Near-field Radiative Transfer Research Articles

Drift current-induced tunable near-field energy transfer between twist magnetic Weyl semimetals and graphene

Abstract Both the nonreciprocal surface modes in Weyl semimetal (WSM) with a large anomalous Hall effect and the nonreciprocal photon occupation number on a graphene surface induced by the drift current provide a promising way to manipulate the nonreciprocal near-field energy transfer. Interestingly, the interactions between nonreciprocities are highly important for research in (thermal) photonics but remain challenging. In this study, we theoretically investigated the near-field radiative heat flux transfer between a graphene heterostructure supported by a magnetic WSM and a twist-Weyl semimetal (T-WSM). The nonreciprocal surface mode could be changed by the separation space between two Weyl nodes and the twist angle. Notably, we found that in the absence of a temperature difference between two parallel plates, nonequilibrium fluctuations caused by drift currents led to the transfer of near-field radiative heat flux. Furthermore, these nonreciprocal surface modes interacted with the nonreciprocal photon occupation number in graphene to achieve flexible manipulation of the near-field heat flux size and direction. Additionally, graphene adjustable flux in the case of a temperature difference between the two plates was also discussed. Our scheme can provide a reference for near-field heat flux regulation in nonequilibrium systems.

Dirac semimetal-assisted near-field radiative thermal rectifier and thermostat based on phase transition of vanadium dioxide.

The near-field thermal radiation has broad application prospects in micro-nano-scale thermal management technology. In this paper, we report the Dirac semimetal-assisted (AlCuFe quasicrystal) near-field radiative thermal rectifier (DSTR) and thermostat (DST), respectively. The DSTR is made of a Dirac semimetal-covered vanadium dioxide (VO2) plate and silicon dioxide (SiO2) plate separated by a vacuum gap. The left and right sides of DST are consisted of the SiO2 covered with Dirac semimetal, and the intermediate plate is the VO2. The strong coupling of the surface electromagnetic modes between the Dirac semimetal, SiO2, and insulating VO2 leads to enhance near-field radiative transfer. In the DSTR, the net radiative heat flux of VO2 in the insulating state is much larger than that in metallic state. When the vacuum gap distance d=100 nm, Fermi level EF=0.20 eV, and film thickness t=12 nm, the global rectification factor of DSTR is 3.5, which is 50% higher than that of structure without Dirac semimetal. In the DST, the equilibrium temperature of the VO2 can be controlled accurately to achieve the switching between the metallic and insulating state of VO2. When the vacuum gap distance d=60 nm, intermediate plate thickness δ=30 nm, and film thickness t=2 nm, with the modulation of Fermi level between 0.05-0.15 eV, the equilibrium temperature of VO2 can be controlled between 325-371 K. In brief, when the crystalline state of VO2 changes between the insulating and metallic state with temperature, the active regulation of near-field thermal radiation can be realized in both two-body and three-body parallel plate structure. This work will pave a way to further improve performance of near-field radiative thermal management and modulation.

Residual surface charge mediated near-field radiative energy transfer: A topological insulator analog

We study the modifications of near-field radiative energy transfer (NFRET) caused by residual surface charges, which are common in micro- and nano-systems like NEMS/MEMS. The host object with the residual surface charges and the inherent bulk state can be treated as an analog of the real three-dimensional topological insulator, which is inherent of also both surface states and bulk states and is promising to modulate NFRET. Through constructing such a topological insulator analog, we aim to modulate NFRET concerning only common trivial materials. Besides the well-known resonant modes (surface polariton and localized surface polariton) supported by the bulk state, the residual surface charges give rise to an additional temperature-dependent mode providing a new heat flux channel. For low temperatures we find a giant surface-charge-induced enhancement of the NFRET due to a good match between the surface-charge-induced resonance and the Planck window. However, for relative high temperatures where the Fröhlich resonance dominates the heat transfer rather the surface-charge-induced resonance, the residual charges result in a weakening of the NFRET. This work paves way for understanding and modulating the near-field radiative energy transfer for micro- and nano-systems.

Near-Field Energy Transfer between Graphene and Magneto-Optic Media.

We consider the near-field radiative energy transfer between two separated parallel plates: graphene supported by a substrate and a magneto-optic medium. We first study the scenario in which the two plates have the same temperature. An electric current through the graphene gives rise to nonequilibrium fluctuations and induces energy transfer. Both the magnitude and direction of the energy flux can be controlled by the electric current and an in-plane magnetic field in the magneto-optic medium. This is due to the interplay between the nonreciprocal photon occupation number in the graphene and nonreciprocal surface modes in the magneto-optic plate. Furthermore, we report that a tunable thermoelectric current can be generated in the graphene in the presence of a temperature difference between the two plates.

Integrated Near-Field Thermophotovoltaic Device Overcoming Blackbody Limit

Near-field thermal radiation transfer overcoming the blackbody limit has attracted significant attention in recent years owing to its potential for drastically increasing the output power and conve...

Thermal equilibrium spin torque: Near-field radiative angular momentum transfer in magneto-optical media

Spin and orbital angular momentum of light plays a central role in quantum nanophotonics as well as topological electrodynamics. Here, we show that the thermal radiation from finite-sized bodies comprising of nonreciprocal magneto-optical materials can exert a spin torque even in global thermal equilibrium. Moving beyond the paradigm of near-field heat transfer, we calculate near-field radiative angular momentum transfer between finite-sized nonreciprocal objects by combining Rytov's fluctuational electrodynamics with the theory of optical angular momentum. We prove that a single magneto-optical cubic particle in non-equilibrium with its surroundings experiences a torque in the presence of an applied magnetic field (T-symmetry breaking). Furthermore, even in global thermal equilibrium, two particles with misaligned gyrotropic axes experience equal magnitude torques with opposite signs which tend to align their gyrotropic axes parallel to each other. Our results are universally applicable to semiconductors like InSb (magneto-plasmas) as well as Weyl semi-metals which exhibit the anomalous Hall effect (gyrotropy) at infrared frequencies. Our work paves the way towards near-field angular momentum transfer mediated by thermal fluctuations for nanoscale devices.

Enhancement in the multi-junction thermophotovoltaic system based on near-field heat transfer and hyperbolic metamaterial

A multi-junction thermophotovoltaic system based on near-field radiation and hyperbolic metamaterial was developed. The multilayer near-field system is studied through the fluctuational electrodynamics based on dyadic Green's function and a recursive calculation model, whereas the external quantum efficiency, open-circuit voltage and fill factor are considered to evaluate the performance of the photovoltaic cell. The system expands the available spectrum to 1720–3650 nm so that more photons can be recovered. The near-field radiative transfer is further improved owing to the infinite wavevector in the hyperbolic metamaterial. The results demonstrate that the multi-junction system generates 4.7236 × 106 W/m2 of electricity with a conversion efficiency of 45.26%, achieving 1.3 times (2.38 times) more electricity with 15.3% (25.95%) higher conversion efficiency than the single InGaAsSb (InAs) cell. The study further improves the performance of the multi-junction thermophotovoltaic and paves the way to design a more efficient near-field multi-junction thermophotovoltaic system.

Near-field radiative transfer by bulk hyperbolic polaritons across vacuum gap

Thermal radiation has always been treated as a surface phenomenon. A recent study has demonstrated that new radiation channels, the hyperbolic phonon polaritons, can contribute to heat transfer inside bulk hyperbolic materials with a heat flux comparable to conduction. For near-field radiative transfer across a vacuum gap between two hyperbolic materials, hyperbolic phonon polaritons have been considered as surface modes that are responsible for heat transfer enhancement. Here, we analyze near-field radiative transfer due to hyperbolic phonon polaritons, driven by temperature gradient inside the bulk materials. We develop a mesoscale many-body scattering approach to account for the role of hyperbolic phonon polaritons in radiative transfer in the bulk and across a vacuum gap. Our study points out the equivalency between the bulk-generated mode and the surface mode in the absence of a temperature gradient in the material, and hence provide a unified framework for near-field radiative transfer by hyperbolic phonon polaritons. The results also elucidate contributions of the bulk-generated mode and the bulk temperature profile in the enhanced near-field radiative transfer.

Enhancement of spectrally controlled near-field radiation transfer by magnetic polariton generated by metal–insulator–metal structures

Near-field radiation transfer between a metal–insulator–metal (MIM)-structured emitter and receiver was investigated through numerical simulation using a finite difference time domain (FDTD) method. Both the emitter and receiver consist of a squared-island-type metal array composed of nickel (Ni), an insulator layer composed of silicon dioxide (SiO2), and a metal substrate composed of nickel (Ni). The emitter and receiver were set up with a vacuum gap of a few hundred nanometers. The results showed that the near-field radiation flux was enhanced by a factor of approximately four, when compared to that between blackbody surfaces. Simultaneously, the enhancement was spectrally controlled over frequencies ranging from 9 × 1014 to 20 × 1014 rad/s (approximating to a wavelength range of 0.9 to 2.1 μm) depending on the size of the squared island. Images of the magnetic field in both the emitter and receiver clarified that the spectrally enhanced radiation flux was caused by the magnetic polariton (MP)) generated in the insulator between the squared-island metal and the substrate metal when the frequency of the MP coincided with that of the near-field radiation. Moreover, the resonance mode between the frequencies of the MP and near-field radiation was predicted by an impedance model that considered the capacitance between the islands in the emitter and receiver, in addition to the conventional circuit of a MIM structure.

Retrieval of Uniaxial Permittivity and Permeability for the Study of Near-Field Radiative Transport Between Metallic Nanowire Arrays

Abstract Recently metamaterials made of periodic nanowire arrays, multilayers, and grating structures have been studied for near-field thermal radiation with enhanced coupling of evanescent waves due to surface plasmon/phonon polariton, hyperbolic mode, epsilon-near-zero and epsilon-near-pole (ENP) modes, guided mode, and wave interference. In this work, both effective uniaxial electric permittivity and magnetic permeability of a nanowire-based metamaterial are retrieved theoretically through the far-field radiative properties obtained by finite difference time-domain (FDTD) simulations. The artificial magnetic response of metamaterials, which cannot be obtained by traditional effective medium theory (EMT) based on electric permittivity of constitutes only, is successfully captured by the nonunity magnetic permeability, whose resonant frequency is verified by an inductor-capacitor model. By incorporating the retrieved electric permittivity and magnetic permeability into fluctuational electrodynamics with multilayer uniaxial wave optics, the near-field radiative heat transfer between the metallic nanowire arrays is theoretically studied and spectral near-field heat enhancements are found for both transverse electric and magnetic waves due to artificial magnetic resonances. The understanding and insights obtained here will facilitate the application of metamaterials in near-field radiative transfer.

COMPUTATIONAL NEAR-FIELD RADIATIVE TRANSFER AND NF-RT-FDTD ALGORITHM

Understanding the fundamentals of near-field radiative transfer is essential for future development of new sensors and energy harvesting devices. Simulations of such problems would require a coupled solution of the electromagnetic wave equations along with the expressions for thermal emission from a body at finite temperature. Versatile computational tools, which account for the intricate physics and the computational challenges of the problems, are likely to help to the future nanomanufacturing systems and processes. These simulation methodologies should be valid for one-, two-, and three-dimensional geometries with inhomogeneities and arbitrary edges and should be applicable to different materials. In this chapter, we briefly review the recent works on numerical methods used to solve the computational near-field radiative transfer (NFRT) problems. Each of these methods has its own advantages and disadvantages, and no single technique can provide the complete and robust solution for all problems at hand. Then we outline an algorithm based on the finite difference time domain (FDTD) method for one- and two-dimensional NFRT problems. For this, we discuss the details of NF-RT-FDTD algorithm and show how this approach can be applied to surfaces covered with particles as well as with thin films with inhomogeneities. We also present simulations for more complicated biomimetic structures inspired by nature for possible sensing and energy harvesting applications.

Giant near-field radiative heat transfer between ultrathin metallic films.

Understanding energy transfer via near-field thermal radiation is essential for applications such as near-field imaging, thermophotovoltaics and thermal circuit devices. Evanescent waves and photon tunneling are responsible for the near-field energy transfer. In bulk noble metals, however, surface plasmons do not contribute efficiently to the near-field energy transfer because of the mismatch of wavelength. In this paper, a giant near-field radiative heat transfer rate that is orders-of-magnitude greater than the blackbody limit between two ultrathin metallic films is demonstrated at nanoscale separations. Moreover, different physical origins for near-field thermal radiation transfer for thick and thin metallic films are clarified, and the radiative heat transfer enhancement in ultrathin metallic films is proved to come from the excitation of surface plasmons. Meanwhile, because of the inevitable high sheet resistance of ultrathin metal films, the heat transfer coefficient is 4600 times greater than the Planckian limit for the separation of 10 nm in ultrathin metallic films, which is the same order or even greater than that in other 2D materials with low carrier density. Our work shows that ultrathin metallic films are excellent materials for radiative heat transfer, which may find promising applications in thermal nano-devices and thermal engineering.

Correction to Electronic Modulation of Near-Field Radiative Transfer in Graphene Field Effect Heterostructures.

Correction to Electronic Modulation of Near-Field Radiative Transfer in Graphene Field Effect Heterostructures.

Electronic Modulation of Near-Field Radiative Transfer in Graphene Field Effect Heterostructures.

Manipulating heat flow in a controllable and reversible manner is a topic of fundamental and practical interest. Numerous approaches to perform thermal switching have been reported, but they typically suffer from various limitations, for instance requiring mechanical modulation of a submicron gap spacing or only operating in a narrow temperature window. Here, we report the experimental modulation of radiative heat flow by electronic gating of a graphene field effect heterostructure without any moving elements. We measure a maximum heat flux modulation of 4 ± 3% and an absolute modulation depth of 24 ± 7 mW m-2 V-1 in samples with vacuum gap distances ranging from 1 to 3 μm. The active area in the samples through which heat is transferred is ∼1 cm2, indicating the scalable nature of these structures. A clear experimental path exists to realize switching ratios as large as 100%, laying the foundation for electronic control of near-field thermal radiation using 2D materials.

One-Chip Near-Field Thermophotovoltaic Device Integrating a Thin-Film Thermal Emitter and Photovoltaic Cell.

Thermal radiation transfer between two objects separated by a subwavelength gap (near-field thermal radiation transfer) can be orders of magnitude larger than that in free space, which is attracting increasing attention with respect to both fundamental nanoscience and its potential for high-power-density and high-efficiency conversion of heat to electricity in thermophotovoltaic (TPV) systems. However, the realization of near-field thermal radiation transfer in TPV systems involves significant challenges because it requires a subwavelength gap and large temperature difference between the emitter and the PV cell while minimizing the heat transfer that does not contribute to the photocurrent generation. To overcome these challenges, here we demonstrate a one-chip near-field TPV device consisting of a thin-film Si emitter and InGaAs PV cell with an intermediate Si substrate, which enables the suppression of the heat transfer due to sub-bandgap radiation by free carriers and surface modes. Through the one-chip integration and thermal isolation using Si process technologies, we realize a deep subwavelength gap (<150 nm) between the emitter and the intermediate substrate without using any external positioners while maintaining a large temperature difference (>700 K). Compared to the equivalent device operating in the far-field regime, we achieve 10-fold enhancement of the photocurrent in the PV cell without degrading the open-circuit voltage and fill factor, demonstrating the potential of our one-chip device for the future applications of near-field thermal radiation transfer.

Tunable near-field radiative transfer by III–V group compound semiconductors

Near-field radiative transfer (NFRT) refers to the energy transfer mechanism which takes place between media separated by distances comparable to or much smaller than the dominant wavelength of emission. NFRT is due to the contribution of evanescent waves and coherent nature of the energy transfer within nano-gaps, and can exceed Planck’s blackbody limit. As researchers further investigate this phenomenon and start fabrication of custom-made platforms, advances in utilization of NFRT in energy harvesting applications move forward day by day. In designing and manufacturing such harvesting devices, chemical and physical properties of surfaces and wafers are important for development of effective solutions. In this work, we compare several III–V group compound semiconductor wafers (mainly GaAs, InSb, and InP) from fabrication point of view, in order to explore their possible use in future devices. The results presented here show that the type of dopant, wafer temperature, and gap size are very important factors as they affect the NFRT rates. GaAs, InSb, and InP wafers significantly enhance the near-field fluxes beyond the blackbody rates, and n-type InSb yields to the highest enhancement. For GaAs, p-type yielded a higher radiative flux compared to n-type GaAs, as oppose to n-type InSb outperforming its p-type and undoped counterparts. Furthermore, the possible use of n-InSb as the TPV cell at 550 K is discussed for effective energy harvesting. These findings can be useful for determination of the proper material type for emitting and non-emitting NFRT-based energy harvesting devices.

Performance enhancement of near-field thermoradiative devices using hyperbolic metamaterials.

We analyze a near-field thermoradiative device that consists of an indium arsenide-based photodiode under negative illumination. We analyze a possible enhancement of conversion efficiency by use of hyperbolic metamaterial (HMM) in place of bulk metallic heat sink. A stack of alternating thin-films of metal [zirconium carbide (ZrC)] and dielectric [silicon dioxide (SiO2)] is chosen to be the HMM under investigation. The presence of hyperbolic modes creates additional channels of near-field radiative transfer. An increased power density is predicted without a compromise in system efficiency.

A biomimicry design for nanoscale radiative cooling applications inspired by Morpho didius butterfly

In nature, novel colors and patterns have evolved in various species for survival, recognizability or mating purposes. Investigations of the morphology of various butterfly wings have shown that in addition to the pigmentation, micro and nanostructures within the wings have also allowed better communication systems and the pheromone-producing organs which are the main regulators of the temperature within butterfly wings. Within the blue spectrum (450–495 nm), Morpho didius butterfly exhibit iridescence in their structure-based wings’ color. Inspired by the rich physics behind this concept, we present a designer metamaterial system that has the potential to be used for near-field radiative cooling applications. This biomimicry design involves SiC palm tree-like structures placed in close proximity of a thin film in a vacuum environment separated by nanoscale gaps. The near-field energy exchange is enhanced significantly by decreasing the dimensions of the tree and rotating the free-standing structure by 90 degrees clockwise and bringing it to the close proximity of a second thin film. This exchange is calculated by using newly developed near-field radiative transfer finite difference time domain (NF-RT-FDTD) algorithm. Several orders of enhancement of near-field heat flux within the infrared atmospheric window (8–13 μm bandwidth) are achieved. This spectrally selective enhancement is associated with the geometric variations, the spatial location of the source of excitation and the material characteristics, and can be tuned to tailor strong radiative cooling mechanisms.

Magnetic Field Effect of Near-Field Radiative Heat Transfer for SiC Nanowires/Plates

The SiC micro/nano-scale structure has advantages for enhancing nonreciprocal absorptance for photovoltaic use due to the magneto optical effect. In this work, we demonstrate the near-field radiative transfer between two aligned SiC nanowires/plates under different magnetic field intensities, in which Lorentz-Drude equations of the dielectric constant tensor are proposed to describe the dielectric constant as a magnetic field applied on the SiC structure. The magnetic field strength is qualified in this study. Using local effective medium theory and the fluctuation-dissipation theorem, we evaluate the near-field radiation between SiC nanowires with different filling ratios and gap distances under an external magnetic field. Compared to the near-field heat flux between two SiC plates, the one between SiC nanowires can be enhanced with magnetic field intensity, a high filling ratio, and a small gap distance. The electric field intensity is also presented for understanding light coupling, propagation, and absorption nature of SiC grating under variable incidence angles and magnetic field strengths. This relative study is useful for thermal radiative design in optical instruments.

Near-field radiative heat transfer enhancement using natural hyperbolic material

Hyperbolic materials have been newly discovered that can enhance near-field radiative transfer. Multilayer and three-dimensional hyperbolic materials require challenging fabrication techniques. From a practical point of view, natural hyperbolic materials serve better in applications requiring near-field radiative transfer enhancement. In this study, we investigate enhancement of near-field radiative transfer using natural hyperbolic material – calcite, and compare with that from (also natural) polar dielectric materials, SiC, where phonon polaritons are responsible for enhanced near-field heat transfer. Spectral analyses of enhanced near-field radiative transfer from hyperbolic modes in hyperbolic materials and surface phonon polaritons in polar dielectric materials are carried out and compared, which determine the origin and amount of near-field radiative transfer enhancement in these materials.