arXiv subjects

Sheila Edalatpour

Publications and source records attributed to Sheila Edalatpour.

16 recordsLinked to original sources

arXiv2026

Optimized Nanogap Thermophotovoltaic Devices for Waste Heat Recovery

Nanogap thermophotovoltaic (TPV) devices can deliver high power densities even with the medium-temperature heat sources. As such, these devices are very promising for recovering industrial waste heat. So far, the demonstrated nanogap TPVs have shown performances far below optimal. The objective of this study is to identify the optimal designs for nanogap TPV devices targeted for industrial waste heat recovery. Optimal configurations for maximal power density, maximal efficiency, and a trade-off between the two are determined as a function of the size of the vacuum gap between the emitter and the photovoltaic (PV) cell. The effects of adding a metallic cover to the PV cell, as well as introducing an air gap between the PV cell and the reflector, are also studied through this optimization framework. Results show that the optimal device configuration is highly sensitive to the vacuum gap size. A metallic cover enhances power density for gaps below 125nm due to surface plasmon-polariton coupling, but significantly reduces efficiency due to its parasitic absorption. To realize the benefits of air gaps, ultrathin PV cells requiring mechanical support by a substrate are needed. The presence of the substrate, however, diminishes the benefits of the air gap rendering them ineffective. ITO and InAs are found as optimal materials for the emitter and PV cell, respectively, owing to tunable plasma frequency of ITO and low bandgap of InAs.

physics.optics↗

arXiv2025

Interfacial Heat Transport via Evanescent Radiation by Hot Electrons

We predict an additional thermal transport pathway across metal/non-metal interfaces with large electron-phonon non-equilibrium via evanescent radiative heat transfer. In such systems, electron scattering processes vary drastically and can be leveraged to guide heat across interfaces via radiative heat transport without engaging the lattice directly. We employ the formalism of fluctuational electrodynamics to simulate the spectral radiative heat flux across the interface of a metal film and a non-metal substrate. We find that the radiative conductance can exceed 300 MW m$^{-2}$ K$^{-1}$ at an electron temperature of 5000 K for an emitting tungsten film on a hexagonal boron nitride substrate, becoming comparable to its conductive counterpart. This allows for a more holistic approach to the heat flow across interfaces, accounting for electron-phonon non-equilibrium and ultrafast near-field phonon-polariton coupling.

cond-mat.mes-hall↗

arXiv2024

Enhancing Near-Field Radiative Heat Transfer between Dissimilar Dielectric Media by Coupling Surface Phonon Polaritons to Graphenes Plasmons

Dielectric media are very promising for near-field radiative heat transfer (NFRHT) applications as these materials can thermally emit surface phonon polaritons (SPhPs) resulting in large and quasi-monochromatic heat fluxes. Near-field radiative heat flux between dissimilar dielectric media is much smaller than that between similar dielectric media and is also not quasi-monochromatic. This is due to the mismatch of the SPhP frequencies of the two heat-exchanging dielectric media. Here, we experimentally demonstrate that NFRHT between dissimilar dielectric media increases substantially when a graphene sheet is deposited on the medium with the smaller SPhP frequency. An enhancement of 2.7 to 3.2 folds is measured for the heat flux between SiC and LiF separated by a vacuum gap of size 100 to 140 nm when LiF is covered by a graphene sheet. This enhancement is due to the coupling of SPhPs and surface plasmon polaritons (SPPs). The SPPs of graphene are coupled to the SPhPs of LiF resulting in coupled SPhP-SPPs with a dispersion branch monotonically increasing with the wavevector. This monotonically increasing branch of dispersion relation intersects the dispersion branch of the SPhPs of SiC causing the coupling of the surface modes across the vacuum gap, which resonantly increases the heat flux at the SPhP frequency of SiC. This surface phonon-plasmon coupling also makes NFRHT quasi-monochromatic, which is highly desired for applications such as near-field thermophotovoltaics and thermophotonics. This study experimentally demonstrates that graphene is a very promising material for tuning the magnitude and spectrum of NFRHT between dissimilar dielectric media.

physics.optics↗

arXiv2024

Phonon Thermal Transport between Two in-Plane, Two-Dimensional Nanoribbons in the Extreme Near-Field Regime

The phonon thermal conductance of sub-nanometric vacuum gaps between two in-plane nanoribbons of two-dimensional materials (graphene and silicene) is analyzed using the atomistic Green's function method and by employing the Tersoff and Lennard-Jones potentials for describing the interatomic interactions. It is found that the phonon conductance decays exponentially with the size of the gap. Three exponential regimes have been identified. In the regime where the Lennard-Jones (L-J) potential is driven by the repulsive interatomic forces, caused by the overlap of electronic orbits, there is a sharp exponential decay in conductance as the gap increases (exp(-10.0d) for graphene). When both the repulsive and attractive (van der Waals) interatomic forces contribute to the L-J potential, the decay rate of the conductance significantly reduces to exp(-2.0d) for graphene and exp(-2.5d) for silicene. In the regime where attractive van der Waals forces dominate the L-J potential, phonon conductance has the slowest exponential decay as exp(-1.3d) for both silicene and graphene. It is also found that the contribution from the optical phonons to the conductance is non-negligible only for very small gaps between graphene nanoribbons (d < 1.6 Å). The phonon conductance of the gap is shown to vary with the width of the nanoribbon very modestly, such that the thermal conductivity of the gap linearly increases with the nanoribbon widths. The results of this study are of significance for fundamental understanding of heat transfer in the extreme near-field regime and for predicting the effect of interfaces and defects on heat transfer.

cond-mat.mes-hall↗

arXiv2023

Probing Near-Field Thermal Emission of Localized Surface Phonons from Silicon Carbide Nanopillars

Thermal emission of localized surface phonons (LSPhs) from nanostructures of polaritonic materials is a promising mechanism for tuning the spectrum of near-field thermal radiation. Previous studies have theoretically shown that thermal emission of LSPhs results in narrow-band peaks in the near-field spectra, whose spectral locations can be modulated by changing the dimensions of the nanostructure. However, near-field thermal emission of LSPhs has not been experimentally explored yet. In this study, we measure the spectrum of near-field thermal radiation from arrays of 6H-silicon carbide (6H-SiC) nanopillars using an internal-reflection-element based spectroscopy technique. We present an experimental demonstration of thermal emission of the transverse dipole, quadrupole, and octupole, as well as longitudinal monopole from 6H-SiC nanopillars at a near-field distance from the array. We show that the spectral location of the longitudinal monopole and transverse dipole are significantly affected by the near-field coupling between neighboring nanopillars as well as the intercoupling of the nanopillars and the substrate. We also experimentally demonstrate that the spectrum of near-field thermal radiation from 6H-SiC nanopillar arrays can be tuned by varying the dimensions of the nanopillars, providing an opportunity for designing emitters with tailored near-field thermal radiation.

cond-mat.mes-hall↗

arXiv2022

The Effect of Non-Local Electrical Conductivity on Near-Field Radiative Heat Transfer between Graphene Sheets

Graphene's near-field radiative heat transfer is determined from its electrical conductivity, commonly modeled using the local Kubo and Drude formulas. In this letter, we analyze the non-locality of graphene's electrical conductivity using the Lindhard model combined with the Mermin relaxation time approximation. We also study how the variation of electrical conductivity with wavevector affects near-field radiative conductance between two graphene sheets separated by a vacuum gap. It is shown that the variation of electrical conductivity with wavevector, $k_ρ$, is appreciable for $k_ρ$s greater than $100k_0$, where $k_0$ is the magnitude of the wavevector in the free space. The Kubo electrical conductivity provides an accurate estimation of the spectral radiative conductance between two graphene sheets except for around the surface-plasmon-polariton frequency of graphene and at separation gaps smaller than 20 nm where there is a non-negligible contribution from modes with $k_ρ>100k_0$ to the radiative conductance. The Drude formula proves to be inaccurate for modeling the electrical conductivity and radiative conductance of graphene except for at temperatures much below the Fermi temperature and frequencies much smaller than $2μ_c/{\hbar}$, where $μ_c$ and ${\hbar}$ are the chemical potential and reduced Planck's constant, respectively. It is also shown that the electronic scattering processes should be considered in the Lindhard model properly, such that the local electron number is conserved. A substitution of $ω$ by $ω+iγ$ ($ω$, $i$, and $γ$ being the angular frequency, imaginary unit, and scattering rate, respectively) in the collisionless Lindhard model does not satisfy the conservation of the local electron number and results in significant errors in computing graphene's electrical conductivity and radiative conductance.

cond-mat.mes-hall↗

arXiv2021

The Quantum Confinement Effect on the Spectrum of Near-Field Thermal Radiation by Quantum Dots

The quantum confinement effect on the spectrum of near-field thermal radiation by periodic and random arrays of quantum dots (QDs) is investigated. The local density of states (LDOS) thermally emitted by QD arrays made of three lead chalcogenides, namely, lead sulfide, lead selenide, and lead telluride, is computed at a near-field distance from the arrays. The dielectric function of the QDs is extracted from their absorption spectra by utilizing an optimization technique. The thermal discrete dipole approximation is used for computing the LDOS. It is shown that the peak wavenumber of near-field LDOS emitted by periodic arrays of lead chalcogenide QDs can be significantly modulated (up to 4490 cm-1) by varying the size of the dots. The LDOS is proportional to the imaginary part of the QDs' polarizability which peaks at the bandgap energy of the QDs. The bandgap energy of the QDs (and thus the LDOS peak) is significantly affected by the quantum confinement effect which is size-dependent. While the magnitude of thermal radiation by random arrays of QDs can be different from the periodic arrays with the same filling factor by up to 26%, the LDOS spectrum and peak location are the same for both periodic and random arrays. The peak wavenumber of near-field radiative heat transfer between the QD arrays is also strongly affected by quantum confinement in the QDs, and thus it can be tuned by changing the size of the QDs.

cond-mat.mes-hall↗

arXiv2020

Spatial Coherence of Thermal Emission by a Sphere

Analytical expressions for calculating the energy density and spatial correlation function of thermal emission by a homogeneous, isothermal sphere of arbitrary size and material are presented. The spectral distribution and the power law governing the distance-dependent energy density are investigated in the near-field and far-field regimes for silicon carbide, silicon and tungsten spheres of various size parameters ranging from X = 0.002 to 5. The spatial coherence of thermal field emitted by spheres is also studied in both radial and polar directions. The energy density follows a power law of d^-2 (d is the observation distance) in the far field for all sizes and materials. The power law in the near field is strongly dependent on the material, size parameter, and the ratio d/a (a is the sphere radius). In the near field, the energy density follows a power law of d^-6 when X<<1 and d/a>>1 (similar to an electric point dipole). With increasing X or decreasing d/a, the contribution of multipoles to the energy density increases resulting in an increase in the power of d until the power law converges to that for a semi-infinite medium. The spatial correlation length in the radial direction is in the orders of $λ$, 0.1$λ$, and 0.001$λ$ in the far field, intermediate near field, and extreme near field, respectively. The correlation angle in the extreme near field is strongly dependent on the sphere size parameter, such that it decreases by three orders of magnitude (from 0.5$π$ to 0.001$π$) when X increases from 0.002 to 5. In the intermediate near field and far field, the correlation angle retains the same order of magnitude (0.15$π$ - 0.7$π$) for all considered Xs. While the excitation of dipolar localized surface phonons (LSPhs) does not affect the correlation length and angle, the multipolar LSPhs reduce the spatial coherence in both directions.

physics.optics↗

arXiv2019

The Measurement of Near-Field Thermal Emission Spectra using an Infrared Waveguide

We describe a simple and robust method using an internal reflection element acting as an infrared waveguide to measure the spectra of near-field thermal emission. We experimentally demonstrate the spectrally-narrow peaks of near-field thermal emission by isotropic media due to the excitation of surface phonon-polaritons in quartz and amorphous silica and due to the frustrated total-internal-reflection modes in amorphous silica and polytetrafluoroethylene. Additionally, we demonstrate the broadband near-field thermal emission of hyperbolic modes in hexagonal boron nitride which is an anisotropic uniaxial medium. We also present a theoretical approach based on the fluctuational electrodynamics and dyadic Green's functions for one-dimensional layered media for accurate modeling of the measured spectra.

physics.ins-det↗

arXiv2019

Near-Field Thermal Emission by Periodic Arrays

Near-field thermal emission can be engineered by using periodic arrays of sub-wavelength emitters. The array thermal emission is dependent on the shape, size, and materials properties of the individual elements as well as the period of the array. Designing periodic arrays with desired properties requires models that relate the array geometry and material properties to its near-field thermal emission. In this study, a periodic method is presented for modeling two-dimensional periodic arrays of sub-wavelength emitters. This technique only requires discretizing one period of the array, and thus is computationally beneficial. In this method, the energy density emitted by the array is expressed in terms of array's Green's functions. The array Green's functions are found using the discrete dipole approximation in a periodic manner by expressing a single point source as a series of periodic arrays of phase-shifted point sources. The presented method can be employed for modeling periodic arrays made of inhomogeneous and complex-shape emitters with non-uniform temperature distribution. The proposed technique is verified against the non-periodic thermal discrete-dipole-approximation simulations, and it is demonstrated that this method can serve as a versatile and reliable tool for studying near-field thermal emission by periodic arrays.

physics.comp-ph↗

arXiv2019

Spectral redshift of the thermal near field scattered by a probe

The physics underlying spectral redshift of thermally generated surface phonon-polaritons (SPhPs) observed in near-field thermal spectroscopy is investigated. Numerically exact fluctuational electrodynamics simulations of the thermal near field emitted by a silicon carbide surface scattered in the far zone by an intrinsic silicon probe show that SPhP resonance redshift is a physical phenomenon. A maximum SPhP redshift of 19 cm-1 is predicted for a 200-nm-diameter hemispherical probing tip and a vacuum gap of 10 nm. Resonance redshift is mediated by electromagnetic gap modes excited in the vacuum gap separating the probe and the surface when the probing tip is much larger than the gap size. The impact of gap modes on the scattered field can be mitigated with a probing tip size approximately equal to or smaller than the vacuum gap. However, sharp probing tips induce important spectral broadening of the scattered field. It is also demonstrated that a dipole approximation with multiple reflections cannot be used for explaining the physics and predicting the amount of redshift in near-field thermal spectroscopy. This work shows that the scattered field in the far zone is a combination of the thermal near field emitted by the surface, and electromagnetic interactions between the probe and the surface. Spectroscopic analysis of near-field thermal emission thus requires a numerically exact fluctuational electrodynamics framework for modeling probe-surface interactions.

physics.app-ph↗

arXiv2018

Apparent spectral shift of thermally generated surface phonon-polariton resonance mediated by a non-resonant film

The physical origin of spectral shift of thermally generated surface phonon-polariton (SPhP) resonance of a silicon carbide (SiC) bulk mediated by a non-resonant film is elucidated. The local density of electromagnetic states (LDOS) in a non-resonant intrinsic silicon (Si) film due to thermal emission by SiC, derived using fluctuational electrodynamics, exhibits a local maximum near SPhP resonant frequency in addition to a lower frequency resonance generated by gap modes emerging in the vacuum gap separating the SiC and Si layers. Multiple reflections within the vacuum gap also induce a LDOS drop around SPhP resonant frequency. As a result, depending on the film thickness to vacuum gap ratio and the location where the LDOS is calculated in the film, the low-frequency resonance can dominate the LDOS, such that SPhP resonance appears to be redshifted. A similar spectral behavior is observed on the monochromatic radiative heat flux absorbed by the Si film. It is shown that apparent spectral (red and blue) shift of SPhP resonance mediated by a non-resonant film is bounded by the transverse and longitudinal optical phonon frequencies of SiC. This work is of importance in applications involving dissimilar materials, such as thermophotovoltaics and thermal rectification, where gap modes may significantly disrupt flux resonance. Gap modes may also be at the origin of the resonance redshift systematically observed in near-field thermal spectroscopy.

physics.optics↗

arXiv2016

Near-field radiative heat transfer between arbitrarily-shaped objects and a surface

A fluctuational electrodynamics-based formalism for calculating near-field radiative heat transfer between objects of arbitrary size and shape and an infinite surface is presented. The surface interactions are treated analytically via Sommerfeld's theory of electric dipole radiation above an infinite plane. The volume integral equation for the electric field is discretized using the thermal discrete dipole approximation (T-DDA). The framework is verified against exact results in the sphere-surface configuration, and is applied to analyze near-field radiative heat transfer between a complex-shaped probe and an infinite plane both made of silica. It is found that when the probe tip size is approximately equal to or smaller than the gap d separating the probe and the surface, coupled localized surface phonon (LSPh)-surface phonon-polariton (SPhP) mediated heat transfer occurs. In this regime, the net spectral heat rate exhibits four resonant modes due to LSPhs along the minor axis of the probe while the net total heat rate in the near field follows a d -0.3 power law. Conversely, when the probe tip size is much larger than the separation gap d, heat transfer is mediated by SPhPs resulting in two resonant modes in the net spectral heat rate corresponding to those of a single emitting silica surface while the net total heat rate approaches a d -2 power law. It is also demonstrated that a complex-shaped probe can be approximated by a prolate spheroidal electric dipole when the thermal wavelength is larger than the major axis of the spheroidal dipole and when the separation gap d is much larger than the radius of curvature of the dipole tip facing the surface.

physics.comp-ph↗

arXiv2015

Near-field thermal electromagnetic transport: An overview

A general near-field thermal electromagnetic transport formalism that is independent of the size, shape and number of heat sources is derived. The formalism is based on fluctuational electrodynamics, where fluctuating currents due to thermal agitation are added to Maxwell's curl equations, and is thus valid for heat sources in local thermodynamic equilibrium. Using a volume integral formulation, it is shown that the proposed formalism is a generalization of the classical electromagnetic scattering framework in which thermal emission is implicitly assumed to be negligible. The near-field thermal electromagnetic transport formalism is afterwards applied to a problem involving three spheres with size comparable to the wavelength, where all multipolar interactions are taken into account. Using the thermal discrete dipole approximation, it is shown that depending on the dielectric function, the presence of a third sphere slightly affects the spatial distribution of power absorbed compared to the two-sphere case. A transient analysis shows that despite a non-uniform spatial distribution of power absorbed, the sphere temperature remains spatially uniform at any instant due to the fact that the thermal resistance by conduction is much smaller than the resistance by radiation. The formalism proposed in this paper is general, and could be used as a starting point for adapting solution methods employed in traditional electromagnetic scattering problems to near-field thermal electromagnetic transport.

physics.comp-ph↗

arXiv2015

Convergence analysis of the thermal discrete dipole approximation

The thermal discrete dipole approximation (T-DDA) is a numerical approach for modeling near-field radiative heat transfer in complex three-dimensional geometries. In this work, the convergence of the T-DDA is investigated by comparison against the exact results for two spheres separated by a vacuum gap. The error associated with the T-DDA is reported for various sphere sizes, refractive indices and vacuum gap thicknesses. The results reveal that for a fixed number of subvolumes, the accuracy of the T-DDA degrades as the refractive index and the sphere diameter to gap ratio increase. A converging trend is observed as the number of subvolumes increases. The large computational requirements associated with increasing the number of subvolumes, and the shape error induced by large sphere diameter to gap ratios, are mitigated by using a nonuniform discretization scheme. Nonuniform discretization is shown to significantly accelerate the convergence of the T-DDA, and is thus recommended for near-field thermal radiation simulations. Errors less than 5% are obtained in 74% of the cases studied by using up to 82712 subvolumes. Additionally, the convergence analysis demonstrates that the T-DDA is very accurate when dealing with surface polariton resonant modes dominating radiative heat transfer in the near field.

physics.comp-ph↗

arXiv2013

The Thermal Discrete Dipole Approximation (T-DDA) for near-field radiative heat transfer simulations in three-dimensional arbitrary geometries

A novel numerical method called the Thermal Discrete Dipole Approximation (T-DDA) is proposed for modeling near-field radiative heat transfer in three-dimensional arbitrary geometries. The T-DDA is conceptually similar to the Discrete Dipole Approximation, except that the incident field originates from thermal oscillations of dipoles. The T-DDA is described in details in the paper, and the method is tested against exact results of radiative conductance between two spheres separated by a sub-wavelength vacuum gap. For all cases considered, the results calculated from the T-DDA are in good agreement with those from the analytical solution. When considering frequency-independent dielectric functions, it is observed that the number of sub-volumes required for convergence increases as the sphere permittivity increases. Additionally, simulations performed for two silica spheres of 0.5 micrometer-diameter show that the resonant modes are predicted accurately via the T-DDA. For separation gaps of 0.5 micrometer and 0.2 micrometer, the relative differences between the T-DDA and the exact results are 0.35% and 6.4%, respectively, when 552 sub-volumes are used to discretize a sphere. Finally, simulations are performed for two cubes of silica separated by a sub-wavelength gap. The results revealed that faster convergence is obtained when considering cubical objects rather than curved geometries. This work suggests that the T-DDA is a robust numerical approach that can be employed for solving a wide variety of near-field thermal radiation problems in three-dimensional geometries.

physics.comp-ph↗