We consider the space of all causal bulk materials, 2D materials, and metamaterials for maximum near-field radiative heat transfer (RHT). Causality constrains the bandwidth over which plasmonic response can occur, explaining two key traits in ideal materials: small background permittivities (minimal high-energy transitions in 2D materials), and Drude-like free-carrier response, which together optimally yield 10X enhancements beyond the theoretical state-of-the-art. We identify transparent conducting oxides, III-Nitrides, and graphene as materials that should offer nearly ideal near-field RHT rates, if doped to exhibit plasmonic resonances at what we term "near-field Wien frequencies." Deep-subwavelength patterning can provide marginal further gains, at the expense of extremely small feature sizes. Optimal materials have moderate loss rates and plasmonic response at 19 μm for 300K temperature, suggesting a new opportunity for plasmonics at mid- to far-infrared wavelengths, with low carrier concentrations and no requirement to minimize loss.
Near-field radiative heat transfer between close objects may exceed the far-field blackbody radiation in orders of magnitude when exploiting polaritonic materials. Great efforts have been made to experimentally measure this fundamental stochastic effect but mostly based on simple materials. In this work, we foster an all-optical method to characterize the heat transfer between less explored plasmon-phonon hybrid polaritonic systems made of graphene-SiC heterostructures. A large heat flux about 26 times of the blackbody radiation limit is obtained over a 150-nm vacuum gap, attributed to the cou…
We report a detailed formalism aimed at the thermal modeling and heat mitigation in high-power double-clad fiber amplifiers. Closed form analytical formulas are developed that take into account the spatial profile of the amplified signal and pump in the double-clad geometry, the presence of the amplified spontaneous emission, and the possibility of radiative cooling due to anti-Stokes fluorescence emission. The formalism is applied to a high-power Yb-doped silica fiber amplifier. The contributions to the heat-load from the pump-signal quantum defect, as well as the pump and signal parasitic ab…
A thermostat senses the temperature of a physical system and switches heating or cooling devices on or off, regulating the flow of heat to maintain the system's temperature near a desired setpoint. Taking advantage of recent advances in radiative heat transfer technologies, here we propose a passive radiative "thermostat" based on phase-change photonic nanostructures for thermal regulation at room temperature. By self-adjusting their visible to mid-IR absorptivity and emissivity responses depending on the ambient temperature, the proposed devices use the sky to passively cool or heat during da…
The study of radiative heat transfer in particulate system is usually based on radiative transfer equation (RTE) with effective radiative properties. However, for non-random, densely and regularly packed particulate systems, the applicability of RTE is questionable due to dependent scattering and weak randomness of particle arrangement. In this paper, a new continuum approach that does not explicitly rely on the RTE is proposed for radiative heat transfer in the densely packed particulate system. The new approach is based on the generalization of the concept of radiation distribution factor (R…
We study the effect of particles shape on the radiative heat transfer in a three-body system. It is found that the radiative heat flux between two nanoparticles in a three body system can be tuned by the shape of the third particle. In particular, we show that the heat flux is very sensitive to the particle shapes and slight mismatches of shapes results in either enhanced or suppressed heat flux.
We develop an analytical framework for nonreciprocal radiative heat transfer in two-body planar systems. Based on our formalism, we identify effects that are uniquely nonreciprocal in near-field heat transfer in planar systems. We further introduce a general thermodynamic constraint that is applicable for both reciprocal and nonreciprocal planar systems, in agreement with the second law of thermodynamics. We numerically demonstrate our findings in an example system consisting of magneto-optical materials. Our formalism applies to both near- and far-field regimes, opening opportunities for expl…
We demonstrate the existence of a shuttling effect for the radiative heat flux exchanged between two bodies separated by a vacuum gap when the chemical potential of photons or the temperature difference is modulated. We show that this modulation typically gives rise to a supplementary flux which superimposes to the flux produced by the mean gradient, enhancing the heat exchange. When the system displays a negative differential thermal resistance, however, the radiative shuttling contributes to insulate the two bodies from each other. These results pave the way for a novel strategy for an activ…
Near-field radiative heat transfer allows heat to propagate across a small vacuum gap in quantities that are several orders of magnitude greater then the heat transfer by far-field, blackbody radiation. Although heat transfer via near-field effects has been discussed for many years, experimental verification of this theory has been very limited. We have measured the heat transfer between two macroscopic sapphire plates, finding an increase in agreement with expectations from theory. These experiments, conducted near 300 K, have measured the heat transfer as a function of separation over mm to …
In this work, we study the near-field radiative heat transfer between two suspended sheets of anisotropic 2D materials. It is found that the radiative heat transfer can be enhanced with orders-of-magnitude over the blackbody limit for nanoscale separation. The enhancement is attributed to the excitation of anisotropic and hyperbolic plasmonic modes. Meanwhile, a large thermal modulation effect, depending on the twisted angle of principal axes between the upper and bottom sheets of anisotropic 2D materials, is revealed. The near-field radiative heat transfer for different concentrations of elec…
Micro-nanoparticle systems have wide applications in thermal science and technology. In dense particulate system, the particle separation distance may be less than the characteristic thermal wavelength and near field effect will be significant and become a key factor to influence thermal radiation transfer in the system. In this study, radiative heat transfer (RHT) between two metallic nanoparticles clusters are explored using many-body radiative heat transfer theory implemented with the coupled electric and magnetic dipole (CEMD) approach, which effectively takes into account the contribution…
We study the photon thermal Hall effect and the persistent heat current in radiative heat transfer. We show that the photon thermal Hall effect is not a uniquely nonreciprocal effect; it can arise in some reciprocal systems with broken mirror symmetry. This is in contrast with the persistent heat current, which is a uniquely non-reciprocal effect that can not exist in any reciprocal system. Nevertheless, for a specific class of systems with $C_4$ rotational symmetry, we note that the photon thermal Hall effect is uniquely nonreciprocal, and moreover there is a direct connection between the per…
We demonstrate in this work that the use of metasurfaces provides a viable strategy to largely tune and enhance near-field radiative heat transfer between extended structures. In particular, using a rigorous coupled wave analysis, we predict that Si-based metasurfaces featuring two-dimensional periodic arrays of holes can exhibit a room-temperature near-field radiative heat conductance much larger than any unstructured material to date. We show that this enhancement, which takes place in a broad range of separations, relies on the possibility to largely tune the properties of the surface plasm…
In this letter, we study the near-field radiative heat transfer between two metamaterial substrates coated with silicon carbide (SiC) thin films. It is known that metamaterials can enhance the near-field heat transfer over ordinary materials due to excitation of magnetic plasmons associated with s polarization, while strong surface phonon polariton exists for SiC.By careful tuning of the optical properties of metamaterial it is possible to excite electrical and magnetic resonance for the metamaterial and surface phonon polaritons for SiC at different spectral regions, resulting in the enhanced…
We present a theoretical analysis that demonstrates that the far-field radiative heat transfer between objects with dimensions smaller than the thermal wavelength can overcome the Planckian limit by orders of magnitude. We illustrate this phenomenon with micron-sized structures that can be readily fabricated and tested with existing technology. Our work shows the dramatic failure of the classical theory to predict the far-field radiative heat transfer between micro- and nano-devices.
A parametric study of Drude and Lorentz models performances in maximizing near-field radiative heat transfer between two semi-infinite planes separated by nanometric distances at room temperature is presented in this paper. Optimal parameters of these models that provide optical properties maximizing the radiative heat flux are reported and compared to real materials usually considered in similar studies, silicon carbide and heavily doped silicon in this case. Results are obtained by exact and approximate (in the extreme near-field regime and the electrostatic limit hypothesis) calculations. T…
We compute the radiative heat transfer between nanostructured gold plates in the framework of the scattering theory. We predict an enhancement of the heat transfer as we increase the depth of the corrugations while keeping the distance of closest approach fixed. We interpret this effect in terms of the evolution of plasmonic and guided modes as a function of the grating's geometry.
Through quantum mechanical considerations, we optimize the material response factor $|χ|^2/\text{Im}\left[χ\right]$, which plays a pivotal role in the fundamental limits of near-field radiative heat transfer (RHT). A comparison of the limits obtained to experimental data for select materials shows that current materials fall several orders of magnitude short of the optimized values, suggesting the possibility of significant improvement in the rate of radiative heat transfer between two bodies. This work informs material design efforts that seek to optimize RHT, as well as provides insights int…
In the present work, we theoretically demonstrate, for the first time, that near field radiative transport between 1D periodic grating microstructures separated by subwavelength vacuum gaps can be significantly enhanced by exciting magnetic resonance or polariton. Fluctuational electrodynamics that incorporates scattering matrix theory with rigorous coupled wave analysis is employed to exactly calculate the near field radiative heat flux between two SiC gratings. Besides the well known coupled surface phonon polaritons (SPhP), an additional spectral radiative heat flux peak, which is due to ma…
We study the constraints on reciprocal and non-reciprocal many-body radiative heat transfer imposed by symmetry and the second law of thermodynamics. We show that the symmetry of such a many-body system in general forms a magnetic group, and the constraints of the magnetic group on the heat transfer can be derived using a generalized reciprocity theorem. We also show that the second law of thermodynamics provides additional constraints in the form of a nodal conservation law of heat flow at equilibrium. As an application, we provide a systematic approach to determine the existence of persisten…
Here, we outline a theory of radiative heat transfer based on an equivalent electrical network representation for the hot material slabs in an arbitrary multilayered environment with arbitrary distribution of temperatures and electromagnetic properties among the layers. Our approach is fully equivalent to the known theories operating with the fluctuating current density, while being significantly simpler in analysis and applications.
An exact analytical description of the internal radiative field inside an emitting-absorbing gray semi-transparent medium enclosed in a two-dimensional parallelogram cavity is proposed. The expressions of the incident radiation and the radiative flux field are angularly and spatially discretized with a double Gauss quadrature, and the temperature field is obtained by using an iterative process. Some numerical solutions are tabulated and graphically presented as the benchmark solutions. Temperature and two components of the radiative flux are finally sketched on the whole domain. It is shown th…
In this letter, we study the radiative heat transfer between two nanoparticles in the near field and in the far field. We find that the heat transfer is dominated by the electric dipole-dipole interaction for dielectric particles and by the magnetic dipole-dipole interaction for metallic nanoparticles. We introduce polarizabilities formulas valid for arbitrary values of the skin depth. While the heat transfer mechanism is different for metallic and dielectric nanoparticles, we show that the distance dependence is the same. However, the dependence of the heat flux on the particle radius is diff…
We present a theoretical study of the effect of an external dc magnetic field in the near-field radiative heat transfer between two one-dimensional magneto-photonic crystals with unit cells comprising a magneto-optical layer made of n-doped InSb and a dielectric layer. We find that in absence of an external field, and depending on the gap size, the radiative heat transfer between these multilayer structures can be larger or smaller than that of the case of two InSb infinite plates. On the other hand, when an external magnetic field is applied, the near-field radiative heat transfer is reduced …
Condensation and sublimation of water vapors (and CO2, CH4, N2O vapors also) in the Earth atmosphere must be accompanied by emission of latent heats on characteristic frequencies marked in absorption spectra. Calculated wave lengths completely explain all peaks observed for these gases in the near IR. Established phenomena require further investigations, re-estimation of atmospheric heat balances and so on. Investigation of analogical peaks in atmospheres of other planets can be used for analyses of their structures.
Here, we develop a theory of radiative heat transfer based on an equivalent electrical network representation for the hot material slabs in an arbitrary multilayered environment with arbitrary distribution of temperatures and electromagnetic properties among the layers. Our approach is fully equivalent to the known theories operating with the fluctuating current density, while being significantly simpler in analysis and applications. A practical example of the near-infrared heat transfer through the micron gap filled with an indefinite metamaterial is considered using the suggested method. The…
We calculate numerically the heat transfer rate between a spatially dispersive sphere and a half-space. By utilising Huygens' principle and the extinction theorem, we derive the necessary reflection coefficients at the sphere and the plate without the need to resort to additional boundary conditions. We find for small distances $d\sim 1$nm a significant modification of the spectral heat transfer rate due to spatial dispersion. As a consequence, the spurious divergencies that occur in spatially local approach are absent.
Localized cavity resonances due to nanostructures at material surfaces can greatly enhance radiative heat transfer (RHT) between two closely placed bodies owing to stretching of cavity states in momentum space beyond light line. Based on such understanding, we numerically demonstrate the possibility of ultra-broadband super-Planckian RHT between two plates patterned with trapezoidal-shaped hyperbolic metamaterial (HMM) arrays. The phenomenon is rooted not only in HMM's high effective index for creating sub-wavelength resonators, but also its extremely anisotropic isofrequency contour. The two …
We derive shape-independent limits to the spectral radiative heat-transfer rate between two closely spaced bodies, generalizing the concept of a black body to the case of near-field energy transfer. Through conservation of energy and reciprocity, we show that each body of susceptibility $χ$ can emit and absorb radiation at enhanced rates bounded by $|χ|^2 / \textrm{Im} χ$, optimally mediated by near-field photon transfer proportional to $1/d^2$ across a separation distance $d$. Dipole--dipole and dipole--plate structures approach restricted versions of the limit, but common large-area structur…
The control of electric currents in solids is at the origin of the modern electronics revolution which has driven our daily life since the second half of 20th century. Surprisingly, to date, there is no thermal analog for a control of heat flux. Here, we summarize the very last developments carried out in this direction to control heat exchanges by radiation both in near and far-field in complex architecture networks.
Modulation of near-field radiative heat transfer (NFRHT) with rotated anisotropic hBN and MoO3 nanoparticles (NPs) are investigated. The spectral heat power, total heat power and electric field energy density are calculated at different particle orientations. Numerical results show that the modulation factor of the MoO3 NPs could be up to ~12000 with particle radius of 40 nm at a gap distance of 200 nm, which is ~ 10-fold larger than the state of the art. Excitation of localized hyperbolic phonon polaritons (LHPPs) in different particle orientations allow for the excellent modulation. The modu…
The properties of thermal radiation exchange between hot and cold objects can be strongly modified if they interact in the near field where electromagnetic coupling occurs across gaps narrower than the dominant wavelength of thermal radiation. Using a rigorous fluctuational electrodynamics approach, we predict that ultra-thin films of plasmonic materials can be used to dramatically enhance near-field heat transfer. The total spectrally integrated film-to-film heat transfer is over an order of magnitude larger than between the same materials in bulk form and also exceeds the levels achievable w…
We study in this work the near-field radiative heat transfer between two semi-infinite parallel planes of highly n-doped semiconductors. Using a nonlocal model of the dielectric permittivity, usually used for the case of metallic planes, we show that the radiative heat transfer coefficientsaturates as the separation distance is reduced for high doping concentration. These results replace the 1/d${}^2$ infinite divergence obtained in the local model case. Different features of the obtained results are shown to relate physically to the parameters of the materials, mainly the doping concentration…
Although near-field radiative heat transfer was introduced in the 1950's, interest in the field has recently revived, as the effect promises improved performance in various applications where contactless temperature regulation in the small-scale is a requirement. With progress in computational electromagnetics as well as in nanoinstrumentation, it has become possible to simulate the effect in complex configurations and to measure it with high precision. In this Perspective, we highlight key theoretical and experimental advances in the field, and we discuss important developments in tailoring a…
Here, we study the role of topological edge states of light in the transport of thermally generated radiation in a closed cavity at a thermodynamic equilibrium. It is shown that even in the zero temperature limit - when the field fluctuations are purely quantum mechanical - there is a persistent flow of electromagnetic momentum in the cavity in closed orbits, deeply rooted in the emergence of spatially separated unidirectional edge state channels. It is highlighted the electromagnetic orbital angular momentum of the system is nontrivial, and that the energy circulation is towards the same dire…
We predict a huge anisotropic thermal magnetoresistance (ATMR) in the near-field radiative heat transfer between magneto-optical particles when the direction of an external magnetic field is changed with respect to the heat current direction. We illustrate this effect with the case of two InSb spherical particles where we find that the ATMR amplitude can reach values of up to 800% for a magnetic field of 5 T, which is many orders of magnitude larger than its spintronic analogue in electronic devices. This thermomagnetic effect could find broad applications in the fields of ultrafast thermal ma…
We study the heat transfer between two parallel metallic semi-infinite media with a gap in the nanometer-scale range. We show that the near-field radiative heat flux saturates at distances smaller than the metal skin depth when using a local dielectric constant and investigate the origin of this effect. The effect of non-local corrections is analysed using the Lindhard-Mermin and Boltzmann-Mermin models. We find that local and non-local models yield the same heat fluxes for gaps larger than 2 nm. Finally, we explain the saturation observed in a recent experiment as a manifestation of the skin …
Metamaterials possess artificial bulk and surface electromagnetic states. Tamed dispersion properties of surface waves allow one to achieve controllable super-Planckian radiative heat transfer (RHT) process between two closely spaced objects. We numerically demonstrate enhanced RHT between two 2D grooved metal plates by a full-wave scattering approach. The enhancement originates from both transverse magnetic spoof surface plasmon polaritons and a series of transverse electric bonding- and anti-bonding waveguide modes at surfaces. The RHT spectrum is frequency-selective, and highly geometricall…
Topological phonon polaritons (TPhPs) are promising optical modes relevant in long-range radiative heat transfer, information processing and infrared sensing, whose topological protection is expected to enable their robust existence and transport. In this work we show that TPhPs can be supported in one-dimensional (1D) bichromatic silicon carbide nanoparticle (NP) chains, and demonstrate that they can considerably enhance radiative heat transfer for an array much longer than the wavelength of radiation. By introducing incommensurate or commensurate modulations on the interparticle distances, t…
Radiative heat transfer between uniform plates is bounded by the narrow range and limited contribution of surface waves. Using a combination of analytical calculations and numerical gradient-based optimization, we show that such a limitation can be overcome in complicated multilayer geometries, allowing the scattering and coupling rates of slab resonances to be altered over a broad range of evanescent wavevectors. We conclude that while the radiative flux between two inhomogeneous slabs can only be weakly enhanced, the flux between a dipolar particle and an inhomogeneous slab---proportional to…
Very recently it has been predicted that the far-field radiative heat transfer between two macroscopic systems can largely overcome the limit set by Planck's law if one of their dimensions becomes much smaller than the thermal wavelength ($λ_{\rm Th} \approx 10\, μ$m at room temperature). To explore the ultimate limit of the far-field violation of Planck's law, here we present a theoretical study of the radiative heat transfer between two-dimensional (2D) materials. We show that the far-field thermal radiation exchanged by two coplanar systems with a one-atom-thick geometrical cross section ca…
We present here a generalization of the thermal discrete dipole approximation (TDDA) that allows us to describe the near-field radiative heat transfer between finite objects of arbitrary shape that exhibit magneto-optical (MO) activity. We also extend the TDDA approach to describe the thermal emission of a finite object with and without MO activity. Our method is also valid for optically anisotropic materials described by an arbitrary permittivity tensor and we provide simple closed formulas for the basic thermal quantities that considerably simplify the implementation of TDDA method. Moreover…
We show in this article that phase change materials (PCM) exhibiting a phase transition between a dielectric state and a metallic state are good candidates to perform modulation as well as amplification of radiative thermal flux. We propose a simple situation in plane parallel geometry where a so-called radiative thermal transistor could be achieved. In this configuration, we put a PCM between two blackbodies at different temperatures. We show that the transistor effect can be achieved easily when this material has its critical temperature between the two blackbody temperatures. We also see, t…
