Tailoring near-field thermal radiation between metallo-dielectric multilayers using coupled surface plasmon polaritons

Enhanced Far-Field Thermal Radiation through a Polaritonic Waveguide

We experimentally demonstrate the enhancement of the far-field thermal radiation between two nonabsorbent Si microplates coated with energy-absorbent silicon dioxide (SiO_{2}) nanolayers supporting the propagation of surface phonon polaritons. By measuring the radiative thermal conductance between two coated Si plates, we find that its values are twice those obtained without the SiO_{2} coating. This twofold increase results from the hybridization of polaritons with guided modes inside Si and is well predicted by fluctuational electrodynamics and an analytical model based on a two-dimensional density of polariton states. These findings could be applied to thermal management in microelectronics, silicon photonics, energy conversion, atmospheric sciences, and astrophysics.

Corner- and edge-mode enhancement of near-field radiative heat transfer

It is well established that near-field radiative heat transfer (NFRHT) can exceed Planck's blackbody limit1 by orders of magnitude owing to the tunnelling of evanescent electromagnetic frustrated and surface modes2-4, as has been demonstrated experimentally for NFRHT between two large parallel surfaces5-7 and between two subwavelength membranes8,9. However, although nanostructures can also sustain a much richer variety of localized electromagnetic modes at their corners and edges10,11, 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.

Observation of Near-Field Thermal Radiation between Coplanar Nanodevices with Subwavelength Dimensions

With the continuous advancement of nanotechnology, nanodevices have become crucial components in computing, sensing, and energy conversion applications. The structures of nanodevices typically possess subwavelength dimensions and separations, which pose significant challenges for understanding energy transport phenomena in nanodevices. Here, on the basis of a judiciously designed thermal photonic nanodevice, we report the first measurement of near-field energy transport between two coplanar subwavelength structures over temperature bias up to ∼190 K. Our experimental results demonstrate a 20-fold enhancement in energy transfer beyond blackbody radiation. In contrast with the well-established near-field interactions between two semi-infinite bodies, the subwavelength confinements in nanodevices lead to increased polariton scattering and reduction of supporting photonic modes and, therefore, a lower energy flow at a given separation. Our work unveils exciting opportunities for the rational design of nanodevices, particularly for coplanar near-field energy transport, with important implications for the development of efficient nanodevices for energy harvesting and thermal management.

Transient measurement of near-field thermal radiation between macroscopic objects

The involvement of evanescent waves in the near-field regime could greatly enhance spontaneous thermal radiation, offering a unique opportunity to study nanoscale photon-phonon interaction. However, accurately characterizing this subtle phenomenon is very challenging. This paper proposes a transient all-optical method for rapidly characterizing near-field radiative heat transfer (NFRHT) between macroscopic objects, using the first law of thermodynamics. Significantly, a full measurement at a fixed gap distance is completed within tens of seconds. By simplifying the configuration, the transient all-optical method achieves high measurement accuracy and reliable reproducibility. The proposed method can effectively analyze the NFRHT in various material systems, including SiO2, SiC, and Si, which involve different phonon or plasmon polaritons. Experimental observations demonstrate significant super-Planckian radiation, which arises from the near-field coupling of bounded surface modes. Furthermore, the method achieves excellent agreement with theory, with a minimal discrepancy of less than 2.7% across a wide temperature range. This wireless method could accurately characterize the NFRHT for objects with different sizes or optical properties, enabling the exploration of both fundamental interests and practical applications.

Lattice Mie resonances and emissivity enhancement in mid-infrared iron pyrite metasurfaces

High-refractive-index antennas with characteristic dimensions comparable to wavelength have a remarkable ability to support pronounces electric and magnetic dipole resonances. Furthermore, periodic arrangements of such resonant antennas result in narrow and strong lattice resonances facilitated by the lattice. We design iron pyrite antennas operating in the mid-infrared spectral range due to the material's low-energy bandgap and high refractive index. We utilize Kirchhoff's law, stating that emissivity and absorptance are equal to each other in equilibrium, and we apply it to improve the thermal properties of the iron pyrite metasurface. Through the excitation of collective resonances and manipulation of the antenna lattice's period, we demonstrate our capacity to control emissivity peaks. These peaks stem from the resonant excitation of electric and magnetic dipoles within proximity to the Rayleigh anomalies. In the lattice of truncated-cone antennas, we observe Rabi splitting of electric and magnetic dipole lattice resonances originating from the antennas' broken symmetry. We demonstrate that the truncated-cone antenna lattices support strong out-of-plane magnetic dipole lattice resonances at oblique incidence. We show that the truncated-cone antennas, as opposed to disks or cones, facilitate a particularly strong resonance and bound state in the continuum at the normal incidence. Our work demonstrates the effective manipulation of emissivity peaks in iron pyrite metasurfaces through controlled lattice resonances and antenna design, offering promising avenues for mid-infrared spectral engineering.

Measurement of Near-Field Radiative Heat Transfer at Deep Sub-Wavelength Distances using Nanomechanical Resonators

Near-field radiative heat transfer (NFRHT) measurements often rely on custom microdevices that can be difficult to reproduce after their original demonstration. Here we study NFRHT using plain silicon nitride (SiN) membrane nanomechanical resonators─a widely available substrate used in applications such as electron microscopy and optomechanics─and on which other materials can easily be deposited. We report measurements down to a minimal distance of 180 nm between a large radius of curvature (15.5 mm) glass radiator and a SiN membrane resonator. At such deep sub-wavelength distance, heat transfer is dominated by surface polariton resonances over a (0.25 mm)2 effective area, which is comparable to plane-plane experiments employing custom microfabricated devices. We also discuss how measurements using nanomechanical resonators create opportunities for simultaneously measuring near-field radiative heat transfer and thermal radiation forces (e.g., thermal corrections to Casimir forces).

Regulation of Near-Field Radiative Heat Transfer between Multilayer BP/hBN Heterostructures

As an allotrope of phosphorus and a promising 2D semiconductor, black phosphorus (BP) exhibits in-plane anisotropy along its armchair and zigzag crystal directions, allowing for efficient regulation of near-field radiative heat transfer (NFRHT). In this work, we investigate the NFRHT between two multilayer BP/hBN heterostructures and theoretically demonstrate that thermal regulation can be realized by tuning the electron density and rotation angle of BP. Results show that a larger electron density leads to the coupling of anisotropic surface plasmon polaritons (SPPs) of BP with hyperbolic modes of hBN, and rotation of BP changes the anisotropic characteristic of coupled SPPs on both sides, whereby a regulation ratio of 5.8 can be obtained. We also analyze the effects of period number, hBN layer thickness, and topmost-layer material on the NFRHT. This work may be beneficial for efficient nanoscale thermal management and physical understanding of radiative heat transfer based on anisotropic SPPs.

Nonlocal Near-Field Radiative Heat Transfer by Transdimensional Plasmonics

Using transdimensional plasmonic materials (TDPM) within the framework of fluctuational electrodynamics, we demonstrate nonlocality in dielectric response alters near-field heat transfer at gap sizes on the order of hundreds of nanometers. Our theoretical study reveals that, opposite to the local model prediction, propagating waves can transport energy through the TDPM. However, energy transport by polaritons at shorter separations is reduced due to the metallic response of TDPM stronger than that predicted by the local model. Our experiments conducted for a configuration with a silica sphere and a doped silicon plate coated with an ultrathin layer of platinum as the TDPM show good agreement with the nonlocal near-field radiation theory. Our experimental work in conjunction with the nonlocal theory has important implications in thermophotovoltaic energy conversion, thermal management applications with metal coatings, and quantum-optical structures.

Probing the Limits to Near-Field Heat Transfer Enhancements in Phonon-Polaritonic Materials

Near-field radiative heat transfer (NFRHT) arises between objects separated by nanoscale gaps and leads to dramatic enhancements in heat transfer rates compared to the far-field. Recent experiments have provided first insights into these enhancements, especially using silicon dioxide (SiO₂) surfaces, which support surface phonon polaritons (SPhP). Yet, theoretical analysis suggests that SPhPs in SiO₂ occur at frequencies far higher than optimal. Here, we first show theoretically that SPhP-mediated NFRHT, at room temperature, can be 5-fold larger than that of SiO₂, for materials that support SPhPs closer to an optimal frequency of 67 meV. Next, we experimentally demonstrate that MgF₂ and Al₂O₃ closely approach this limit. Specifically, we demonstrate that near-field thermal conductance between MgF₂ plates separated by 50 nm approaches within nearly 50% of the global SPhP bound. These findings lay the foundation for exploring the limits to radiative heat transfer rates at the nanoscale.

Near-Field Radiative Heat Transfer Modulation with an Ultrahigh Dynamic Range through Mode Mismatching

Modulating near-field radiative heat transfer (NFRHT) with a high dynamic range is challenging in nanoscale thermal science and engineering. Modulation depths [(maximum value - minimum value)/(maximum value + minimum value) × 100%] of ≈2% to ≈15.7% have been reported with matched modes, but breaking the constraint of mode matching theoretically allows for higher modulation depth. We demonstrate a modulation depth of ≈32.2% by a pair of graphene-covered SU8 heterostructures at a gap distance of ≈80 nm. Dissimilar Fermi levels tuned by bias voltages enable mismatched surface plasmon polaritons which improves the modulation. The modulation depth when switching from a matched mode to a mismatched mode is ≈4.4-fold compared to that when switching between matched modes. This work shows the importance of symmetry in polariton-mediated NFRHT and represents the largest modulation depth to date in a two-body system with fixed gap distance and temperature.

Enhancement and Manipulation of Near-Field Thermal Radiation Using Hybrid Hyperbolic Polaritons

Owing to a high electromagnetic confinement and a strong photonic density of states, hyperbolic surface plasmon polaritons (HSPPs) provide a fascinating promise for applications in thermal photonics. In this work, we theoretically predict a possibility for the improvement of the near-field radiative heat transfer on the basis of tailoring the electromagnetic state of hyperbolic metasurfaces by the uniaxial hyperbolic substrate. By using the photonic tunneling coefficient and the polaritons dispersion, we present a comprehensive study of the hybrid effect of the hyperbolic substrate on HSPPs. We find that due to the hybrid effect of the hyperbolic substrate, the anisotropy surface state of hyperbolic metasurfaces would undergo significant deformations and even topological transition. Moreover, we systematically exhibit the evolution of such hybrid hyperbolic mode with different thicknesses of the hyperbolic substrate and analyze the thickness effect on radiative properties of the hybrid system. It is shown that the resulting heat transfer with the assistance of the hybrid hyperbolic mode by optimizing the substrate parameters is many times stronger than that of monolayer hyperbolic metasurface at the same vacuum gap. Taken together, our results provide a platform to tailor 2D hyperbolic plasmons as a potential strategy toward passive or active control of the near-field heat transfer, and the hybrid hyperbolic mode presented here may facilitate the system design for near-field energy harvesting, thermal imaging, and radiative cooling applications.

Enhanced Near-Field Radiative Heat Transfer between Graphene/hBN Systems

Near-field radiative heat transfer (NFRHT) can exceed the blackbody radiation limit owing to the coupled evanescent waves, implying a significant potential for energy conversion and thermal management. Coupled surface plasmon polaritons (SPPs) and hyperbolic phonon polaritons (HPPs) with small ohmic losses enable a long propagation wavelength that is essential in NFRHT. However, so far, there still lacks knowledge about the experimental investigation of the coupling of SPPs and HPPs in terms of NFRHT. In this study, the NFRHT between graphene/hexagonal boron nitride (hBN) systems that can be readily transferred onto various substrates, with a gap space of ≈400 nm is measured. NFRHT enhancements in the order of three and six times higher than the blackbody limit for graphene/hBN heterostructures and graphene/hBN/graphene multilayers, respectively are demonstrated. In addition, the largest ever radiative heat flux using graphene/hBN/graphene multilayers under similar gap space of 400 nm is obtained. Consequently, analyzing the photon tunneling modes reveal that these phenomena are consequences of coupled SPPs of graphene and HPPs of hBN.

Research Progress of Plasmonic Nanostructure-Enhanced Photovoltaic Solar Cells

Enhancement of the electromagnetic properties of metallic nanostructures constitute an extensive research field related to plasmonics. The latter term is derived from plasmons, which are quanta corresponding to longitudinal waves that are propagating in matter by the collective motion of electrons. Plasmonics are increasingly finding wide application in sensing, microscopy, optical communications, biophotonics, and light trapping enhancement for solar energy conversion. Although the plasmonics field has relatively a short history of development, it has led to substantial advancement in enhancing the absorption of the solar spectrum and charge carrier separation efficiency. Recently, huge developments have been made in understanding the basic parameters and mechanisms governing the application of plasmonics, including the effects of nanoparticles' size, arrangement, and geometry and how all these factors impact the dielectric field in the surrounding medium of the plasmons. This review article emphasizes recent developments, fundamentals, and fabrication techniques for plasmonic nanostructures while investigating their thermal effects and detailing light-trapping enhancement mechanisms. The mismatch effect of the front and back light grating for optimum light trapping is also discussed. Different arrangements of plasmonic nanostructures in photovoltaics for efficiency enhancement, plasmonics' limitations, and modeling performance are also deeply explored.

Effective Approximation Method for Nanogratings-induced Near-Field Radiative Heat Transfer

Nanoscale radiative thermal transport between a pair of metamaterial gratings is studied within this work. The effective medium theory (EMT), a traditional method to calculate the near-field radiative heat transfer (NFRHT) between nanograting structures, does not account for the surface pattern effects of nanostructures. Here, we introduce the effective approximation NFRHT method that considers the effects of surface patterns on the NFRHT. Meanwhile, we calculate the heat flux between a pair of silica (SiO₂) nanogratings with various separation distances, lateral displacements, and grating heights with respect to one another. Numerical calculations show that when compared with the EMT method, here the effective approximation method is more suitable for analyzing the NFRHT between a pair of relatively displaced nanogratings. Furthermore, it is demonstrated that compared with the result based on the EMT method, it is possible to realize an inverse heat flux trend with respect to the nanograting height between nanogratings without modifying the vacuum gap calculated by this effective approximation NFRHT method, which verifies that the NFRHT between the side faces of gratings greatly affects the NFRHT between a pair of nanogratings. By taking advantage of this effective approximation NFRHT method, the NFRHT in complex micro/nano-electromechanical devices can be accurately predicted and analyzed.

Optical axis-driven modulation of near-field radiative heat transfer between two calcite parallel structures

Recently, the increasing research on the anisotropic optical axis (OA) has provided a novel way to control light. However, this method is rarely applied to modulate the near-field radiative heat...

Optimized Colossal Near-Field Thermal Radiation Enabled by Manipulating Coupled Plasmon Polariton Geometry

Collective optoelectronic phenomena such as plasmons and phonon polaritons can drive processes in many branches of nanoscale science. Classical physics predicts that a perfect thermal emitter operates at the black body limit. Numerous experiments have shown that surface phonon polaritons allow emission two orders of magnitude above the limit at a gap distance of ≈50 nm. This work shows that a supported multilayer graphene structure improves the state of the art by around one order of magnitude with a ≈1129-fold-enhancement at a gap distance of ≈55 nm. Coupled surface plasmon polaritons at mid- and far-infrared frequencies allow for near-unity photon tunneling across a broad swath of k-space enabling the improved result. Electric tuning of the Fermi-level allows for the detailed characterization and optimization of the colossal nanoscale heat transfer.

High-performance photonic transformers for DC voltage conversion

Direct current (DC) converters play an essential role in electronic circuits. Conventional high-efficiency DC voltage converters, especially step-up type, rely on switching operation, where energy is periodically stored within and released from inductors and/or capacitors connected in a variety of circuit topologies. Since these energy storage components, especially inductors, are fundamentally difficult to scale down, miniaturization of switching converters proves challenging. Furthermore, the resulting switching currents produce significant electromagnetic noise. To overcome the limitations of switching converters, photonic transformers, where voltage conversion is achieved through light emission and detection processes, have been demonstrated. However, the demonstrated efficiency is significantly below that of the switching converter. Here we perform a detailed balance analysis and show that with a monolithically integrated design that enables efficient photon transport, the photonic transformer can operate with a near-unity conversion efficiency and high voltage conversion ratio. We validate the theory with a transformer constructed with off-the-shelf discrete components. Our experiment showcases near noiseless operation and a voltage conversion ratio that is significantly higher than obtained in previous photonic transformers. Our findings point to the possibility of a high-performance optical solution to miniaturizing DC power converters and improving the electromagnetic compatibility and quality of electrical power.

Conventional DC-DC converters rely on switching operations and energy storing components which face both noise and scaling difficulties. Here, the authors present an alternative design for a DC-to-DC converter based on closely coupled LEDs and photovoltaic cells, which exhibits high efficiency, low noise, and miniaturizability.

Radiative heat transfer at the nanoscale: experimental trends and challenges

Energy transport theories are being revisited at the nanoscale, as macroscopic laws known for a century are broken at dimensions smaller than those associated with energy carriers. For thermal radiation, where the typical dimension is provided by Wien's wavelength, Planck's law and associated concepts describing surface-to-surface radiative transfer have to be replaced by a full electromagnetic framework capturing near-field radiative heat transfer (photon tunnelling between close bodies), interference effects and sub-wavelength thermal emission (emitting body of small size). It is only during the last decade that nanotechnology has allowed for many experimental verifications - with a recent boom - of the large increase of radiative heat transfer at the nanoscale. In this minireview, we highlight the parameter space that has been investigated until now, showing that it is limited in terms of inter-body distance, temperature and object size, and provide clues about possible thermal-energy harvesting, sensing and management applications. We also provide an outlook on open topics, underlining some difficulties in applying single-wavelength approaches to broadband thermal emitters while acknowledging the promise of thermal nanophotonics and observing that molecular/chemical viewpoints have been hardly addressed.

Degenerate quasi-normal mode theory for near-field radiation between plasmonic structures

Near-field radiation can exceed the blackbody radiation limit due to the contributions from evanescent waves. One promising approach to further enhance near-field radiation beyond existing bulk materials is to utilize metamaterials or metasurfaces made from subwavelength plasmonic structures. In this work, we investigate the near-field thermal radiation between complex plasmonic structures with higher-order symmetry and degeneracy, which is crucial for understanding the radiative heat exchange between metamaterials or metasurfaces at extremely small gaps. We demonstrate that the introduction of degeneracy can drastically boost near-field thermal radiation between plasmonic structures. The enhancement of near-field thermal radiation originates from the emergence of degenerate resonance modes and the secondary emission of thermal photons due to the nonzero coupling between the degenerate modes. Our study provides new pathways for designing high-intensity near-field thermal emitters and absorbers for thermophotovoltaics, thermal management, and infrared spectroscopy.

Radiative thermal switch driven by anisotropic black phosphorus plasmons

Black phosphorus (BP), as a two-dimensional material, has exhibited unique optoelectronic properties due to its anisotropic plasmons. In the present work, we theoretically propose a radiative thermal switch (RTS) composed of BP gratings in the context of near-field radiative heat transfer. The simply mechanical rotation between the gratings enables considerable modulation of radiative heat flux, especially when combined with the use of non-identical parameters, i.e., filling factors and electron densities of BP. Among all the cases including asymmetric BP gratings, symmetric BP gratings, and BP films, we find that the asymmetric BP gratings possess the most excellent switching performance. The optimized switching factors can be as high as 90% with the vacuum separation d=50 nm and higher than 70% even in the far-field regime d=1 µm. The high-performance switching is basically attributed to the rotatable-tunable anisotropic BP plasmons between the asymmetric gratings. Moreover, due to the twisting principle, the RTS can work at a wide range of temperature, which has great advantage over the phase change materials-based RTS. The proposed switching scheme has great significance for the applications in optoelectronic devices and thermal circuits.

Colossal Enhancement of Near-Field Thermal Radiation Across Hundreds of Nanometers between Millimeter-Scale Plates through Surface Plasmon and Phonon Polaritons Coupling

Coupling modes between surface plasmon polaritons (SPPs) and surface phonon polaritons (SPhPs) play a vital role in enhancing near-field thermal radiation but are relatively unexplored, and no experimental result is available. Here, we consider the NFTR enhancement between two identical graphene-covered SiO₂ heterostructures with millimeter-scale surface area and report an experimentally record-breaking ∼64-fold enhancement compared to blackbody (BB) limit at a gap distance of 170 nm. The energy transmission coefficient and radiation spectra show that the physical mechanism behind the colossal enhancement is the coupling between the surface plasmon and phonon polaritons.

Near-field radiative heat transfer in multilayered graphene system considering equilibrium temperature distribution

In the present work, the near-field radiative heat transfer of a multilayered graphene system is investigated within the framework of the many-body theory. For the first time, the temperature distribution corresponding to the steady state of the system is investigated. Unique temperature steps are observed near both boundaries of the system, especially in the strong near-field regime. By utilizing the effective radiative thermal conductance, the thermal freedom of heat flux in different regions of the system is analyzed quantitatively, and the cause of various temperature distributions is explained accordingly. To characterize the heat transfer ability of the whole system, we evaluate the system with two heat transfer coefficients (HTC), transient heat transfer coefficient (THTC), and steady heat transfer coefficient (SHTC). A unique many-body enhancement is observed, which causes a red-shift of resonance peak corresponding to graphene surface plasmon polaritons. Furthermore, a three-body enhancement of SHTC emerges thanks to the relay effect and the complexity of the system. The regime of heat transport can be tuned by changing the chemical potentials of graphene and undergoes a transition from diffusive to quasi-ballistic transport in the strong near-field regime.

A near-field radiative heat transfer device

Recently, several reports have experimentally shown near-field radiative heat transfer (NFRHT) exceeding the far-field blackbody limit between planar surfaces^1-5. 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 conversion^6,7 and thermal management^8-10. 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.

Harvesting the Electromagnetic Energy Confined Close to a Hot Body

In the close vicinity of a hot body, at distances smaller than the thermal wavelength, a high electromagnetic energy density exists due to the presence of evanescent fields radiated by the partial charges in thermal motion around its surface. This energy density can surpass the energy density in vacuum by several orders of magnitude. By approaching a photovoltaic (PV) cell with a band gap in the infrared frequency range, this nonradiative energy can be transferred to it by photon tunnelling and surface mode coupling. Here we review the basic ideas and recent progress in near-field energy harvesting.