Far-field Thermal Radiation Research Articles

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.

Far-Field Radiative Thermal Rectification Based on Asymmetric Emissivity.

This experimental study investigates thermal rectification via asymmetric far-field thermal radiation on a fused silica slab. An asymmetrical distribution of surface emissivity is created over the device by partially covering the fused silica with a 100 nm thick aluminum film. The slab is subjected to a thermal bias, and when this bias is reversed, a small temperature difference is observed between the different configurations. This temperature difference arises from the difference in emissivity between the aluminum layer and fused silica, resulting in the transfer of thermal energy to the surrounding environment through radiation. Experimental findings are supported by finite element simulations, which not only confirm the measured values but also provide valuable insights into the rectification efficiency of the system. The rectification efficiency is found to be approximately 50% at room temperature for a thermal bias of 140 K. Simulations, which are performed by considering different environmental conditions experienced by the radiation and free convection processes, provide further insight into the underlying thermal rectification mechanism. These simulations consider an environmental temperature of 4 K for thermal radiation and an ambient temperature of 294 K for free convection and reveal an enhanced rectification effect with a rectification efficiency up to 600% when a thermal bias of 195 K is applied. This result emphasizes the significance of considering both convection and radiation in the thermal management and rectification of asymmetric systems. The outcomes of this study further our understanding of the thermal rectification phenomenon. They also show the importance of system asymmetry, emissivity disparities, environmental conditions, and the interplay between convection and radiation. Furthermore, the findings have implications for heat transfer and rectification in asymmetric systems, offering potential applications in areas such as energy harvesting, thermal management, and heat transfer optimization in electronic devices.

Far-field thermal radiation from short-pitch silicon-carbide nanopillar arrays

Silicon carbide (SiC) supports surface phonons in the infrared region of the electromagnetic spectrum where these modes can be thermally emitted. Additionally, the magnitude, spectrum, and direction of thermal radiation from SiC can be controlled by engineering this material at the sub-wavelength scale. For these reasons, SiC nanopillars are of high interest for thermal-radiation tuning. So far, theoretical and experimental studies of thermal emission from SiC nanopillars have been limited to long-pitch arrays with a microscale interpillar spacing. It is not clear how far-field thermal emission from SiC nanopillars is affected when the interparticle spacing reduces to the nanometer scale, where the near-field interaction between adjacent nanopillars arises and the array becomes zero order. In this Letter, we study physical mechanisms of far-field thermal radiation from zero-order arrays of silicon-carbide nanopillars with a nanoscale interpillar spacing. We show that the increased volume of thermal emitters and thermal radiation of the hybrid waveguide-surface-phonon-polariton mode from zero-order arrays increase the spectral emissivity of silicon carbide to values as large as 1 for a wide range of angles. The enhanced, dispersion-less thermal emission from a zero-order SiC array of nano-frustums with an optimized interspacing of 300 nm is experimentally demonstrated. Our study provides insight into thermal radiation from dense nanostructures and has significant implications for thermal management of electronic devices and energy harvesting applications.

Far-field thermal radiation properties of graphene under uniaxial strain

Electronic band structure and optical conductivity of single-layer graphene could be altered by applied uniaxial strain. Valley and space inversion symmetries are broken. Dirac cones are deformed. We investigate the effect of uniaxial strain on the radiative properties of graphene from the perspective of direction and modulus. Optical conductivity exhibits wealthy phenomenon due to the degeneracy of the energy band broken by strain. The total energy radiation exhibits a novel behavior of periodicity in , in accordance with the symmetry of the hexagonal honeycomb lattice.

Transition from Phononic to Geometrical Mie Modes Measured in Single Subwavelength Polar Dielectric Spheres

Spherical dielectric resonators are highly attractive for light manipulation, thanks to their intrinsic electric and magnetic resonances. Here, we present measurements of the mid-infrared far-field thermal radiation of single subwavelength dielectric spheres deposited on a gold substrate, of radii ranging from 1 to 2.5 μm, which agree quantitatively with simulated absorption cross sections. For SiO2 microspheres, we evidence the excitation of both surface phonon-polariton (SPhP) modes and geometrical electric and magnetic Mie modes. The transition from a phonon-mode-dominated to a Mie-mode-dominated emission spectrum is observed, with a threshold radius of ∼1.5 μm. We also show that the presence of the metallic substrate augments the computed spheres absorption cross-section due to increased local field enhancement, arising from the near-field interaction of the spheres oscillating charges with their image in the metallic mirror. In contrast, measurements of single subwavelength SPhP-inactive PTFE spheres reveal that the mid-infrared response of such lossy spheres is dominated by their bulk absorption. Our results demonstrate how engineering the geometrical and dielectric properties of subwavelength scatterers can enable the control of thermal emission near room temperature, with exciting perspectives for applications such as radiative cooling.

Broadband directional control of thermal emission.

Controlling the directionality of emitted far-field thermal radiation is a fundamental challenge. Photonic strategies enable angular selectivity of thermal emission over narrow bandwidths, but thermal radiation is a broadband phenomenon. The ability to constrain emitted thermal radiation to fixed narrow angular ranges over broad bandwidths is an important, but lacking, capability. We introduce gradient epsilon-near-zero (ENZ) materials that enable broad-spectrum directional control of thermal emission. We demonstrate two emitters consisting of multiple oxides that exhibit high (>0.7, >0.6) directional emissivity (60° to 75°, 70° to 85°) in the p-polarization for a range of wavelengths (10.0 to 14.3 micrometers, 7.7 to 11.5 micrometers). This broadband directional emission enables meaningful radiative heat transfer primarily in the high emissivity directions. Decoupling the conventional limitations on angular and spectral response improves performance for applications such as thermal camouflaging, solar heating, radiative cooling, and waste heat recovery.

Near-field radiation assisted smart skin for spacecraft thermal control

Thermal control is of critical importance for normal operation of spacecraft. Given thermal radiation is the only means of heat dissipation in space, an efficient thermal control approach for spacecraft is to coat the radiator with a tunable-emittance “skin” that can tune its heat dissipation according to various thermal conditions. The existing schemes solely relying on far-field thermal radiation, which are based on mechanical, electrochromic or thermochromic working principles, are difficult to combine the advantages of all-solid-state structure, actively and accurate tuning, and large tuning range of heat flux. In this work, we propose a near-field radiation assisted (NFRA) smart skin for thermal control which can tune the heat rejection accurately and in a large range. It contains a metal-insulator-semiconductor (MIS) structure, where the carrier distribution in the semiconductor layer can be electrically altered. In this way, the near-field heat flux, and ultimately the skin emission power expressed using effective emittance, can be controlled as a function of the applied voltage. The variation range of the effective emittance can exceed 0.7 when adjusting the applied voltage from −10 V to 100 V with our preliminary design. This work opens a new way of smart skin design for active spacecraft thermal control.

Scale Law of Far-Field Thermal Radiation from Plasmonic Metasurfaces.

Although metamaterials or metasurfaces consisting of patterned subwavelength structures have been widely employed for thermal emission control, the collective behavior of the emitter array in a metasurface still remains unclear. Here, based on the quasinormal mode theory, we derive a new scale law to elucidate the far-field thermal emission from a metasurface composed of densely packed plasmonic nanoemitters. The tight binding method is used to approximate the collective resonant mode of the emitter array. Because of in-phase near-field interaction, the thermal radiation from a single emitter in a metasurface is suppressed by its adjacent emitters. We find that the overall far-field thermal radiation from a metasurface can be either positively or negatively correlated with the packing density of the emitters, depending on the mode properties of the single emitter. This new scale law thus serves as a general guideline for designing metasurfaces with desired thermal emission properties.

Thermal emission at the optical topological transition

We develop a theoretical description of far-field thermal radiation near optical topological transitions. Our results show a strong asymmetric pattern of reduced emission near the transition from type-I hyperbolic to dielectric responses and are applicable in both naturally available and composite media.

Non-Hermitian Selective Thermal Emitters using Metal-Semiconductor Hybrid Resonators.

All open systems that exchange energy with their environment are non-Hermitian. Thermal emitters are open systems that can benefit from the rich set of physical phenomena enabled by their non-Hermitian description. Using phase, symmetry, chirality, and topology, thermal radiation from hot surfaces can be unconventionally engineered to generate light with new states. Such thermal emitters are necessary for a wide variety of applications in sensing and energy conversion. Here, a non-Hermitian selective thermal emitter is experimentally demonstrated, which exhibits passive PT-symmetry in thermal emission at 700 °C. Furthermore, the effect of internal phase of the oscillator system on far-field thermal radiation is experimentally demonstrated. The ability to tune the oscillator phase provides new pathways for both engineering and controlling selective thermal emitters for applications in sensing and energy conversion.

Enhanced thermal radiation via interweaved L slots.

The rate of heat transfer by thermal radiation is a function of the number of channels that carry the electromagnetic energy, and the capacity of each channel to convey the electromagnetic energy. In this research, we show that we can increase the number of these channels for a given emitter volume, and accordingly, we can enhance both near- and far-field thermal radiation exchange. We increase the number of channels by carving a variety of slots with different sizes. Using a modified finite-difference time-domain simulation, we show that the interweaved L slots achieved higher rates of heat transfer than the flat slab and straight slots (all having the same volume) by 15 and 2.5 times, respectively, for far-field thermal radiation (separation gap dc = 30 μm), and 5.6730 and 1.145 times for near-field thermal radiation (dc = 0.5 μm).

Exploring the Limits of Super-Planckian Far-Field Radiative Heat Transfer Using 2D Materials

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 (λTh ≈ 10 μm 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 can be more than 7 orders of magnitude larger than the theoretical limit set by Planck’s law for blackbodies and can be comparable to the heat transfer of two parallel sheets at the same distance. In particular, we illustrate this phenomenon with different materials such as graphene, where the radiation can also be tuned by a external gate, and single-layer black phosphorus. In both cases the far-field radiative heat transfer is domina...

Modulation and amplification of radiative far field heat transfer: Towards a simple radiative thermal transistor

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 that the more the material is reflective in the metallic state, the more switching effect is realized, whereas the more PCM transition is stiff in temperature, the more thermal amplification is high. We finally take the example of VO2 that exhibits an insulator-metallic transition at 68 °C. We show that a demonstrator of a radiative transistor could easily be achieved in view of the heat flux levels predicted. Far-field thermal radiation experiments are proposed to back the results presented.

Stokes parameters and partial polarization of far-field radiation emitted by hot bodies

The polarization properties of far-field thermal radiation emitted by hot bodies at uniform temperature is explored. Some novel results are obtained for the Stokes parameters for emission from large, smooth, opaque bodies of convex shape and also for small particles in the Rayleigh region. For the case of large, smooth, opaque bodies, the results may be readily extended to include nonuniform temperature distributions that vary slowly on the scale of a wavelength. The formulas obtained are particularly simple and straightforward to apply, and examples of their application to emission from a large smooth cylinder and from a small ellipsoidal particle are described.