Thermal Emitters Research Articles

Tight Bounds and the Role of Optical Loss in Polariton-Mediated Near-Field Heat Transfer

We introduce an analytical framework for near-field radiative heat transfer in bulk plasmonic and polar media. Considering material dispersion, we derive a closed-form expression for the radiative thermal conductance, which disentangles the role of optical loss from other material dispersion characteristics, such as the spectral width of the Reststrahlen band in polar dielectrics, as well as from the temperature. We provide a universal condition for maximizing heat transfer that defines the optimal interplay between a material's optical loss and polariton resonance frequency, based on which we introduce tight bounds to near-field heat transfer. With this formalism, one can quantitatively evaluate all polaritonic materials in terms of their performance as near-field thermal emitters.

Manipulating Coherence of Near-Field Thermal Radiation in Time-Modulated Systems.

We show that the spatial coherence of thermal radiation can be manipulated in time-modulated photonic systems supporting surface polaritons. We develop a fluctuational electrodynamics formalism for such systems to calculate the cross-spectral density tensor of the emitted thermal electromagnetic fields in the near-field regime. Our calculations indicate that, due to time-modulation, spatial coherence can be transferred between different frequencies, and correlations between different frequency components become possible. All these effects are unique to time-modulated systems. We also show that the decay rate of optical emitters can be controlled in the proximity of such time-modulated structure. Our findings open a promising avenue toward coherence control in thermal radiation, dynamical thermal imaging, manipulating energy transfer among thermal or optical emitters, efficient near-field radiative cooling, and engineering spontaneous emission rates of molecules.

Nonreciprocal thermal emitter for near perpendicular incident light with cascade grating involving weyl semimetal

A nonreciprocal thermal emitter, consisting of a high refractive dielectric grating atop a Weyl semimetal (WS) grating and a metallic reflector, is proposed and studied. The emitter exhibits unequal directional absorption and emission, which breaks the traditional Kirchhoff's law and leads to the realization of strong nonreciprocal radiation. What's more, near-perfect nonreciprocity is realized at an angle of only 1o, which breaks the constraint of large operating angle confronted by existing nonreciprocal emitters and significantly enlarges the angular range. The physical origin of such phenomenon is disclosed by analyzing the magnetic field amplitude distribution and further confirmed by the impedance matching theory. To further confirm the range of angles that can be used, the strong nonreciprocity at several other angles is investigated. Last, the influence of the axial vector of the WS and the structure dimensions on the nonreciprocity is also investigated. We believed that the results should be useful for improving the efficiency of energy harvesting structures and developing novel thermal devices.

Bifacial Thermophotovoltaic Energy Conversion

Thermophotovoltaic (TPV) energy conversion efficiency has recently surpassed 30%. The key behind such high efficiency is the inclusion of a highly efficient mirror in the rear of the TPV cell that turns back to the thermal emitter the outband energy photons. Efficiencies over 50% could be theoretically attainable by approaching a mirror reflectance of 100%. However, the very few percent of outband absorption significantly deteriorate the conversion efficiency, especially at low emitter temperatures. Thus, current research focuses on developing advance mirror designs able to reach an extreme high outband reflectance over 95%. In this article I propose a bifacial TPV cell that enables very efficient photon recycling without using mirrors and that is less sensitive to outband optical losses. The key to this design is that the cell is introduced in a thermal emitter enclosure where it is irradiated from both sides. Then, outband photons transmit through the cell and are reabsorbed in the emitter. Therefore, the optical losses linked to the mirror/cell interface are eliminated, potentially enabling higher photon recycling efficiencies. This article presents a detailed balance simulation of an edge-cooled bifacial TPV cell to demonstrate that bifacial configuration enables higher conversion efficiencies and twice much as power density than monofacial designs, the latter being an advantage for moderate temperature and low-cost TPV power generation. Therefore, bifacial TPV cells are appealing for developing practical high-efficient and low-cost TPV devices for power generation in an extended range of heat source temperatures.

Module-Level Polaritonic Thermophotovoltaic Emitters via Hierarchical Sequential Learning.

Thermophotovoltaic (TPV) generators provide continuous and high-efficiency power output by utilizing local thermal emitters to convert energy from various sources to thermal radiation matching the bandgaps of photovoltaic cells. Lack of effective guidelines for thermal emission control at high temperatures, poor thermal stability, and limited fabrication scalability are the three key challenges for the practical deployment of TPV devices. Here we develop a hierarchical sequential-learning optimization framework and experimentally realize a 6″ module-scale polaritonic thermal emitter with bandwidth-controlled thermal emission as well as excellent thermal stability at 1473 K. The 300 nm bandwidth thermal emission is realized by a complex photon polariton based on the superposition of Tamm plasmon polariton and surface plasmon polariton. We experimentally achieve a spectral efficiency of 65.6% (wavelength range of 0.4-8 μm) with statistical deviation less than 4% over the 6″ emitter, demonstrating industrial-level reliability for module-scale TPV applications.

High-quality factor mid-infrared absorber based on all-dielectric metasurfaces.

The absorption spectrum of metasurface absorbers can be manipulated by changing structures. However, narrowband performance absorbers with high quality factors (Q-factor) are hard to achieve, mainly for the ohmic loss of metal resonators. Here, we propose an all-dielectric metasurface absorber with narrow absorption linewidth in the mid-infrared range. Magnetic quadrupole resonance is excited in the stacked Ge-Si3N4 nanoarrays with an absorption of 89.6% and a Q-factor of 6120 at 6.612 µm. The separate lossless Ge resonator and lossy Si3N4 layer realize high electromagnetic field gain and absorption, respectively. And the proposed method successfully reduced the intrinsic loss of the absorber, which reduced the absorption beyond the resonant wavelength and improved the absorption efficiency of Si3N4 in the low loss range. Furthermore, the absorption intensity and wavelength can be modulated by adjusting the geometric parameters of the structure. We believe this research has good application prospects in mid-infrared lasers, thermal emitters, gas feature sensing, and spectral detection.

Near perfect and polarization insensitive broadband absorber based on Al2O3 - Ti - Al2O3 triple layer structure

In this paper, a polarization-insensitive and broadband absorber based on Al 2 O 3 - Ti - Al 2 O 3 rectangular periodic sandwich structure has been studied. Simulations indicate the absorbance remains over 97% at the wavelength from 599 nm to 1912 nm, and the corresponding average absorption is up to 98.5% in this range. Moreover, the broadband absorption is background refractive index and polarization insensitive. The absorption mechanism of the broad-band is studied by impedance-matching theory and finite difference time domain (FDTD) method, which is mainly originated from the cavity plasmonic resonance, surface plasmonic dipolar resonance, and near field plasmonic coupling effect. The proposed broadband absorber is expected to play a role in solar cell, which can be applied in the areas of solar cell design, imaging, and thermal emitters. Moreover, the use of Ti material and Al 2 O 3 instead of noble metals has the advantages of thermal robustness, and lower cost.

Nonequilibrium Hanbury Brown-Twiss interferometry: Theory and application to binary stars

Intensity interferometry based on Hanbury Brown and Twiss's seminal experiment for determining the radius of the star Sirius formed the basis for developing the quantum theory of light. To date, the principle of this experiment is used in various forms across different fields of quantum optics, imaging, and astronomy. Although the technique is powerful, it has not been generalized for objects at different temperatures. Here, we address this problem using a generating functional formalism by employing the $P$-function representation of quantum-thermal light. Specifically, we investigate the photon coincidences of a system of two extended objects at different temperatures using this theoretical framework. We show two unique aspects in the second-order quantum coherence function: interference oscillations and a long-baseline asymptotic value that depends on the observation frequency, temperatures, and size of both objects. We apply our approach to the case of binary stars and discuss the advantages of measuring these two features in an experiment. In addition to the estimation of the radii of each star and the distance between them, we also show that the present approach is suitable for the estimation of temperatures as well. To this end, we apply it to the practical case of binary stars Luhman 16 and Spica $\ensuremath{\alpha}$ Vir. We find that for currently available telescopes, an experimental demonstration is feasible in the near term. Our work contributes to the fundamental understanding of intensity interferometry of quantum-thermal light and can be used as a tool for studying two-body thermal emitters, from binary stars to extended objects.

Laser Constructing Short‐Range Disordered Metagratings for Visible Near‐Infrared Polarization‐Independent Absorption

AbstractSubwavelength‐structured metasurfaces working in visible and near‐infrared bands present a high challenge in large‐scale device fabrication. In this study, a scalable, high‐efficient, and low‐cost laser‐induced nanopatterning technique is exploited to fabricate a kind of short‐range disordered metagratings, which enables broadband polarization‐independent absorption in the visible to near‐infrared wavelength. The short‐range disorder of the laser‐induced nanogratings originates from the laser‐induced thermal effect and can be spontaneously organized during laser nanopatterning. The unique disorder can break the unidirectional characteristics of the nanogratings, which empowers the metagratings to excite coupled resonance modes. This helps to achieve near‐perfect absorption in the visible to near‐infrared band (400–1100 nm) with an excellent angular tolerance (up to 60°) and average absorptivities of 96.3% and 93.5% under the TM and TE modes, respectively. These low‐cost metagratings can potentially inspire wide applications in the fields of solar cells, sensing, and thermal emitters.

Janus Interface Engineering Boosting Visibly Transparent Radiative Cooling for Energy Saving.

Visibly transparent radiative cooling (VTRC) shows great potential in energy-saving buildings or car glasses for lighting and cooling. How to balance the lighting and cooling performance is of significance to VTRC. In addition, the thermal radiative performance on the inner side should also be determined for cooling. Here, we designed a Janus VTRC coating consisting of a thermal emitter, PDMS, and a transparent near-infrared reflector, TiO2/Ag/TiO2. On the outer side, the visible transmittance T̅vis = 0.70, while the solar reflectance R̅solar = 0.40, and the thermal emittance in the atmospheric window ε̅LWIR = 0.94 can be achieved experimentally. On the inner side, the thermal emittance ε̅IR can be 0.90 or 0.01 depending on the substrate (glass or near-infrared reflector), which acts as the radiative conductor or barrier for energy saving in hot or cold internal situations. Compared with glass, the designed PDMS/NIR/glass achieves an average temperature drop of 14.6 °C experimentally. The energy-saving calculation based on seven cities in China shows that the VTRC coating can save 34-44% of the annual cooling energy consumption. This Janus visibly transparent radiative cooling technology with internal and external regulation provides a potential strategy for energy saving under the requirement of simultaneous lighting and cooling.

Realization of an efficient radiative cooling emitter with double layer inorganic SiO2 and TiO2 metamaterial

A novel double-layer thermal emitter consisting of inorganic SiO2 and TiO2 metamaterials is proposed and numerically investigated. It can achieve ultrahigh absorption of 96.42 % and 92.97 % in the first and second atmospheric transparency window simultaneously, which helps to achieve efficient radiative cooling. On the premise of obtaining high absorption, a comparative study on the design with and without the metal substrate has been conducted, verifying our simple design can be adopted without the metal substrate. Meanwhile, the high emissivity still persists under different incident angles, air mass and water vapor column. Under the influence of convection coefficients, a net cooling power of 164.39 W/m2 is achieved. Finally, we establish a steady heat balance system to mimic the thermal emitter and obtain an average temperature drop of 9.6℃ with the Jacobian iteration method. Therefore, our design with excellent cooling capacity may provide a new guidance in the application of radiative cooling device.

Facile Fabrication of Titanium Nitride Nanoring Broad-Band Absorbers in the Visible to Near-Infrared by Shadow Sphere Lithography

Plasmonic broad-band absorbers have received much attention because of their high absorption and potential applications for light-absorbing devices such as thermophotovoltaics, solar energy harvesting, and thermal emitters. However, the fabrication of complex structures in a large area and thermostability remains a great challenge. Here, we report a titanium nitride nanoring broad-band absorber that has over 95% average absorption in the visible and near-infrared regions (400-900 nm). Nanoring structures in a large area (inch2) are fabricated by shadow sphere lithography, which can innovatively increase fabrication efficiency. The nanoring absorber showed over 2.3 times higher-temperature increases than flat film under the irradiation of light. These large-scale and broad-band absorbers have potential applications for solar energy conversion devices such as thermophotovoltaics and photothermal devices.

Wide-angle and broadband nonreciprocal thermal emitter with cascaded dielectric and Weyl semimetal grating structure

The recent review [Phys. Rev. Appl. 18, 027001 (2022)] has considered that the existing schemes of nonreciprocal radiation are greatly limited by the narrow-operated bandwidth and small angular range. To address these key challenges, here, the wide-angle and broadband nonreciprocal radiation based on cascaded dielectric and Weyl semimetal (WS) grating atop a thick continuous metal film is investigated. It is shown that strong nonreciprocal radiation with nonreciprocity larger than 0.9 is achieved in the spectral range of 14.77–16.175 μm for the angle of 59°. The physical origin behind this broadband nonreciprocal radiation is revealed through investigating the magnetic field distributions at several selected wavelengths and is also confirmed by the impedance matching theory. In addition, the broadband nonreciprocal radiation performance remains stably in a wide parameter space. Furthermore, it is found that the broadband spectral nonreciprocity can be maintained well in a wide angular range, in particular, above 0.7 nonreciprocity can be realized in the wavelength range of 14.5–16.5 μm for the angle between 36° and 64.5°. Both features make the proposed scheme very attractive for real production. Finally, the broadband spectral nonreciprocity can be flexibly controlled through change in the axial vector of the WS. We believe that the conclusions will pave the way for designing energy harvesting and conversion devices with improved efficiency.

Weyl semimetal mediated epsilon-near-zero hybrid polaritons and the induced nonreciprocal radiation.

Polaritonic excitation and management in ultra-thin polar crystals has recently received significant attention and holds new promise for epsilon-near-zero (ENZ) modes. However, manipulation of the ENZ mode via anisotropic magneto-optic (MO) material remains elusive. Herein, we provide an effective strategy for constructing an ENZ polar thin film with dependence on Weyl semimetals (WSM). The thermal radiation of the proposed device is explored with electromagnetic (EM) simulations that utilize the anisotropic rigorous coupled-wave analysis (aRCWA) method. Strong coupling of the ENZ mode to WSM polaritons has been demonstrated, and the structural parameters hold tolerance on the order of hundreds of nanometers, which is highly favorable for low-cost fabrication and high-performance application. By changing both the azimuthal angle (ϕ) and angle of incidence (θ), the nonreciprocity (η) can be effectively influenced. The distribution of η is symmetrical with ϕ = 180°, η = 0 when ϕ = 90° and ϕ = 270°. The mechanism of this proposal is owing to the hybrid polaritons supported by the polar thin film and nonreciprocal radiation of WSM, which is validated by examining the amplitude distribution of the magnetic field. The nonreciprocal emitter described herein allows simultaneous control of spectral distribution and polarization of radiation, which will facilitate the active design and application of mid-infrared (MIR) thermal emitters.

Selective Emitter for Solar Thermophotovoltaic Applications

Selective Emitters (SEs) are the main components of solar thermophotovoltaic (STPV) systems; they act as intermediate thermal radiation emitters to shape the incident solar spectrum to match the wavelengths useful for the PV cell. In this work, we present the design, optimisation, fabrication, and characterisation of an SE based on a multilayer design made of SiNx, SiO2, and TiO2 layers. The SE is optimised to work with PV cells based on III-V semiconductors, such as GaSb, InGaAs, and InGaAsS, the bests suitable for SPTV applications. The fabricated SE shows an emitter efficiency (ηSE) of 50% if matched with a PV cell with an energy bandgap of 0.63 eV.

Spectrally stable thermal emitters enabled by material-based high-impedance surfaces.

Radiative thermal engineering with subwavelength metallic bodies is a key element for heat and energy management applications, communication and sensing. Here, we numerically and experimentally demonstrate metallic thermal emitters with narrowband but extremely stable emission spectra, whose resonant frequency does not shift with changes on the nanofilm thickness, the angle of observation and/or polarization. Our devices are based on epsilon-near-zero (ENZ) substrates acting as material-based high-impedance substrates. They do not require from complex nanofabrication processes, thus being compatible with large-area and low-cost applications.

Thermophotovoltaic conversion efficiency measurement at high view factors

A standardized method for measuring thermophotovoltaic (TPV) efficiency has not been yet established, which makes the reported results difficult to compare. Besides, most of the TPV efficiencies reported to date have been obtained using small view factors, i.e., large cell-to-emitter distances, so the impact of the series resistance is usually underestimated, and the optical cavity effects, i.e., the multiple reflections taking place between the emitter and the cell, are not accounted for experimentally. In this work, we present an experimental setup able to measure the TPV efficiency under high view factors (up to 0.98), by using small emitter-to-cell distances (< 1 mm). This allows a more accurate direct measurement of the TPV efficiency at higher power densities than previous works. As a result, a TPV efficiency of 26.4+-0.1 % and a power density of 4.3+-0.8 W/cm2 have been obtained for an InGaAs TPV cell with a back surface reflector irradiated by a graphite thermal emitter at 1592 C.

Tunable near-perfect nonreciprocal radiation with a Weyl semimetal and graphene.

A tunable near-perfect nonreciprocal thermal emitter, consisting of a dielectric plane and a monolayer graphene sandwiched between a subwavelength grating and a Weyl semimetal plane, is proposed and investigated. Near-complete nonreciprocal radiation can be achieved at resonance, breaking the traditional Kirchhoff's law. The underlying physical mechanism, resulting from a guided mode resonance, is disclosed by illustrating the magnetic field distribution. Moreover, the strong nonreciprocity remains well within a wide range of geometrical parameters. What's more, the performance of the near-perfect spectral nonreciprocity can be flexibly controlled in a wide spectral range through varying the Fermi level of graphene and the axial vector of the Weyl semimetal, which reduces the cost and should be interesting for real application. The conclusions of this paper should prompt the further development of tunable nonreciprocal thermal emitters.

Design of Wavelength Selectively Metamaterial Antenna for Thermal Camouflage

Phase transition materials offer exciting opportunities to dynamically control the optical properties of photonic devices. As a special phase change material (PCM), vanadium dioxide (VO2) is an excellent alternative for thermal camouflage technology research. Due to the unique property of VO2, coupled with the metallic structure can enable the design of thermally reconfigurable absorbers that allow resonance tuning. We propose a very simple, multi-layered with metamaterial excellent thermal emitter nanoantenna. The size of our design is only 1365 nm for the infrared wavelength range from 0.8 to 12 µm. Concept silver (Ag), VO2, dielectric separator and silver (Ag) configuration are used for finite difference time domain (FDTD) analysis. The absorption band can be further expanded with more deposition layers with various metals. In this paper, a thermal camouflage mid-infrared adaptive metamaterial antenna based on a PCM metamaterial is designed to fit perfectly into atmospheric windows. The spectral properties of the structure were calculated using FDTD method. The distinctive feature of the proposed structure is that adaptations can be made by placing an ultra-thin VO2 interlayer between the metallic structures. We believe that the proposed design is very promising in terms of simple fabrication and modifiable aspects for wide-area broadband unity absorption.

Nonreciprocal Thermal Emission Using Spatiotemporal Modulation of Graphene

We present a nonreciprocal thermal emitter based on the dynamic modulation of graphene. A graphene ribbon grating situated on a dielectric slab is designed to excite high-quality resonances in the long-IR region. We show that upon space–time modulation of the Fermi energy of graphene, asymmetric modal splitting results in large nonreciprocity, leading to a strong violation of Kirchhoff’s law of thermal radiation. We further show that the graphene system allows the creation of “absorptivity holes” and “emissivity holes” in the spectrum that are asymmetric. By changing the modulation frequency, the location of these holes can be adjusted, giving rise to a new dimension of tunable thermal emission in nonreciprocal systems. In this system, while Kirchhoff’s law is violated, we numerically observe certain symmetry properties between absorption and emission. We establish that these symmetry properties follow compound symmetry considerations.