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Using Rytov’s fluctuational electrodynamics framework, Polder and Van Hove predicted that radiative heat transfer between planar surfaces separated by a vacuum gap smaller than the thermal wavelength exceeds the blackbody limit due to tunnelling of evanescent modes. This finding has led to the conceptualization of systems capitalizing on evanescent modes such as thermophotovoltaic converters and thermal rectifiers. Their development is, however, limited by the lack of devices enabling radiative transfer between macroscale planar surfaces separated by a nanosize vacuum gap. Here we measure radiative heat transfer for large temperature differences (∼120 K) using a custom-fabricated device in which the gap separating two 5 × 5 mm 2 intrinsic silicon planar surfaces is modulated from 3,500 to 150 nm. A substantial enhancement over the blackbody limit by a factor of 8.4 is reported for a 150-nm-thick gap. Our device paves the way for the establishment of novel evanescent wave-based systems. Evanescent coupling between surfaces separated by a distance smaller than the thermal wavelength can lead to radiative heat transfer greater than the blackbody limit. Here, the authors demonstrate this between two macroscopic-scale surfaces, paving the way to harnessing the effect in thermal devices.

The impacts of radiative, electrical and thermal losses on the performances of nanoscale-gap thermophotovoltaic (nano-TPV) power generators consisting of a gallium antimonide cell paired with a broadband tungsten and a radiatively-optimized Drude radiator are analyzed. Results reveal that surface mode mediated nano-TPV power generation with the Drude radiator outperforms the tungsten radiator, dominated by frustrated modes, only for a vacuum gap thickness of 10 nm and if both electrical and thermal losses are neglected. The key limiting factors for the Drude- and tungsten-based devices are respectively the recombination of electron-hole pairs at the cell surface and thermalization of radiation with energy larger than the cell absorption bandgap. A design guideline is also proposed where a high energy cutoff above which radiation has a net negative effect on nano-TPV power output due to thermal losses is determined. It is shown that the power output of a tungsten-based device increases by 6.5% while the cell temperature decreases by 30 K when applying a high energy cutoff at 1.45 eV. This work demonstrates that design and optimization of nano-TPV devices must account for radiative, electrical and thermal losses.

Recently, several reports have experimentally shown near-field radiative heat transfer (NFRHT) exceeding the far-field blackbody limit between planar surfaces1–5. However, owing to the difficulties associated with maintaining the nanosized gap required for measuring a near-field enhancement, these demonstrations have been limited to experiments that cannot be implemented in large-scale devices. This poses a bottleneck to the deployment of NFRHT concepts in practical applications. Here, we describe a device bridging laboratory-scale measurements and potential NFRHT engineering applications in energy conversion6,7 and thermal management8–10. We report a maximum NFRHT enhancement of approximately 28.5 over the blackbody limit with devices made of millimetre-sized doped Si surfaces separated by vacuum gap spacings down to approximately 110 nm. The devices use micropillars, separating the high-temperature emitter and low-temperature receiver, manufactured within micrometre-deep pits. These micropillars, which are about 4.5 to 45 times longer than the nanosize vacuum spacing at which radiation transfer takes place, minimize parasitic heat conduction without sacrificing the structural integrity of the device. The robustness of our devices enables gap spacing visualization by scanning electron microscopy (SEM) before performing NFRHT measurements.

The possibility of controlling near-field radiative heat transfer with the use of silicon carbide thin films supporting surface phonon–polaritons in the infrared spectrum is explored. For this purpose, the local density of electromagnetic states is calculated and analyzed within the nanometric gap formed between two SiC films as well as the radiative heat flux exchanged between the thin layers.

The radiative heat exchange on the nanoscale can be tuned using polar dielectric nanostructures.

It is well established that near-field radiative heat transfer (NFRHT) can exceed Planck’s blackbody limit 1 by orders of magnitude owing to the tunnelling of evanescent electromagnetic frustrated and surface modes 2 – 4 , as has been demonstrated experimentally for NFRHT between two large parallel surfaces 5 – 7 and between two subwavelength membranes 8 , 9 . However, although nanostructures can also sustain a much richer variety of localized electromagnetic modes at their corners and edges 10 , 11 , the contributions of such additional modes to further enhancing NFRHT remain unexplored. Here we demonstrate both theoretically and experimentally a physical mechanism of NFRHT mediated by the corner and edge modes, and show that it can dominate the NFRHT in the ‘dual nanoscale regime’ in which both the thickness of the emitter and receiver, and their gap spacing, are much smaller than the thermal photon wavelengths. For two coplanar 20-nm-thick silicon carbide membranes separated by a 100-nm vacuum gap, the NFRHT coefficient at room temperature is both predicted and measured to be 830 W m − 2 K − 1 , which is 5.5 times larger than that for two infinite silicon carbide surfaces separated by the same gap, and 1,400 times larger than the corresponding blackbody limit accounting for the geometric view factor between two coplanar membranes. This enhancement is dominated by the electromagnetic corner and edge modes, which account for 81% of the NFRHT between the silicon carbide membranes. These findings are important for future NFRHT applications in thermal management and energy conversion. Near-field radiative heat transfer between two coplanar silicon carbide membranes in close proximity is enhanced by the electromagnetic corner and edge modes.

The effect of orientation on near-field radiative heat transfer between two complex-shaped superellipsoid particles of SiO2 is presented. The particles under study are 50 nm in radius and of variable concavity. Orientation is characterized by the degree of rotational symmetry in the two-particle systems, and the radiative conductance is calculated using the discrete system Green's function approach to account for all electromagnetic interactions. Results reveal that the total conductance in some orientations can be up to twice that of other orientations when particles are at center-of-mass separation distances of 110 nm. Orientation effects are not significantly correlated with system rotational symmetries but are strongly correlated with the minimum vacuum gap distance between particles. As such, orientation effects on near-field radiative heat transfer are a consequence of particle topology, with more extreme topologies leading to a continuation of orientation effects at larger particle center-of-mass separation distances. The concave superellipsoid particles display significant orientation effects up to a center-of-mass separation distance approximately equal to 3.9 times the particle radius, while the convex superellipsoid particles display significant orientation effects up to a center-of-mass separation distance approximately equal to 3.2 times the particle radius. In contrast to previous anisotropic, spheroidal dipole studies, these results of complex-shaped superellipsoid particles suggest that orientation effects become negligible when heat transfer is a volumetric process for all orientations. This work is essential for understanding radiative transport between particles that have non-regular geometries or that may have geometrical defects or abnormalities.