Metamaterial Emitters Research Articles

Thermal stability of tungsten based metamaterial emitter under medium vacuum and inert gas conditions

Commercial deployment of thermophotovoltaics (TPV) is lacking behind the implementation of solar PV technology due to limited thermal stability of the selective emitter structures. Most of the TPV emitters demonstrated so far are designed to operate under high vacuum conditions (~10−6 mbar vacuum pressure), whereas under medium vacuum conditions (~10−2 mbar vacuum pressure), which are feasible in technical implementations of TPV, these emitters suffer from oxidation due to significant O2 partial pressure. In this work, the thermal stability of 1D refractory W-HfO2 based multilayered metamaterial emitter structure is investigated under different vacuum conditions. The impact of the O2 partial pressure on thermal stability of the emitters is experimentally quantified. We show that, under medium vacuum conditions, i.e. ~10−2 mbar vacuum pressure, the emitter shows unprecedented thermal stability up to 1300 °C when the residual O2 in the annealing chamber is minimized by encapsulating the annealing chamber with Ar atmosphere. This study presents a significant step in the experimental implementation of high temperature stable emitters under medium vacuum conditions, and their potential in construction of economically viable TPV systems. The high TPV efficiency, ~50% spectral efficiency for GaSb PV cell at 1300 °C, and high temperature stability make this platform well suited for technical application in next-generation TPV systems.

Design of an All-Semiconductor Selective Metamaterial Emitter in the Mid-IR Regime with Larger Feature Sizes for Thermophotovoltaic Energy Conversion Applications

A thermophotovoltaic (TPV) system converts heat that is absorbed via conduction, convection, and/or radiation to electricity. The efficiency of TPV energy conversion can be improved with a narrowband selective emitter that emits photons at just above the bandgap energy towards the TPV photodiode. We numerically report a selective metamaterial (MM) emitter design with a single layer of cylindrical structures of p-type silicon (boron-doped). Our design (substrate-free) features a peak absorbance of 94.8% at the wavelength of 3.47 μm with the smallest lateral dimension of 0.8 μm. The absorption is found to be due to the resonance of electric and magnetic fields in the structure. The larger dimensions of our selective MM emitter design make it significantly easier to pattern than many of previously reported selective MM emitters operating at similar wavelengths to that of our work. We believe that our work demonstrates a path forward for future research on larger-area all-semiconductor selective MM emitters with a variety of peak absorbance wavelengths for TPV applications.

Evaluation of performance of near-field thermophotovoltaic systems based on entropy analysis

Near-field radiative heat transfer can significantly improve the output power of thermophotovoltaic (TPV) systems. Therefore, it is crucial to explore how to increase the energy conversion efficiencies of near-field TPV systems. In this study, based on the fluctuation–dissipation theorem with the effective medium theory, we evaluate the performance of a near-field TPV system from the formulation of thermodynamics. It is found that a near-field TPV system consisting of InAs or GaSb cell can achieve higher heat flux or efficiency limit, respectively. Moreover, the TPV system with a hyperbolic metamaterial (HMM) emitter composed of nanowire or nanohole arrays can achieve higher heat flux and efficiency limit compared to that of the bulk reference. This is attributable to the HMMs being able to support hyperbolic modes, and the radiation with the HMM emitter exhibits a favorable entropy content for the energy conversion efficiency. This work provides an approach to determine the efficiency limit and establish a target for efficiency of the near-field TPV system without considering how the system works. The results of this study will facilitate the design and application of the HMM emitter and material of the TPV cell to improve the efficiency of near-field TPV systems.

Multiple Resonance Metamaterial Emitter for Deception of Infrared Emission with Enhanced Energy Dissipation.

Artificial camouflage surfaces for assimilating with the environment have been utilized for controlling optical properties. Especially, the optical properties of infrared (IR) camouflage materials should be satisfied with two requirements: deception of IR signature in a detected band through reduced emissive energy and dissipation of reduced emissive energy for preventing thermal instability through an undetected band. Most reported articles suggest the reduction of emissive energy in the detected band; however, broadband emission for enough energy dissipation through the undetected band simultaneously is still a challenging issue. Here, we demonstrate the multiresonance emitter for broadband emission with IR camouflage utilizing the electromagnetic properties of dielectric material. We reveal that the interaction between the magnetic resonance and dielectric layer's property in a metal-dielectric-metal structure induces the multiple resonance at the specific band. We present an IR camouflage behavior of multiresonance emitter on a curved surface through the IR camera (8-14 μm). We evaluate the energy dissipation in the undetected band, which is 1613% higher than metal and 26% higher than conventional selective emitters. This study paves the way to develop broadband emitters for radiative cooling and thermophotovoltaic applications.

High-performance near-field thermophotovoltaic device with CaF<sub>2</sub>/W multilayer hyperbolic metamaterial emitter

Thermophotovoltaic (TPV) device is a thermoelectric conversion method with great application prospects. In the far-field regime, the thermoelectric power is usually small due to the Planck blackbody radiation limit, but can be substantially enhanced in the near-field regime where evanescent waves will participate in the heat transfer by tunneling. In this aspect, the surface optical density of state is a key factor that will determine the transfer conversion of thermal photons. Plasmonic or phononic resonance materials have been discussed in the literature to acquire large heat flux. Besides, metamaterial is another way to pursue the design freedoms for the same purpose. In this work, we propose a [CaF<sub>2</sub>/W]<i><sub>n</sub></i> multilayer based infrared hyperbolic metamaterial (HMM) with high surface density of states as an emitter of a high-performance TPV cell made of an InSb p-n junction (energy bandgap = 0.17 eV). The effective medium theory (EMT) is utilized to describe the electromagnetic behavior of the HMM. The near-field heat flux is calculated based on electrodynamic wave theory and Green's function method, and the photocurrent of thermophotovoltaic device is derived using diffusion equation for semiconductor. For comparison, we design three different radiators, i.e. tungsten film (W), [GaF<sub>2</sub>/W]<i><sub>n</sub></i> multilayer hyperbolic metamaterial (HMM), and tungsten-grounded HMM (WHMM). Compared with the pure tungsten radiator, the artificial structure exhibits the hyperbolic dispersion characteristic in a wide frequency range, which gives rise to a higher local density of states, in particular in the hyperbolic-to-elliptic spectral transition region. As a result, the radiation power and the energy conversion efficiency are greatly enhanced, which are more easily realized by a matched emission band achieved by the structural design. We find that the thermophotovoltaic device with WHMM radiator has a similar power and conversion efficiency to that with the HMM radiator. The influence of the substrate can be ignored when the hyperbolic metamaterial is thicker than 140 nm, very beneficial to the actual fabrication of the device. By our system, with multilayer hyperbolic metamaterial (HMM) radiator, a high electric power >1 W/m<sup>2</sup> and a conversion efficiency about 11% can be obtained at a bias temperature of 200 K and a 100 nm vacuum gap. Compared with nanowire arrays or natural hyperbolic material, the multilayer structure proposed in this paper has obvious advantages in bandwidth and manufacturing and may find important applications in near-field thermophotovoltaic device and other relevant areas.

Theoretical study of multilayer ring metamaterial emitter for a low bandgap TPV cell

To improve the conversion efficiency of InGaAsSb, we designed a multi-layer ring metamaterial (MTM) emitter with a cut-off wavelength of 2.34 µm based on theoretical calculation. By adjusting the geometric parameters, such as the width of the periodic structure, the number of layers, the radii and thickness of the multilayer rings, and the thickness of the dielectric substrate, an optimal structure that matched the external quantum efficiency of the InGaAsSb cell was obtained. Through numerical simulation, the emitter demonstrated a mean emittance of 0.91 in the wavelength range of 0.5–2.4 µm. Additionally, our emitter can obtain a spectral efficiency of 85.6% for the InGaAsSb cell at 1600 K, thereby significantly improving the efficiency of the TPV.

Non-tapered metamaterial emitters for radiative cooling to low temperature limit

A selective thermal emitter is a requisite for approaching the radiative cooling limit, while the tapered metamaterial emitters, thus far, have difficulties in scalable manufacturing, hindering the field tests. In this work, we presented a methodology for designing the non-tapered metamaterial emitters without excluding the tapered and the inverted ones considering experimental imperfections. The bandwidths were not governed by the ratio of top to bottom width of arrays any more as conventional tapered metamaterials, while the non-tapered ones were only dependent on the width of vertical pillars and the thicknesses of dielectric building blocks. The sensitivity of the emission spectral characteristics of the non-tapered metamaterial emitters to the tilt of side walls was also investigated and the optimization strategy was proposed. The non-tapered metamaterial emitters could achieve an average emissivity of 0.8 at the atmospheric window with wavelengths of 8 to 13 μm, which would bring about a temperature reduction of 55 K in the absence of parasitic heat. The thickness-tailored bandwidth features in the non-tapered metamaterial emitters, which provided an in-depth insight into conventional tapered metamaterial emitters, mitigating complexities in practical production and paving the way for experimental access to more significant temperature reduction.

Metamaterial emitter for thermophotovoltaics stable up to 1400\u2009\xb0C

High temperature stable selective emitters can significantly increase efficiency and radiative power in thermophotovoltaic (TPV) systems. However, optical properties of structured emitters reported so far degrade at temperatures approaching 1200 °C due to various degradation mechanisms. We have realized a 1D structured emitter based on a sputtered W-HfO2 layered metamaterial and demonstrated desired band edge spectral properties at 1400 °C. To the best of our knowledge the temperature of 1400 °C is the highest reported for a structured emitter, so far. The spatial confinement and absence of edges stabilizes the W-HfO2 multilayer system to temperatures unprecedented for other nanoscaled W-structures. Only when this confinement is broken W starts to show the well-known self-diffusion behavior transforming to spherical shaped W-islands. We further show that the oxidation of W by atmospheric oxygen could be prevented by reducing the vacuum pressure below 10−5 mbar. When oxidation is mitigated we observe that the 20 nm spatially confined W films survive temperatures up to 1400 °C. The demonstrated thermal stability is limited by grain growth in HfO2, which leads to a rupture of the W-layers, thus, to a degradation of the multilayer system at 1450 °C.

A Pragmatic Bilayer Selective Emitter for Efficient Radiative Cooling under Direct Sunlight.

Radiative cooling can make the selective emitter cool below ambient temperature without any external energy. Recent advances in photonic crystal and metamaterial technology made a high-efficiency selective emitter achievable by precisely controlling the emitter’s Infrared emission spectrum. However, the high cost of the photonic crystals and meta-materials limit their application. Herein, an efficient bilayer selective emitter is prepared based on the molecular vibrations of functional nanoparticles. By optimizing the volume fraction of the functional nanoparticles, the bilayer selective emitter can theoretically cool 36.7 °C and 25.5 °C below the ambient temperature in the nighttime and daytime, respectively. Such an efficient cooling performance is comparable with the published photonic crystal and metamaterial selective emitters. The rooftop measurements show that the bilayer selective emitter is effective in the ambient air even under direct sunlight. The relatively low cost and excellent cooling performance enable the bilayer selective emitter to have great potential for a practical purpose.

Broadband switching of mid-infrared atmospheric windows by VO2-based thermal emitter.

Atmospheric windows play an important role in the field of infrared detection and radiative cooling. In this paper, the development of VO2-based metamaterial emitter brings broadband thermal-switching light to mid-infrared atmospheric windows. At room temperature, the emitter radiates light in both 3-5μm and 8-14μm atmospheric windows. At high temperature, the radiation peaks move out of the atmospheric windows and result a strong radiation at 5-8μm. The underlying mechanism relies on the relationship between VO2 metal-insulator transition (MIT) and resonant absorption modes coupling. Corresponding thermal imaging experiment exhibits two distinct phenomena. One is the observation of unchanged thermal radiation around MIT temperature. The other phenomenon regards the concealment of the emitter from Al background at specific temperatures. These two phenomena show potential application in infrared anti-detection.

Performance Analysis of a Metamaterial-Based Near-Field Thermophotovoltaic System Considering Cooling System Energy Consumption

A mathematical physical model of a near-field thermophotovoltaic (TPV) system containing a metamaterial emitter comprised of W-nanowire arrays embedded in a SiC host is constructed. It is notable that this model incorporates a cooling system. On this basis, the influence of the emitter temperature and filling ratio, the cell and emitter thicknesses, and the emitter cell vacuum gap on the TPV system output performance is analyzed. It is found that the cooling device energy consumption increases by two orders of magnitude with decreased emitter cell gap size in the near-field; the highest possible value is 267.74 W·cm−2. However, the maximum net system efficiency is only 11.79 %. The emitter radiation capability can be enhanced by increasing the emitter temperature, but the cooling system energy consumption remains a significant problem. When the emitter temperature increases to 2200 K, the net system efficiency and net output density reach maximum values of only 6.22 % and 36.28 W·cm−2, respectively. Further investigation demonstrates that a large emitter thickness can induce a surface disturbance phenomenon, resulting in rapid decreases in the net system output power density and net system efficiency to − 251.85 W·cm−2 and − 12.86 %, respectively. However, when the cell thickness increases beyond 1000 nm, the net system efficiency and output power density are stable at 2.13 % and 24.12 W·cm−2, respectively. Finally, the emitter filling ratio should be increased as much as possible to maintain good system performance.

Enhanced low-gap thermophotovoltaic cell efficiency for a wide temperature range based on a selective meta-material emitter

The concept of thermophotovoltaic system, based on a compact selective meta-material film structure acting as a selector thermal blackbody radiation emitter, is presented. The meta-material (MTM) film exhibits broad-band thermal radiation absorption with selective narrow-band thermal infrared emission, and can thus be efficiently coupled to a tandem (InAs/GaInAsSb) thermophotovoltaic (TPV) cell, which has based on the experimentally sub-cells data. The TPV tandem cell combined with the selective emitter achieves highest TPV conversion efficiency of 41% for black body radiation temperature (TBB) = 1500 °C, and lowest TPV conversion efficiency of 11.82% for TBB = 300 °C. The TPV combined system thus operates in wide temperature range of 300 °C to 1500 °C, and the combined system shows higher performance when the MTM film is included. It also shows much higher performance than literature values reported for GaSb/GaInAsSb tandem thermophotovoltaic cells having no MTM film.

High-efficiency thermophotovoltaic energy conversion enabled by a metamaterial selective emitter

Thermophotovoltaics (TPV) is the process by which photons radiated from a thermal emitter are converted into electrical power via a photovoltaic cell. Selective thermal emitters that can survive at temperatures at or above ∼1000°C have the potential to greatly improve the efficiency of TPV energy conversion by restricting the emission of photons with energies below the photovoltaic (PV) cell bandgap energy. In this work, we demonstrated TPV energy conversion using a high-temperature selective emitter, dielectric filter, and 0.6 eV In0.68Ga0.32As photovoltaic cell. We fabricated a passivated platinum and alumina frequency-selective surface by conventional stepper lithography. To our knowledge, this is the first demonstration of TPV energy conversion using a metamaterial emitter. The emitter was heated to >1000°C, and converted electrical power was measured. After accounting for geometry, we demonstrated a thermal-to-electrical power conversion efficiency of 24.1±0.9% at 1055°C. We separately modeled our system consisting of a selective emitter, dielectric filter, and PV cell and found agreement with our measured efficiency and power to within 1%. Our results indicate that high-efficiency TPV generators are possible and are candidates for remote power generation, combined heat and power, and heat-scavenging applications.

Tunable dual-band thermal emitter consisting of single-sized phase-changing GST nanodisks.

Thermal emission control has been attracting increased attention in both fundamental science and many applications including infrared sensing, radiative cooling and thermophotovoltaics. In this paper, a tunable dual-band thermal emitter including phase-changing material Ge2Sb2Te5 (GST) is experimentally demonstrated. Two emission peak wavelengths are at 7.36 μm and 5.40 μm at amorphous phase, and can be continuously tuned to 10.01 μm and 7.56 μm while GST is tuned to crystalline phase. Compared with other dual-band metamaterial emitters, this tunable dual-band thermal emitter is only composed of an array of single-sized GST nanodisks (on a gold film), which can greatly simplify the design and manufacturing process, and pave the way towards dynamical thermal emission control.

Highly efficient and broadband mid-infrared metamaterial thermal emitter for optical gas sensing.

Development of a novel, cost-effective, and highly efficient mid-infrared light source has been identified as a major scientific and technological goal within the area of optical gas sensing. We have proposed and investigated a mid-infrared metamaterial thermal emitter based on micro-structured chromium thin film. The results demonstrate that the proposed thermal light source supports broadband and wide angular absorption of both TE- and TM-polarized light, giving rise to broadband thermal radiation with averaged emissivity of ∼0.94 in a mid-infrared atmospheric window of 8-14μm. The proposed microphotonic concept provides a promising alternative mid-infrared source and paves the way towards novel optical gas sensing platforms for many applications.

On-Chip Narrowband Thermal Emitter for Mid-IR Optical Gas Sensing

Efficient light sources compatible to complementary metal oxide semiconductor (CMOS) technology are key components for low-cost, compact mid-infrared gas sensing systems. In this work we present an on-chip narrowband thermal light source for the mid-infrared wavelength range by combining microelectromechanical system (MEMS) heaters with metamaterial perfect emitter structures. Exhibiting a resonance quality factor of 15.7 at the center wavelength of 3.96 μm and an emissivity of 0.99, the demonstrated emitter is a spectrally narrow and efficient light source. We show temperature-stable (resonance wavelength shift 0.04 nm/°C) and angular-independent emission characteristics up to angles of 50° and provide an equivalent circuit model illustrating the structure’s resonance behavior. Owing to its spectrally tailored, nondispersive emission, additional filter elements in a free-space optical gas sensing setup become obsolete. In a proof-of-concept demonstration of such a filter-free gas sensing system with CO2 ...

Performance optimization analysis of solar thermophotovoltaic energy conversion systems

By applying metamaterial absorber and emitter, the STPV system with unity emitter-absorber area ratio exhibits conversion efficiency between 8% and 10% with concentration factor varying between 20 and 200. However, the performance of the STPV system with unity emitter-absorber area ratio becomes unacceptably bad with higher concentration factor mainly because of the greatly enhanced radiation energy loss due to the thermal emission from the absorber. In this case, in order to reduce the radiation energy loss, the emitter-absorber area ratio needs to be enlarged. This work systematically analyze the influence of the emitter-absorber area ratio on the performance of STPV system with concentration factor varying between 200 and 500. Non-ideal conditions such as metamaterial emitter/absorber, non-ideal adiabatic layer and copper plate heat exchanger are also taken into consideration. It turns out that, when the concentration factor ranges from 200 to 500, the conversion efficiency of the STPV system with optimized emitter-absorber area ratio increases from 13% to 14.8% while that with unity emitter-absorber area ratio decreases from 8.6% to 3%, which indicates a maximal difference of around 12% under 500 suns. Furthermore, the output power density for the STPV system with optimized emitter-absorber area ratio is also analyzed. The result shows that the output power density increases from 1650W/m2 to 2300W/m2, which is much larger than the output power density of a conventional Si-substrate single-junction solar cell. In summary, this work demonstrates the advantages of STPV system with optimized emitter-absorber area ratio, which can be applied in the field of designing solar thermophotovoltaic devices.

Enhanced energy transfer by near-field coupling of a nanostructured metamaterial with a graphene-covered plate

Coupled surface plasmon/phonon polaritons and hyperbolic modes are known to enhance radiative transfer across nanometer vacuum gaps but usually require identical materials. It becomes crucial to achieve strong near-field energy transfer between dissimilar materials for applications like near-field thermophotovoltaic and thermal rectification. In this work, we theoretically demonstrate enhanced near-field radiative transfer between a nanostructured metamaterial emitter and a graphene-covered planar receiver. Strong near-field coupling with two orders of magnitude enhancement in the spectral heat flux is achieved at the gap distance of 20nm. By carefully selecting the graphene chemical potential and doping levels of silicon nanohole emitter and silicon plate receiver, the total near-field radiative heat flux can reach about 500 times higher than the far-field blackbody limit between 400K and 300K. The physical mechanism is elucidated by the near-field surface plasmon coupling with fluctuational electrodynamics and dispersion relations. The effects of graphene chemical potential, emitter and receiver doping levels, and vacuum gap distance on the near-field coupling and radiative energy transfer are analyzed in detail.

Densely-tiled metal-insulator-metal metamaterial resonators with quasi- monochromatic thermal emission.

Metal-insulator-metal metamaterial thermal emitters strongly radiate at multiple resonant wavelengths. The fundamental mode, whose wavelength is the longest among resonances, is generally utilized for selective emission. In this paper, we show that parasitic modes at shorter wavelengths are suppressed by newly employed densely-tiled resonators, and that the suppression enables quasi-monochromatic thermal emission. The second-order harmonics, which is excited at half the fundamental wavelength in conventional emitters, shifts toward shorter wavelength. The blue-shift reduces the amplitude of the second-order emission by taking a distance from the Wien wavelength. Other parasitic modes are eliminated by the small spacing between resonators. The densely-tiled resonators are fabricated, and the measured emission spectra agree well with numerical simulations. The methodology presented here for the suppression of parasitic modes adds flexibility to metamaterial thermal emitters.

Performance analysis of solar thermophotovoltaic conversion enhanced by selective metamaterial absorbers and emitters

By converting broadband sunlight into narrowband thermal radiation matched to the bandgap of thermophotovoltaic (TPV) cells, solar thermophotovoltaic (STPV) systems could potentially reach a high conversion efficiency far exceeding the Shockley–Queisser limit. However, actual STPV systems exhibit much lower efficiency due to non-idealities in solar absorbers, thermal emitters and TPV cells. In this work, the STPV system with selective metamaterial solar absorber and emitter is investigated, whose conversion efficiency is between 8% and 10% with concentration factor varying between 20 and 200. This conversion efficiency is remarkably enhanced compared with the conversion efficiency of less than 2.5% for the STPV system employing black absorbers and emitters. The sidewall emission losses from the absorber–emitter module and the non-unity view factor between the thermal emitter and TPV cell will diminish the performance of the STPV system, whose effects are also quantitatively discussed in this work. Furthermore, the non-planar STPV systems with larger emitter–absorber area ratios are investigated, whose conversion efficiency can reach up to 12.6% under 200 suns when the emitter is four times as large as the absorber.