Near-field radiative heat transfer between bodies at the nanoscale can surpass blackbody limits on thermal radiation by orders of magnitude due to contributions from evanescent electromagnetic fields, which carry no energy to the far-field. Thus far, principles guiding explorations of larger heat transfer beyond planar structures have assumed utility in surface nanostructuring, which can enhance the density of states, and further assumed that such design paradigms can approach Landauer limits, in analogy to conduction. We derive fundamental shape-independent limits to radiative heat transfer, applicable in near-through far-field regimes, that incorporate material and geometric constraints such as intrinsic dissipation and finite object sizes, and show that these preclude reaching the Landauer limits in all but a few restrictive scenarios. Additionally, we show that the interplay of material response and electromagnetic scattering among proximate bodies means that bodies which maximize radiative heat transfer actually maximize scattering rather than absorption. Finally, we compare our new bounds to existing Landauer limits, as well as limits involving bodies maximizing far-field absorption, and show that these lead to overly optimistic predictions. Our results have ramifications for the ultimate performance of thermophotovoltaics and nanoscale cooling, as well as related incandescent and luminescent devices.The concept of a blackbody, derived from electromagnetic reciprocity (or detailed balance), has provided a benchmark of the largest emission rates that can be achieved by a heated object: through nanoscale texturing, gray objects can be designed in myriad ways to mimic the response of a blackbody at selective wavelengths [1-3], with implications for a variety of technologies, including high-efficiency solar cells, selective emitters, and thermal sensors [4]. Over the past few decades, much effort has gone toward understanding analogous limits to enhancements of near-field radiative heat transfer (RHT) [5][6][7][8], supported by a rich and growing number of experimental [9-12] and theoretical [13][14][15][16][17] investigations, and motivated by potential applications to thermophotovoltaics [18,19], nanoscale cooling [20], and thermal microscopy [21,22]. A key principle underlying further near-field RHT enhancements is the use of materials supporting bound (plasmon and phonon) polaritons in the infrared, where the Planck distribution peaks at typical temperatures probed in experiments. This leads to strong subwavelength responses tied to corresponding enhancements in the density of states [23][24][25][26]; consequently, the amplified near-field RHT spectrum exhibits a narrow lineshape, justifying focus on selective wavelengths. However, while the properties of such polaritons, particularly their resonance frequencies, associated densities of states, and scattering characteristics can be modified through nanoscale texturing, only recently have computational methods [14][15][16]27] arisen to model RHT between bodi...

We derive bounds on angle-integrated absorption and thermal radiation for arbitrary bodies (of prescribed material susceptibility and domain size) that account for both the per-volume limit on polarization response set by the optical theorem and geometry specific finite size constraints imposed by the interplay of material and radiative losses. We then consider these bounds in a number of common settings, comparing against prior limits as well as nearly optimal structures discovered using topology optimization, and show that they properly capture the transition from the volume scaling characteristics of deeply subwavelength objects (nanoparticle radius or thin film thickness) to the area scaling of the blackbody in the ray optics limit.

We present an approach for computing long-range van der Waals (vdW) interactions between complex molecular systems and arbitrarily shaped macroscopic bodies, melding atomistic treatments of electronic fluctuations based on density functional theory in the former with continuum descriptions of strongly shapedependent electromagnetic fields in the latter, thus capturing many-body and multiple scattering effects to all orders. Such a theory is especially important when considering vdW interactions at mesoscopic scales, i.e., between molecules and structured surfaces with features on the scale of molecular sizes, in which case the finite sizes, complex shapes, and resulting nonlocal electronic excitations of molecules are strongly influenced by electromagnetic retardation and wave effects that depend crucially on the shapes of surrounding macroscopic bodies. We show that these effects together can modify vdW interaction energies and forces, as well as molecular shapes deformed by vdW interactions, by orders of magnitude compared to previous treatments based on Casimir-Polder, nonretarded, or pairwise approximations, which are valid only at macroscopically large or atomic-scale separations or in dilute insulating media, respectively. DOI: 10.1103/PhysRevLett.118.266802 Van der Waals (vdW) interactions play an essential role in noncovalent phenomena throughout biology, chemistry, and condensed-matter physics [1][2][3]. It has long been known that vdW interactions among a system of polarizable atoms are not pairwise additive but instead strongly depend on geometric and material properties [2,4,5]. However, only recently developed theoretical methods have made it possible to account for short-range quantum interactions in addition to long-range many-body screening in molecular ensembles [3,[6][7][8][9][10][11][12][13][14][15], demonstrating that nonlocal many-body effects cannot be captured by simple, pairwise-additive descriptions; these calculations typically neglect electromagnetic retardation effects in molecular systems. Simultaneously, recent theoretical and experimental work has characterized dipolar Casimir-Polder (CP) interactions between macroscopic metallic or dielectric objects and atoms, molecules, or Bose-Einstein condensates, further extending to nonzero temperatures, dynamical situations, and fluctuations in excited states (as in so-called Rydberg atoms) [16][17][18][19][20][21][22][23][24][25]. Yet, while theoretical treatments have thus far accounted for the full electrodynamic response of macroscopic bodies (including retardation), they often treat molecules as point dipoles of some effective bulk permittivities or as collections of noninteracting atomic dipoles, ignoring finite size and other many-body electromagnetic effects.In this Letter, motivated by the aforementioned theoretical developments [1,[16][17][18][24][25][26][27][28], we describe an approach that seamlessly connects atomistic descriptions of large molecules to continuum descriptions of arbitrary macroscopic bodies, characterizing t...

In a complementary article [1], we exploited algebraic properties of Maxwell's equations and fundamental principles such as electromagnetic reciprocity and passivity, to derive fundamental limits to radiative heat transfer applicable in near-through far-field regimes. The limits depend on the choice of material susceptibilities and bounding surfaces enclosing arbitrarily shaped objects. In this article, we apply these bounds to two different geometric configurations of interest, namely dipolar particles or extended structures of infinite area in the near field of one another, and compare these predictions to prior limits. We find that while near-field radiative heat transfer between dipolar particles can saturate purely geometric "Landauer" limits, bounds on extended structures cannot, instead growing much more slowly with respect to a material response figure of merit, an "inverse resistivity" for metals, due to the deleterious effects of multiple scattering; nanostructuring is unable to overcome these limits, which can be practically reached by planar media at the surface polariton condition.Radiative heat transfer (RHT) between two bodies may be written as a frequency integral of the formwhere Π(ω, T ) is the Planck function (and it has been assumed, without loss of generality, that T B > T A so P > 0), and Φ(ω) a dimensionless spectrum of energy transfer. RHT between two objects sufficiently separated in space follows the Planck blackbody law, but in the near-field where separations are smaller than the characteristic thermal wavelength of radiation, contributions to RHT from evanescent modes will dominate, allowing Φ(ω) to exceed the far-field blackbody limits by orders of magnitude. Moreover, because the Planck function decays exponentially with frequency, judicious choice of materials and nanostructured geometries can shift resonances in Φ to lower (especially infrared) frequencies, allowing observation of even larger integrated RHT powers [2-5]. However, after accounting for the effects of such frequency shifts, the degree to which the spectrum Φ at a given frequency can be enhanced remains an open question. The inability of trial-and-error explorations and optimization procedures [6, 7] to saturate prior bounds on Φ based on modal analyses [8][9][10][11] or energy conservation [12] suggests that these prior bounds may be too loose.In a complementary article [1], we derived new bounds that simultaneously account for material and geometric constraints as well as multiple scattering effects. These bounds, valid from the near-through far-field regimes, incorporate the dependence of the optimal modal response of each object on the other while simultaneously being constrained by passivity considerations in isolation. They depend on a general material response factor ("inverse resistivity" for metals) [12],without making explicit reference to specific frequencies or dispersion models, and are domain monotonic, increasing with object volumes independently of their shapes. Consequently, our bounds are applicable ...

Many-body physics aims to understand emergent properties of systems made of many interacting objects. This article reviews recent progress on the topic of radiative heat transfer in many-body systems consisting of thermal emitters interacting in the near-field regime. Near-field radiative heat transfer is a rapidly emerging field of research in which the cooperative behavior of emitters gives rise to peculiar effects which can be exploited to control heat flow at the nanoscale. Using an extension of the standard Polder and van Hove stochastic formalism to deal with thermally generated fields in N -body systems, along with their mutual interactions through multiple scattering, a generalized Landauer-like theory is derived to describe heat exchange mediated by thermal photons in arbitrary reciprocal and non-reciprocal multi-terminal systems. In this review, we use this formalism to address both transport and dynamics in these systems from a unified perspective. Our discussion covers: (i) the description of non-additivity of heat flux and its related effects, including fundamental limits as well as the role of nanostructuring and material choice, (ii) the study of equilibrium states and multistable states, (iii) the relaxation dynamics (thermalization) toward local and global equilibria, (iv) the analysis of heat transport regimes in ordered and disordered systems comprised of a large number of objects, density and range of interactions, and (v) the description of thermomagnetic effects in magneto-optical systems and heat transport mechanisms in non-Hermitian many-body systems. We conclude this review by listing outstanding challenges and promising future research directions.