de celle-ci en fonction du potentiel chimique. Par la suite, nous avons examiné son comportement non-additif et constaté que, pour de petites séparations (d = 60 nm), la non-additivité est relativement faible. En conséquence, nous pouvons utiliser en toute confiance une méthode additive beaucoup plus directe et rapide pour calculer la force avec une précision de quelques pourcents. Cependant, la situation est tout à fait différente pour de plus grandes séparations (d = 200 nm). À cette distance, il y a un effet non-additif significatif, entraînant des écarts allant jusqu'à 30% par rapport à la prédiction additive. Nous avons aussi découvert que cette influence non-additive peut être modulée in situ en ajustant le potentiel chimique du graphène, sans nécessiter de changements dans la géométrie ou les matériaux du système.Après cela, nous avons exploré la CLF en dehors de l'équilibre thermique entre deux couches de graphène à deux températures différentes dans un bain thermique à une troisième température. Contrairement au cas usuel, où la force est toujours attractive dans des conditions d'équilibre, notre étude montre que la force peut être ajustée pour devenir répulsive si l'environnement est à basse température. Nous montrons également que pour obtenir des variations significatives de la CLF, il est essentiel de maintenir une basse température pour les feuilles de graphène, qu'elles soient en équilibre ou hors équilibre thermique. Enfin, nous constatons que la dépendance de la CLF hors équilibre en fonction du potentiel chimique devient non monotone dans une certaine plage de distances.Les différentes approches théoriques et méthodes numériques développées dans cette thèse nous permettrons, dans un futur très proche, d'explorer d'autres situations intéressantes comme le transfert de chaleur et la CLF hors équilibre thermique entre deux réseaux de graphène, la CLF entre un cylindre diélectrique couvert partiellement de graphène et un plan, ou encore le torque de Casimir dans différentes nano-structures à base de graphène.We have studied the Casimir-Lifshitz forces (CLF), a fundamental dispersion force, for graphene-based nano-structures, both under thermal equilibrium and out of thermal equilibrium, i.e., a graphene sheet laid on a dielectric substrate in various thermal conditions and graphene strip-gratings under thermal equilibrium. To analyze this problem, we employed the S-matrix approach, which necessitates efficient methods to determine its elements. Among these methods, we adapted the Fourier Modal Method equipped with Local Basis Functions (FMM-LBF) for our structure undergoing conical diffraction. Additionally, we extended this method to analyze cylindrical structures covered with graphene strips.Concerning the Casimir interaction in graphene gratings laid on dielectric slabs, we observed a notable variation of the CLF with respect to the chemical potential. Subsequently, we examined its non-additive behavior and determined that, for small separations (d = 60 nm), the non-additivity is relatively weak. As a consequence, we can confidently employ a much more direct and rapid approximate additive method to calculate the force with a precision of a few percents.However, the situation is quite different for larger separations (d = 200 nm). At this distance, there is a significant non-additive effect, leading to deviations up to 30% from the additive prediction. Intriguingly, we discovered that this non-additive influence can be modulated in situ by adjusting the graphene chemical potential, without requiring any changes in the system’s geometry or materials.After that, we explored the CLF out of thermal equilibrium (OTE) between two graphene layers at two different temperatures in a thermal bath with a third temperature. Unlike the typical common case, where the force is always attractive under equilibrium conditions, this study shows that the force can be adjusted to switch from attractive to repulsive if the environment is at low temperatures. We also show that in order to achieve significant variations in the CLF, it is essential to maintain low temperatures for the graphene sheets, whether in equilibrium or non- equilibrium conditions. Finally, we found that, for the OTE CLF, the dependence on the chemical potential becomes non-monotonic at a certain range of distances.This work will allow us to study and explore new structures, phenomena, and effects, such as: (i) the heat transfer between two graphene strip gratings (work in progress) since the existing literature predominantly relies on approximate methods like the Effective Medium Theory, (ii) the CLF OTE between graphene strip gratings (work in progress), (iii) the CLF under and out of thermal equilibrium between graphene strips twisted gratings (work in progress) and (iv) The CLF between a cylinder and a plane, between two cylinders or between a cylinder and a graphene grating, and finally (iv) the Casimir torque for graphene based nano-structures
Efficient computation of EM scattering from a dielectric cylinder covered with graphene strips for heat transfer
No full textInternational audienceWe present a numerical approach for the solution of EM scattering from a dielectric cylinder partially covered with graphene. It is based on a clas- sical Fourier-Bessel expansion of the fields inside and outside the cylinder to which we apply the ad-hoc boundary conditions in the presence of graphene. Due to the singular nature of the electric field at the ends of the graphene sheet, we introduce auxiliary boundary conditions to better take this reality into ac- count. The result is a very simple and very efficient method allowing the study of diffraction from such structures. Our ultimate goal is to apply this approach to radiative heat transfer between graphene coated cylinders and planes
Efficient computation of EM scattering from a dielectric cylinder covered with graphene strips
No full textInternational audienceWe present a numerical approach for the solution of EM scattering from a dielectric cylinder partially covered with graphene. It is based on a classical Fourier-Bessel expansion of the fields inside and outside the cylinder to which we apply the ad-hoc boundary conditions in the presence of graphene. Due to the singular nature of the electric field at the ends of the graphene sheet, we introduce auxiliary boundary conditions to better take this reality into account. The result is a very simple and very efficient method allowing the study of diffraction from such structures
Efficient computation of EM scattering from a dielectric cylinder partially covered with a graphene strip
No full textInternational audienceWe present a numerical approach for the solution of EM scattering from a dielectric cylinder partially covered with graphene. It is based on a classical Fourier-Bessel expansion of the fields inside and outside the cylinder to which we apply the ad-hoc boundary conditions in the presence of graphene. Due to the singular nature of the electric field at the ends of the graphene sheet, we introduce auxiliary boundary conditions to better take this reality into account
Electromagnetic scattering by a partially graphene-coated dielectric cylinder : efficient computation and multiple plasmonic resonances
No full textWe present a numerical approach for the solution of electromagnetic scattering from a dielectric cylinder partially covered with graphene. It is based on a classical Fourier-Bessel expansion of the fields inside and outside the cylinder to which we apply ad-hoc boundary conditions in presence of graphene. Due to the singular nature of the electric field at the edges of the graphene sheet, we introduce auxiliary boundary conditions. The result is particularly simple and very efficient method allowing the study of diffraction from such structures. We also highlight the presence of multiple plasmonic resonances that we ascribe to the surface modes of the coated cylinder
Near-field radiative heat transfer between a nanoparticle and a graphene grating
No full textInternational audienceWe investigate the near-field radiative heat transfer between a normally and/or laterally shifted nanoparticle and a planar fused silica slab coated with a strip graphene grating. For this study we develop and use a scattering matrix approach derived from Fourier modal method augmented with local basis functions. We find that adding a graphene sheet coating on the slab can already enhance the heat flux by about 85%. We show that by patterning the graphene sheet coating into a grating, the heat flux is further increased, and this happens thanks to the a topological transition of the plasmonic modes from circular to hyperbolic one, which allows for more energy transfer. The lateral shift affects the accessible range of high- modes and thus affects the heat flux, too. By moving the nanoparticle laterally above the graphene grating, we can obtain an optimal heat flux with strong chemical potential dependance above the strips. For a fixed graphene grating period (m) and not too large normal shift (separation d<800nm), two different types of lateral shift effects (e.g., enhancement and inhibition) on heat transfer have been observed. As the separation is further increased, the lateral shift effect becomes less important. We show that the lateral shift effect is sensitive to the geometric factor . Two distinct asymptotic regimes are proposed: (1) the inhibition regime (d/D<0.85), where the lateral shift reduces the heat transfer, and (2) the neutral regime () where the effect of the lateral shift is negligible. In general, we can say that the geometric factor is a critical point for the lateral shift effect. Our predictions can have relevant implications to the radiative heat transfer and energy management at the nano/micro scale
Near-field radiative heat transfer between a nanoparticle and a graphene grating
Get PDFWe investigate the near-field radiative heat transfer between a normally
and/or laterally shifted nanoparticle and a planar fused silica slab coated
with a strip graphene grating. For this study we develop and use a scattering
matrix approach derived from Fourier modal method augmented with local basis
functions. We find that adding a graphene sheet coating on the slab can already
enhance the heat flux by about 85%. We show that by patterning the graphene
sheet coating into a grating, the heat flux is further increased, and this
happens thanks to the a topological transition of the plasmonic modes from
circular to hyperbolic one, which allows for more energy transfer. The lateral
shift affects the accessible range of high- modes and thus affects the heat
flux, too. By moving the nanoparticle laterally above the graphene grating, we
can obtain an optimal heat flux with strong chemical potential dependance above
the strips. For a fixed graphene grating period (m) and not too large
normal shift (separation nm), two different types of lateral shift
effects (e.g., enhancement and inhibition) on heat transfer have been observed.
As the separation is further increased, the lateral shift effect becomes
less important. We show that the lateral shift effect is sensitive to the
geometric factor . Two distinct asymptotic regimes are proposed: (1) the
inhibition regime (), where the lateral shift reduces the heat
transfer, and (2) the neutral regime () where the effect of the
lateral shift is negligible. In general, we can say that the geometric factor
is a critical point for the lateral shift effect. Our
predictions can have relevant implications to the radiative heat transfer and
energy management at the nano/micro scale.Comment: 12 pages, 13 figure
Numerical assessment of meshless method for studying nanofluid natural convection in a corrugated wall square cavity
No full textThis study aims to investigate the impact of various factors, such as wall shape, Rayleigh number, and volume fraction of nanoparticles, on natural convection in a square cavity that is filled with a mixture of Al2O3 solid particles and liquid water. The research employs numerical simulations based on the radial basis function meshless method and the artificial compressibility technique. The results of the study showed that the temperature distribution in the cavity was mostly uniform, except in the vicinity of the hot wall, while the flow was primarily dominated by convection as the Rayleigh number increased. Furthermore, the heat transfer rate increased with the volume fraction of nanoparticles, indicating the significance of nanoparticles in improving the thermal performance of the system. Additionally, the study found that the average Nusselt number, which characterizes the heat transfer efficiency, was highest when the cavity had a wavy wall. For single and double wavy walls, there were respective enhancements of 32% and 6% compared to a regular wall. Additionally, the Nusselt number increased as the volume fraction of nanoparticles, indicating a significant influence of nanoparticle concentration and wall geometry on the fluid flow and heat transfer characteristics in the square cavity. Consequently, this study's outcomes provide crucial insights into designing and optimizing thermal management systems, particularly those utilizing nanofluids
Effect of graphene grating coating on near-field radiative heat transfer
Get PDFIn this work we analyze the near-field radiative heat transfer (NFRHT)
between finite-thickness planar fused silica slabs coated with graphene
gratings. We go beyond the effective medium approximation by using an exact
Fourier Modal Method (FMM) equipped with specific Local Basis Functions (LBF),
and this is needed for realistic experimental analysis. In general, coating a
substrate with a full graphene sheet has been shown to decrease the NFRHT at
short separations (typically for d<100 nm) compared to the bare substrates,
where the effective medium approximation consistently overestimates the
radiative heat flux, with relative errors exceeding 50%. We show that, by
patterning the graphene sheet into a grating, the topology of the plasmonic
graphene mode changes from circular to hyperbolic, allowing to open more
channels for the energy transfer between the substrates. We show that, at short
separations, the NFRHT between slabs coated with graphene gratings is higher
than that between full-graphene-sheet coated slabs and also than that between
uncoated ones. We show a significant dependence of the radiative heat transfer
on the chemical potential, which can be applied to modulate in situ the
scattering properties of the graphene grating without any geometric
alterations. We also compare the exact calculation with an approximate additive
one and show that this approximation performs quite well for low chemical
potentials. This work has the potential to unveil new avenues for harnessing
non-additive heat transfer effects in graphene-based nanodevices.Comment: 6 pages, 5 figure
Effect of top metallic contacts on energy conversion performances for near-field thermophotovoltaics
No full textInternational audienceWe study Effect of top metallic contacts on energy conversion performances for near-field thermophotovoltaics. We find behaviors differing substantially from those predicted by previous simplistic approaches, with significant impact on the net radiative power absorbed by the cell and, consequently, on the generated electrical power. The design of metallic contact grids on the front side of thermophotovoltaic cells is critical since it can cause significant optical and electrical resistive losses, particularly in the near field. However, from the theoretical point of view, this effect has been either discarded or studied by means of extremely simplified models like the shadowing methods, that consist in simply ignoring the fraction of the semiconductor surface covered by metal. Our study [1], based on a rigorous three-body theoretical framework and implemented using the scattering matrix approach with the Fourier modal method augmented with adaptive spatial resolution, provides deeper insight into the influence of the front metal contact grid. This approach allows direct access to the radiative power absorbed by the semiconductor, enabling the proposal of an alternative definition for the thermophotovoltaic cell efficiency. By modeling this grid as a metallic grating, we demonstrate its significant impact on the net radiative power absorbed by the cell and, consequently, on the generated electrical power. Our analysis reveals behaviors differing substantially from those predicted by previous simplistic approaches. References1.Youssef Jeyar, Kevin Austry, Minggang Luo, Brahim Guizal, Yi Zheng, Riccardo Messina, Rodolphe Vaillon, and Mauro Antezza, arXiv:2412.04258 (2024
