Publications

Topological photonic crystals inherit the unique properties of topological insulators, including topologically protected energy transfer and unidirectional propagation, which offer an excellent platform for exploring exotic physics and developing photonic devices. However, topological photonic crystals possessing mid-infrared edge modes that have potential applications in infrared imaging, biosensing, thermal radiation energy transfer, etc., are seldom brought into focus. In this work, we study the topological properties of a photonic crystal slab (PCS) consisting of silicon square veins in the mid-infrared, which is intended to mimic the two-dimensional Su–Schrieffer–Heeger model. By interfacing topologically trivial and nontrivial PCSs, mid-infrared edge modes can appear at domain wall, according to the principle of bulk-edge correspondence. It is also demonstrated high-efficiency mid-infrared light transport can be achieved by these edge modes. In addition, adjusting the vertical offset near the interface can manipulate the bandwidth for various applications and turns the connected PCS structure to a photonic realization of Rice–Mele model. We further fabricate the PCS and provide an experimental observation of transverse-electric-like edge modes in mid-infrared by using the scattering-type scanning near-field optical microscope. Additionally, we integrate it with phase change material of nanoscale thickness, Ge2Sb2Te5, to realize an ultrafast and switchable topological waveguide with zero static power. This work not only enriches the fundamental understanding of topological physics in mid-infrared optical settings, but also shows promising prospects in compact devices for energy transfer and information processing for light sources in these wavelengths, for instance, thermal radiation.

Topological phonon polaritons (TPhPs) are localized boundary modes arising from the combination of topological photonics and phonon polaritons in micro/nanostructures made of polar dielectrics, which are capable of mediating long-range radiative heat transfer and infrared sensing in a robust manner. This work shows TPhPs can be sustained in a family of one-dimensional quasiperiodic silicon carbide nanoparticle (NP) chains, and can significantly improve radiative heat transfer for these arrays. This family of 1D quasiperiodic lattices is a continuous interpolation between the two paradigmatic limits, viz., the off-diagonal Aubry-André-Harper (AAH) chain and the Fibonacci chain. For different interpolation parameters τ, the band structures are calculated with respect to the modulation phase ϕ as the wavenumber in a synthetic dimension, where a series of gaps and midgap edge states can be observed. These edge states are shown to be topologically protected by applying the gap-labeling theorem and the principle of bulk-boundary correspondence. By means of many-body radiative heat transfer theory for a group of electric dipoles, it is shown quasiperiodicity and topological property can have an important impact on radiative heat transfer. These results can give useful insights on the interplay among topological order, long-range quasiperiodic order, near-field electromagnetic interactions and radiative heat transfer.

Metasurface thermal emitter, whose thermal radiation is tailored by surface micro-nano structures, has been used as a convenient, sensitive and spectrally-efficient radiative source in application of mid-infrared gas sensing. In this study, we numerically and experimentally demonstrate a metasurface thermal emitter based on Au-Al2O3-Au MIM structure which realizes the dual-band spatially-distinguishable emission spectrum. Two strong emission modes are excited in different ranges of emission angles, making it possible to distinguish them by means of their corresponding spatial locations without using optical filters. This emitter promotes a more compact sensing strategy and possibly paves a way to the miniaturization of infrared optical gas sensors.

Selective emitters are crucial in thermophotovoltaic (TPV) systems as they can selectively tailor the incident light to match with the bandgap of photovoltaic (PV) cell, thus greatly increasing the system efficiency. However, further progress has been hampered by the limitation in designing high performance selective emitters with spectrally emissive selectivity and compatible ability to different PV cells. In this study, we propose a design strategy to realize tailorable bandgap-dependent selective emitters based on metasurfaces by combing the two-resonance response of surface plasmon polaritons (SPPs) and magnetic polaritons (MPs). The emission spectrum can be manipulated and matched to various PV cells with different bandgap by simply changing the size and period of the meta-atoms in metasurface. We design and experimentally demonstrate a selective emitter appealing to GaSb PV cells, which presents near-perfect emissivity (above 0.96) above the bandgap wavelength (from 0.9μm to 1.54μm) with polarization-insensitive and angle-insensitive features. Moreover, the emitter shows a high figure of merit (FOM, of 0.85), and the efficiency of TPV system can be expected to be 27% at 1473K. The proposed strategy and designed emitter may pave a reliable route for improving the performance of TPV systems.

It is well known that micro/nanomaterials exhibit many physical properties in the fields of heat transfer, energy conversion and storage, and also have great prospects in nanoelectronics, sensors, photonic devices and biomedical applications [...]

A theoretical model is proposed to investigate the near-field radiative heat transport between two or more SiC/Au nanoparticles inside a cavity configuration composed of two semi-infinite (SiC) plates. We show that the cavity configuration can effectively enhance the near-field radiative heat transfer (NFRHT) between two nanoparticles, and the maximum amplification goes beyond seven orders of magnitude for SiC nanoparticles and two orders of magnitude for Au nanoparticles. Besides, the electric contribution to the NFRHT can exceed the magnetic contribution for Au nanoparticles. We find that the strong coupling between the surface modes of the upper and lower SiC plates plays the major role in this giant amplification. Moreover, the radiative heat transport is known to be super-diffusive in one-dimensional SiC nanoparticle chains and we realize ballistic heat transport by this cavity configuration with small gap. The findings of this work could have potential applications for the thermal management at nanoscale.

Daytime radiative cooling is an innovative passive cooling technology to realize energy saving and counter global warming. To fulfill the demand for aesthetic appearance and effective display in real-life situations, coloration should be incorporated with daytime radiative cooling. Moreover, all of the reported subambient daytime colorful radiative coolers rely on the metal layered reflector, which prevents them from many applications. In this work, we propose a series of colorful radiative cooling coatings consisting of metallic nanoparticles (plain Ag and TiO2@Ag core–shell nanoparticles) for coloration, Y2O3 submicron particles with 30% volume fraction for solar radiation reflection, and polymethyl methacrylate resin as the matrix. A theoretical model on radiative properties of these cooling coatings is proposed by considering the dependent scattering effects among Y2O3 scatterers and localized surface plasmon resonance of metallic nanoparticles. Afterwards, we conduct a parametric study on the coloration and cooling performance of these coatings with different volume fractions and size distributions of embedded particles. Based on numerical simulations, we show these coatings can present a variety of colors and realize temperature drops of 2.0–3.9 °C and net cooling power of 23–47 W/m2 at a total metallic nanoparticles volume fraction of 0.0002%. This work provides pathway for large-scale-manufacturable colorful radiative cooling coatings with potential applications in eco-friendly buildings and outdoor facilities.

Near-field radiative heat transfer (NFRHT) between dissimilar materials is important for near-field applications, such as near-field photonic thermal diode and nanoscale thermal imaging. It is demonstrated that monolayer graphene lead to a significant enhancement in the NFRHT between dissimilar materials. The presence of graphene is not only able to permit a large variation and amplification of NFRHT through its chemical potential, but also to fully compensate the mismatch between the resonance frequencies of the two dissimilar materials. And then we investigate the role of the strong coupling effects among different surface polaritonic modes on NFRHT. Especially, we identify strongly coupled polaritonic modes with considerable Rabi frequencies. Therefore, we analyze the graphene-assisted configurations by calculating both the near-field dispersion relations and Rabi frequencies. To quantify the role played by the monolayer graphene, we have defined two amplification coefficients of NFRHT. Finally, the contributions of thickness of the two different polar dielectrics to the strong coupling effects have been investigated. As a result, we demonstrate that the strong coupling effect we highlight is robust with respect to the mismatch between resonance frequencies, laying the foundation for an active control and enhancement of NFRHT between dissimilar materials.

Thermophotovoltaic systems can harvest electric energy from heat sources with a potential efficiency exceeding the Shockley–Queisser limit due to the selective emission of an elaborate thermal emitter. In this work, a two-dimensional nanodisks/thin-film metamaterial is proposed as a wavelength-selective emitter, which can coordinate well with the photovoltaic cell in a thermophotovoltaic system. Compared to conventional emitters based on surface plasmon polaritons, the emittance peaks of the proposed emitter are realized by the excitations of both electric dipole and anapole modes in silicon nanodisks, which can be easily tailored due to the non-dispersive optical constants of dielectric materials. Meanwhile, the effect of polarization and polar angle on the emittance spectra is also investigated, suggesting that the proposed emitter has high emittance and efficiency not only in the normal direction but also at large oblique angles. Electromagnetic field and current density distributions reveal that the coupling between multipole resonances and the bottom tungsten layer can induce a magnetic dipolar resonance. Therefore, the wavelengths of both emittance peaks are sensitive to the period paralleled to the incident electric field. Besides, the anapole-induced mode can couple with the lattice resonance, resulting in higher emittance. Moreover, the proposed emitter is successfully fabricated, and the measured spectra agree well with the theoretical results. The fundamental understanding and insights obtained here will facilitate the active design and application of novel multipole-based emitters in enhancing energy conversion.

Structural anisotropy exists prevalently in complex materials. Quantitatively extracting structural information is a long-standing challenge in optical imaging. In this paper, we propose an optimized Discrete Dipole Approximation-Monte Carlo (DDA-MC) algorithm to investigate the polarized light transportation in fibrous materials, which is validated by experiments on polystyrene fibers. Based on this model, the impact of structural anisotropy of fibrous scatterers on polarized light transportation has been analyzed quantitatively. In addition, simulation results show that structural anisotropy like the orientation angle of cylinders can be deduced from polarization signatures of backscattering light distribution with different polarized incidences. Furthermore, as a label-free and non-invasive tool, the Mueller matrix polarimetry has recently demonstrated promising potential in biomedical diagnosis. By combining with the Mueller Matrix Transformation (MMT), we present how the structural characteristics of fibrous materials, including the orientation angle and deviation of fibers, can be obtained by a Mueller matrix parameter. Therefore, this study not only provides an understanding of the role of structural anisotropy in polarized light transport but also showcases new insight into the nondestructive polarization imaging to characterize fibrous materials.

As one of the most potential applications of near-field thermal radiation, near-field scanning thermal microscope has its own advantage compared with commercial near-field scanning optical microscope, which takes the thermally excited near-field radiation as light source and does not need external illumination. However, the signal of near-field scanning thermal microscope is very weak. Ones have to heat the tip or sample to improve the signal, or apply a sensitive detector. In this work, we describe a scattering-type multi-probe scanning thermal microscope (s-m-SThM) based on near-field thermal radiation which is configured with multi-probe to improve the signal, and present a theory to determine its far-field signal. The theoretical model is based on dipole approximation and consider the multibody mutual interactions between the probes and the sample. The results demonstrate that multibody mutual interactions between the probes and the sample can effectively improve the far-filed signal of s-m-SThM. For a three-probe (Au) configuration, the results show that the far field signal has been increased more than 200 times compared with a single probe (Au) configuration. This work will help to improve the applicability of scanning thermal microscope based on near-field thermal radiation.

Anisotropic photonic structures play a vital role in tailoring micro- and nanoscale light-matter interactions. In this work, we investigate the interference and localization effects in two-dimensional disordered media containing dipolar scatterers with anisotropic positional correlations. We study the anisotropy-induced frequency shifts in the transmission spectra, which cannot be accounted for by the independent scattering approximation or effective medium theory. We reveal that the distribution of eigenmodes strongly depends on the anisotropic structural correlations, and in all cases, Anderson localized modes are observed although their spatial extent exhibits different degrees of anisotropy. By calculating the level statistics, we demonstrate that the introduction of anisotropy to structural correlations has nontrivial effects on the level spacing statistics, which can be somewhat captured by a critical distribution function that works in the metal–insulator transition regime. This work can provide physical insights into the wave aspects of light transport in disordered media due to anisotropic structural correlations and guide the design of novel nanophotonic devices based on resonant scatterers.

Radiative sky cooling has attracted worldwide attention due to its passive cooling potential with zero energy input. Daytime cooling materials with outstanding spectral properties are developing rapidly. Using these new cooling materials as roof coatings can become one of the most prominent radiative cooling methods integrated with buildings. Current existing models and numerical tools are not able to incorporate the spectral selectivity of radiative cooling materials and local precipitable water vapor, thus failing to accurately predict roof thermal performance. To this end, the paper newly develops a spectral selectivity-based passive radiative roof cooling model to integrate full spectral properties of emerging cooling materials and consider precipitable water vapor-induced atmospheric radiation for precise evaluation of roof cooling performance. A field experiment was carried out to validate the developed model. The mean bias errors (MBEs) of the model in the prediction of rooftop temperatures were found less than 4.6%, verifying the sufficient accuracy of the new model in characterizing roof temperature variations. After validation, the developed model was used to investigate the potential benefits of radiative cooling as a rooftop strategy in the hot and humid region of Hong Kong. The whole year on-site weather data were collected as the model input. Compared to the baseline coating, the reductions in monthly cumulative roof thermal transfer values by applying a porous polymer cooling coating are 18.5–22.3, 33.7–40.5 and 2.9–3.5 kWh/m2 for residential concrete-based, industrial galvanized steel-based, and standard-compliant commercial multilayered roofs, respectively. For the above three roofs, the radiative cooling coating is able to save the annual roof-induced cooling electricity of 54.7–76.6, 97.4–136.4, and 8.8–12.2 kWh/m2, corresponding to the cooling cost saving of 71.8–100.5, 127.8–179.0, 11.5–16.0 HK$/(m2·yr), respectively. The new spectral selectivity-based radiative roof cooling model can be widely employed to accurately evaluate the utilization of daytime radiative coolers as a rooftop strategy in improving building thermal and energy performance.

Light–matter interaction in cold atomic ensembles is one of the central topics in modern quantum and atomic optics with important applications in various quantum technologies. The collective response of dense atomic gases under light excitation depends crucially on the spatial distribution of atoms and the geometry of the ensemble. We analyze near-resonant light transmission in two-dimensional dense ultracold atomic ensembles with short-range positional correlations. Based on coupled-dipole simulations, we show that the collective effects, manifested as notable shifts of transmission resonance frequency and considerable modification of optical depths, are influenced strongly by positional correlations. Mean-field theories such as the Lorentz–Lorenz relation are not capable of describing such collective effects. We also investigate the statistical distribution of eigenstates and provide a connection between the transmission spectra and eigenstate distribution by utilizing the population weighted detuning and decay rate of all eigenstates. We further demonstrate the intricate interplay between dipole–dipole interactions and positional correlations by increasing the number densities of atoms.

Nanofluids show significant enhancement in the absorption of solar energy. However, little work has been done on the effect of nanoparticle aggregation on the thermal radiation properties of nanofluids up to now. In this work, we systematically investigate the thermal radiation properties of nanofluids with nanoparticle aggregation from experiment and theory. On the experimental side, titanium dioxide/silver (TiO2@Ag) plasmonic nanofluids are prepared in distilled water and the spectral transmittance of nanofluids with different degrees of aggregation is measured. The result shows that the size of aggregated nanoparticle cluster has a significant effect on the absorption of nanofluids. On the theoretical side, a theoretical model is developed to calculate the thermal radiation properties of nanofluids with nanoparticle aggregation. By applying the multiple sphere T-matrix (MSTM) method and the Monte Carlo (MC) method, we investigate the effects of nanoparticle aggregation, cluster fractal dimensions and cluster sizes on the thermal radiation properties of nanofluids. The results indicate that our theoretical model can predict the thermal radiation properties of nanofluids with nanoparticle aggregation more reasonably. This is of great significance to understand the mechanism that affects the thermal radiation properties of nanofluids and to actively design nanofluids.

In this study, the propagation of polarized light in anisotropic media composed of prolate ellipsoids oriented in the same direction is investigated. We proposed a DDA-Monte Carlo combined algorithm to solve the radiative transfer problem. By comparisons of backscattering two-dimensional images of Mueller matrix, the results show that, the changing information of aspect ratio and orientation of ellipsoidal scatterers can be well encoded in image-based Mueller matrix. Moreover, the effect of aspect ratio and orientation on diattenuation, retardance, depolarization and linear retardance are demonstrated by decomposing the Mueller matrix. Furthermore, the impact of aspect ratio on the rate of depolarization of linearly and circularly polarized light with optical thickness has been construed. This study has proved that structural anisotropy decelerates the depolarization rate of scattering light and traditional diffusion theory cannot be used to solve polarized light transport in anisotropic media. Our study can promote a further research on the physics of radiative transfer in anisotropic media.

We investigate the topological plasmon polaritons (TPPs) in one-dimensional dimerized doped silicon nanoparticle chains, as an analogy of the topological edge states in the Su–Schrieffer–Heeger model. The photonic band structures are analytically calculated by taking all near-field and far-field dipole–dipole interactions into account. For longitudinal modes, it is demonstrated that the band topology can be well characterized by the complex Zak phase irrespective of the lattice constant and doping concentration. By numerically solving the eigenmodes of a finite system, it is found that a dimerized chain with a nonzero complex Zak phase supports nontrivial topological eigenmodes localized over both edges. By changing the doping concentration of Si, it is possible to tune the resonance frequency of the TPPs from far-infrared to near-infrared, and the localization lengths of the edge modes are also modulated accordingly. In addition, it is shown that a higher working temperature can lead to significantly smaller decay rates. Since these TPPs are highly protected modes that can achieve a strong confinement of electromagnetic waves and are also immune to impurities and disorder, they can provide a potentially tunable tool for robust and enhanced light–matter interactions in the infrared spectrum.

Engineering thermal emission using micro/nanostructures has received growing attention in recent years. A variety of novel coherent, directional, polarized and narrowband thermal light sources as well as wavelength selective emitters have been proposed and investigated. These emitters show a great potential in applications like thermophotovoltaics, solar energy concentration, radiative cooling, thermal detection, sensing, thermal camouflage, thermal rectification, etc. In this article, we give a review on the use of micro/nanostructures to control far-field thermal emission. Thermal emitters based on various micro/nanostructures including gratings, photonic crystals, metamaterials and metasurfaces are reviewed and some important applications are summarized. We expect this article can provide a general overview of this field.

In the quest to enhance light-matter interactions and miniaturize photonics devices, it is crucial to create a strong field enhancement with lower material loss. Here, we combine a plasmonic Fano resonance supported by the silver cluster and anapole states realized by the silicon disk to create a larger field enhancement with less loss through the strong coupling effect. Besides, by varying the gap size, we find that the resonances and the Rabi-splitting energy can be tuned over a wide range of wavelengths, which can achieve a giant splitting over 300 meV. We further demonstrate that it is the interference of magnetic field vortices which induces the strong coupling. Due to the strong coupling, the hybrid nanoantenna can provide both a larger decay rate and higher quantum efficiency, which makes it promising for high-performance miniaturized optical devices.

Although achieving efficient solar-thermal conversion with nanofluids (NFs) plays an important role in developing photothermal technologies, full-spectrum energy usage is still challenging currently. In this work, dimensionless methods of designing NFs are proposed in complex permittivity (ε) space, to make the most of solar energy. Firstly, an effective regime Ω′ for each size parameter x is determined, in which an ideal photothermal efficiency can be guaranteed if only the NPs’ permittivity falls into this area. Besides, on the other hand, by introducing a dimensionless parameter Qi, our methods also allow to actively chose properties of NFs with a desirable solar-to-thermal efficiency, including volume concentration of NPs (fv) and thickness of NFs (L). Based on the proposed principles, results show that SiO2@Ni and SiO2@TiN NFs perform better with efficiency 15–30% improved than other widely used materials, such as SiO2@Ag(Au, Al) counterparts. Further, genetic algorithms are employed to verify the above results. Consistently, it suggests that near-perfect full-spectrum photothermal efficiency of SiO2@Ni or SiO2@TiN NFs can be obtained, reaching to 99% under certain conditions. In addition, regarding these two superior NFs, appreciable performance can also be protected with considerably small values of fv and L, which helps to reduce costs and is attractive in practice. The proposed principles offer a powerful tool to actively design optimized NP-related NFs for achieving perfect photothermal conversion, which will also potentially stimulate the development of other advanced photothermal technologies.

Topological phonon polaritons (TPhPs) are highly localized edge modes that can achieve a strong confinement of electromagnetic waves and are topologically protected to be immune to impurities and disorder. In this paper, we theoretically study the topological phonon polaritons (TPhPs) in one-dimensional (1D) dimerized silicon carbide (SiC) nanoparticle (NP) chains, as an extension of the celebrated Su-Schrieffer-Heeger (SSH) model. We analytically calculate the band structure and complex Zak phase for such chains by taking all near-field and far-field interactions into account. It is found that the 1D dimerized chain supports nontrivial topological states as long as the dimeriza-tion parameter β \> 0.5 and the long-range interactions are weak, although the system is non-Hermitian. By analyzing the distribution of eigenmodes and their participation ratios (PRs), we comprehensively study the effects of disorder on the band structure and midgap modes. We reveal that such TPhPs are very robust under high-degree disorders and even enhanced by the disorder. Through a finite-size scaling analysis, we show this enhancement can be attributed to Anderson localization scheme. These topological phonon polaritonic states provide an efficient interface for thermal radiation control in the mid-infrared.

The sub-ambient clear sky daytime radiative cooling has been achieved recently by applying advanced nanofabrication technologies, which attracted significant research attention in this area. However, there are also several researches which reported that radiative coolers with excellent thermal radiative spectrum failed to achieve sub-ambient radiative cooling. To elucidate the difference, in this work, we perform a detailed investigation of ambient conditions, in particular, water vapor density, on the performance of radiative coolers. We first present the details of how to estimate the performance of radiative coolers under different water vapor concentration and simulate the transmittance and emitted radiation of atmosphere as well as the cooling performance of the ideal selective emitter, which finds the cooling power decreases by 86.6 W/m2 with increasing total water vapor column. Furthermore, the cooling performance of an integrated photonic solar reflector and thermal emitter with atmospheric profiles in California and Hong Kong were calculated and compared with experiment result. To verify our calculation, nighttime on-site cooling performance of a reference film (Al foil) and two selective emitters (DESR-M and double-layer coating) was measured in several ambient conditions. Our research reveals how the ambient humidity affects the radiative cooling performance by both theoretical and experimental analysis and is important for the further application development of radiative coolers.

To further study the heat transfer process inside the functionally graded thermal barrier coatings (FGTBCs) beyond the companion part of the present paper concerning their radiative properties, this paper takes both conductive and radiative heat transfer into account and establishes a comprehensive heat transfer model considering both conductive and radiative heat transfer to estimate the thermal insulation performance of FGTBCs. Thermal conductivities of more than thirty yttria-stabilized zirconia (YSZ)/NiCoCrAlY FGTBC samples were determined based on their thermal diffusivities determined by the laser flash analysis. To predict the overall thermal insulation performance of 8YSZ/NiCoCrAlY FGTBCs, a comprehensive heat transfer model based on the two-flux method and multi-band model was built to solve the stationary temperature distribution and heat flux inside thermal barrier coatings (TBCs) and FGTBCs. Results indicate that FGTBCs show worse thermal insulation performance compared to conventional YSZ TBCs; we also investigated the influence of the number of layers. The temperature of engine blade is increased by 30–50K if coated with an FGTBC instead of a conventional TBC. Conduction heat flux is significantly greater than radiative flux for both TBCs and FGTBCs. This paper gives a detailed analysis by simultaneously considering the heat conduction and thermal radiation in FGTBCs, which is helpful for achieving better thermal design of YSZ/NiCoCrAlY FGTBCs. Along with the Part I, this two-part article presents a deep and thorough study on the thermal insulation performance of YSZ/NiCoCrAlY FGTBCs.

Functionally graded thermal barrier coatings (FGTBCs) are functional composites with multilayered structures whose mechanical properties are better than those of conventional ceramic thermal barrier coatings (TBCs). Although many theoretical and experimental studies concerning both the mechanical properties and heat transfer including heat conduction and thermal radiation of TBCs have been conducted, little attention was paid to heat transfer process, especially the coupled conduction-radiation heat transfer, inside FGTBCs. This two-part paper systematically studied the heat transfer inside YSZ/NiCoCrAlY FGTBCs. In Part I, we investigated their radiative properties through experimental measurements. In Part II, we presented a study on the thermal conductivity of FGTBCs and established a coupled heat transfer model. In this paper, more than thirty yttria-stabilized zirconia (YSZ)/NiCoCrAlY duplex TBC and multilayered FGTBC samples with different multilayer structures, porosities and thicknesses were fabricated via air plasma spraying. The transmittance and reflectance spectra within the 0.3–15μm wavelength range were experimentally obtained by UV–visible spectrometry and Fourier transform infrared spectroscopy. A scanning electron microscope analysis was carried out to characterize the microstructures of FGTBCs. Additionally, we studied each layer of the FGTBCs individually; the four-flux method was employed to obtain the radiative properties based on our experimental measurements. Functionally graded materials (FGM) show larger absorption coefficients and smaller scattering coefficients compared to pure YSZ. The FGTBCs show lower reflectance than YSZ TBCs. We also study the influence of the number of layers on reflectance. The results indicate that FGTBCs have worse thermal insulation ability against radiative flux tconventional ceramic TBCs of the same thickness. This paper provides practical guidance to improve the design of YSZ/NiCoCrAlY FGTBCs and other FGM coatings to reach a balance between good thermal insulation and mechanical strength.

With the rise of artificial magnetism and metamaterials, the toroidal family recently attracted more attention for its unique properties. Here, we propose an all-dielectric pentamer metamolecule consisting of nano-cylinders with two toroidal dipolar resonances, whose frequencies, EM distributions, and Q factor can be efficiently tuned due to the additional electric dipole mode offered by a central cylinder. To further reveal the underlying coupling effects and the formation mechanism of toroidal responses, the multiple scattering theory is adopted. It is found that the first toroidal dipole mode, which can be tuned from 2.21 to 3.55μm, is mainly induced by a collective electric dipolar resonance, while the second one, which can be tuned from 1.53 to 1.84μm, relies on the cross coupling of both electric and magnetic dipolar responses. The proposed low-loss metamolecule and mode coupling analyses may pave the way for the efficient design of toroidal responses in advanced optical devices.

An efficient route is proposed to actively manipulate directional scattering based on magneto-optical (MO) effects and multipolar interferences with a coated MO cylinder. We demonstrate that the presence of an external magnetic field B contributes to breaking the degeneracy of multipoles and making them rotate. The rotated modes further interfere with each other, thus reshaping scattering profiles and introducing novel directional optical switching (OS) phenomena. As a result, we show that forward scattering (FS) and backward scattering (BS) can be flexibly altered by turning on or off the B for both s-polarization and p-polarization, and optical switching exhibits broadband performance as well. More specifically, the proposed nanostructure also operates robust and reliable functionalities when considering realistic MO materials with optical loss and dispersion; therefore, they are ideal candidates to develop reconfigurable, tunable and versatile optical devices.

Thermal radiative transfer can be enhanced significantly when the distance of radiation sources is smaller than the characteristic wavelength of thermal radiation. However, the problem of the near-field thermal radiative transfer (NFTRT) between core-shell nanoparticles has rarely been studied. Moreover, most previous studies investigate the NFTRT between two or three nanoparticles, while few works have been done on real many-body systems where multibody mutual interaction plays a pivotal role. In this study, we choose silicon carbide (SiC) and n-type germanium (Ge) as two constituent materials of the core-shell nanoparticle, and describe a complete theoretical investigation of the NFTRT in many body systems composed of core-shell nanoparticles. The results demonstrate that the core-shell nanoparticles inherit both characteristics of SiC and n-type Ge: quasimonochromaticity and broad-band. What’s more, the spectral features can be tuned by core-shell radius and material composition. Further studies show that multibody mutual interaction has a detrimental effect on the NFTRT between two assembled spherical systems. Still, the enhancement of NFTRTs between a small proportion of nanoparticle dimers inside the systems is observed, which can be attributed to the coherence enhancement of fluctuating thermal fields. And by analyzing the trace of a dyadic Greens functions product, the locations of inherent resonance of nanoparticles and the coherent enhancement due to multibody mutual interaction need to be consistent if we hope strong exaltation effects of NFTRT.

The structural correlations in the microscopic structures of random media can induce the dependent scattering mechanism and thus influence the optical scattering properties. Based on our recent theory on the dependent scattering mechanism in random media composed of discrete dipolar scatterers [B. X. Wang and C. Y. Zhao, Phys. Rev. A 97, 023836 (2018)], in this paper, we study the hard-sphere Yukawa random media, in order to further elucidate the role of structural correlations in the dependent scattering mechanism and hence optical scattering properties. Here, we consider charged colloidal suspensions, whose effective pair interaction between colloids is described by a screened Coulomb (Yukawa) potential. By means of adding salt ions, the pair interaction between the charged particles can be flexibly tailored and therefore the structural correlations are modified. It is shown that this strategy can affect the optical properties significantly. For colloidal TiO2 suspensions, the modification of electric and magnetic dipole excitations induced by the structural correlations can substantially influence the optical scattering properties, in addition to the far-field interference effect described by the structure factor. However, this modification is only slightly altered by different salt concentrations and is mainly because of the packing-density-dependent screening effect. On the other hand, for low refractive index colloidal polystyrene suspensions, the dependent scattering mechanism mainly involves the far-field interference effect, and the effective exciting field amplitude for the electric dipole almost remains unchanged under different structural correlations. The present study has profound implications for understanding the role of structural correlations in the dependent scattering mechanism.

In the last a few decades, the approximate nature of radiative transfer equation (RTE) leads to a bunch of considerations on the effect of dependent scattering in random media, especially in particulate media composed of discrete scatterers. This effect usually indicates those deviations of RTE from experimental and exact numerical results due to electromagnetic wave interference. Here we theoretically and numerically demonstrate the effect of dependent scattering on absorption in disordered media consisting of highly scattering scatterers. By making comparisons between the independent scattering approximation-radiative transfer equation (ISA-RTE) approach and the full-wave coupled dipole method (CDM), we find that deviations between the two approaches increase as the scatterer density increases. The discrepancy also grows with the optical thickness of the whole random media. To quantitatively take dependent scattering effect into account, we develop a theoretical model of the dependent-scattering corrected radiative properties, based on the path-integral diagrammatic technique and the quasi-crystalline approximation (QCA) in the multiple scattering theory. The model results in a more reasonable agreement with numerical simulations. The present work is of practical importance in correctly modeling light absorptance in random media and interpreting the experimental observations in various applications for random media, such as solar energy concentration, micro/nanofluids, structural coloration, etc. It also has profound implications for the coherent scattering physics in random media with absorption.

Negative refraction (NR) metamaterials are featured with unique physical properties and potential to realize full control of electromagnetic waves, which have attracted much attention since the last decade. However, few researches focus on the realization of three-dimensional dielectric NR metamaterials in optic frequency, and the current design methods need further development. In this paper, a three-dimensional all-dielectric NR metamaterial with two NR bands has been realized based on proper excitation of electric and magnetic multipoles. It is also predicted that the coupling of magnetic dipole and electric dipole can lead to the NR bands in near-infrared frequencies, and NR in the visible frequencies can be achieved by the coupling of magnetic quadrupole and electric dipole. Band structures and equal-frequency surfaces of proposed metamaterial arranged in the periodic cubic lattice are solved by adopting the plane wave expansion method, and then the results verify the existence of these two NR frequency bands in periodic metamaterials. In this way, the characteristic parameters such as transmission and absorption of light in two NR bands are also analyzed. In the meantime, the finite-deference time-domain method is used to intuitively display the phenomenon of NR and investigate the effects of disorder in particle arrangement. Besides, it is found that the proposed metamaterials have fine robustness to the disorder in particle arrangement, and these two NR bands can be tuned by adjusting volume fraction. In brief, this work provides means for preliminary designing, profound analysis and intuitively exhibition of NR metamaterials based on dielectric particles.

We have achieved efficient polarization manipulations based on dipolar interferences and lattice couplings in one-dimensional cylindrical metalattices. First, we employ the scattering asymmetry factor g to quantify the directional scattering ability and find the maximum |g|max=1/2 for a cylinder with effective excitations of electric and magnetic dipoles simultaneously. Further, the strong negative–g (gp = −0.38) for p–polarization and positive–g (gs = 0.68) for s–polarization are obtained within a narrow visible band using c-Si with experimental data. Inspired by the polarization-dependent phenomena, we design a metalattice-based linear polarizer considering lattice effects with an optimal particle arrangement. The metalattice performs near-perfect reflection for p–polarized waves but with zero reflection for s–polarized waves with large extinction ratios for transmission (17 dB) and reflection (24 dB). The perfect functionalities can be attributed to the near-field lattice couplings with dipolar interferences. And, we reveal that the polarization-dependent scattering coefficients, which are sensitive to the lattice period, can be largely tuned owing to lattice effects, therefore contributing to modifying far-field scattering patterns. More specifically, the proposed linear polarizers also show robust and reliable functionalities when considering lattice imperfections, the effects of system sizes, oblique incident angles, and the tunbility for different working wavelengths. The present study paves a way to stimulate many advanced practical implements based on multipolar interferences and lattice couplings.

Thermal barrier coatings are common porous materials coated on the surface of devices operating under high temperatures and designed for heat insulation. This study presents a comprehensive investigation on the microstructural effect on radiative scattering coefficient and asymmetry factor of anisotropic thermal barrier coatings. Based on the quartet structure generation set algorithm, the finite-difference-time-domain method is applied to calculate angular scattering intensity distribution of complicated random microstructure, which takes wave nature into account. Combining Monte Carlo method with Particle Swarm Optimization, asymmetry factor, scattering coefficient and absorption coefficient are retrieved simultaneously. The retrieved radiative properties are identified with the angular scattering intensity distribution under different pore shapes, which takes dependent scattering and anisotropic pore shape into account implicitly. It has been found that microstructure significantly affects the radiative properties in thermal barrier coatings. Compared with spherical shape, irregular anisotropic pore shape reduces the forward scattering peak. The method used in this paper can also be applied to other porous media, which designs a frame work for further quantitative study on porous media.

A one-pot emulsion process produces noniridescent supraball inks made of core-shell melanin and silica nanoparticles. Structural colors enable the creation of a spectrum of nonfading colors without pigments, potentially replacing toxic metal oxides and conjugated organic pigments. However, significant challenges remain to achieve the contrast needed for a complete gamut of colors and a scalable process for industrial application. We demonstrate a feasible solution for producing structural colors inspired by bird feathers. We have designed core-shell nanoparticles using high–refractive index (RI) ( 1.74) melanin cores and low-RI ( 1.45) silica shells. The design of these nanoparticles was guided by finite-difference time-domain simulations. These nanoparticles were self-assembled using a one-pot reverse emulsion process, which resulted in bright and noniridescent supraballs. With the combination of only two ingredients, synthetic melanin and silica, we can generate a full spectrum of colors. These supraballs could be directly added to paints, plastics, and coatings and also used as ultraviolet-resistant inks or cosmetics.

Engineering light-matter interaction using cold atomic arrays is one of the central topics in modern optics. Here we have demonstrated the capability of two-dimensional asymmetric cold atomic arrays as microscopic metasurfaces for controlling polarization states of light. The designed linear polarizer can lead to an extinction ratio over 20dB as well as a high transmittance over 0.8 for the permitted polarization at zero detuning. For detuned driving light, changing lattice constants can also achieve high performance linear polarizers. We have also accomplished a circular polarizer by manipulating the phases of transmitted light. A theoretical analysis based on Bloch theorem shows the underlying mechanism for this performance is actually attributed to cooperative effects in periodic lattices. Finally, we discuss in detail the effects of system size, lattice imperfection and nonzero driving light linewidth in practical implementation. The present study paves a way to design extremely miniaturized metasurfaces using cold atoms and other two-level systems, showing great potential in quantum information and quantum metrology sciences as well as the fundamental physics of light-matter interaction.

One passive cooling approach is pumping energy to outer space through thermal radiation. Such a radiative cooling mechanism widely exists in nature and is important to maintain the temperature of the earth. However, natural materials generally have poor radiative cooling efficiency. To better utilize the radiative cooling for thermal management applications, the surface should be designed to have a high reflectivity in the solar spectrum and high emissivity in the "sky window" region (8–13µm in wavelength). In this work, we propose and demonstrate a highly scalable nanoparticle-based double-layer coating to achieve such selective radiative properties. Double-layer coatings consisting of a top reflective layer with high solar albedo and a bottom emissive layer are achieved by properly designed TiO2, SiO2, and SiC nanoparticles. These coatings were fabricated on both low- and high-emissivity substrates and their spectral radiative properties were characterized. The coating composed of TiO2 and SiO2 on a reflective substrate has excellent selective emission property for radiative cooling purpose. Under dry air conditions and assuming non-radiative heat transfer coefficient hc=4W/m2K, TiO2+SiO2 and TiO2+SiC can theoretically achieve about 17°C below ambient at night and 5°C below ambient under direct solar radiation (AM1.5). On-site measurements have also been conducted. Under direct solar irradiation, significant temperature reduction was observed for both aluminum and black substrate after the coating was applied. At nighttime, radiative cooling effect can cool the surface to a few degrees below ambient temperature. Although the theoretical cooling under dry weather condition is not observed, the experiment results can be well explained by theoretical calculations with the consideration of high humidity and non-radiative heat transfer. This nanoparticle-based approach can be easily applied to large area, which is a significant step of achieving large scale application of the radiative cooling technology.

The aim of this study is to present a general method to investigate radiative transfer in disordered media with a subwave-length, anisotropic short-range order and provide a fundamental understanding on the interplay between polarized radiative transfer and microstructural anisotropy as well short-range order. We show the anisotropy of short-range order, described by an anisotropic correlation length in Gaussian random permittivity model, induces a significant anisotropy of radiative properties. Here the photon scattering mean free path is derived using the Feynman diagrammatic expansion of self-energy, and the transport mean free path and phase function are calculated based on the diagrammatic representation of the irreducible vertex in the Bethe-Salpeter equation. We further consider the transport of polarized light in such media by directly solving Bethe-Salpeter equation (BSE) for photons, without the use of traditional vector radiative transfer equation (VRTE). The present method advantageously allows us to elegantly relate anisotropic structural parameters to polarized radiative transport properties and obtain more fundamental physical insights, because the approximations in all steps of our derivation are given explicitly with reasonable explanations from the exact ab-initio BSE. Moreover, through a polarization eigen-channel expansion technique for intensity tensor, we show that values of transport mean free path in different polarization eigen-channels are rather different, which are also strongly affected by structural anisotropy and short-range order. As a conclusion, this study depicts some fundamental physical features of polarized radiative transfer in disordered media, and is also valuable for potential applications of utilizing anisotropic short-range order in disordered media in manipulation of polarized radiative transfer.

In this Letter, we propose a method to design ultrabroadband near-perfect absorbers, consisting of a periodic dielectric–metal multilayer. In the method, the Bloch theorem and optical topological transition (OTT) of iso-frequency surfaces are employed to manipulate the start and end of the near-perfect spectral absorption band, respectively. Moreover, we design and fabricate an ultrabroadband near-perfect absorber utilizing the proposed method. The average absorption of the designed absorber is ∼95% in the focused visible and near-infrared range (0.4–2 μm). This omnidirectional and polarization-independent near-perfect absorber is promising for solar energy harvesting, emissivity control, and thermal imaging.

Most cermet-based coatings achieve their solar selectivities by the tandem interference effect, which has been widely studied. This study focused on the spectral selectivity achieved by the scattering effect of metal particles in cermet-based coatings. Previous research proved that reasonable solar selectivities can be obtained for cermets in the regime of particles with a radius of the order of 100 nm, but their solar absorptance is low (\<90\%). In our research, the effect of metal particles on the spectral selectivity of cermets was studied by the Mie theory and Monte Carlo simulation for Cr, Ni, and W particles with radii of 10 nm, 50 nm, 100 nm, and 200 nm, which were embedded in Al2O3 and occupied 5\% of the volume fraction. It was found that by arranging different particles in different layers, a very high solar absorptance (95.6\%) could be achieved. Since their thermal emittance (∼25\% at 600 °C) was higher than that of normal coatings, these coatings are recommended to be used in solar absorbers that have a high concentration factor. Finally, the dependent scattering effect was qualitatively considered by the coupled-dipole approach. With a metal volume fraction of 5\%, it was found that the effect of dependent scattering was small and should not change the conclusions made based on independent scattering.

Aiming at promoting the fundamental understanding on relationship between the radiative transfer mechanism and microstructure of thermal barrier coatings (TBCs), here in this study, we numerically demonstrate the anisotropy of radiative properties of air plasma sprayed (APS) TBCs for the first time. The anisotropic microstructures of APS TBCs are quantitatively reconstructed based on Ultra-Small-Angle X-Ray Scattering (USAXS) measurement by the Stony Brook University group (Li et al., J. Amer. Ceram. Soc., 92(2) p. 491–500, 2009), in which the microscale pores and cracks are treated as oblate spheroids with a preferred distribution of orientations. The anisotropic mesoscopic radiative properties, including scattering coefficient/mean free path and asymmetry factor, are computed using the discrete dipole approximation (DDA) to solve Maxwell’s equations. To fully depict the effect of anisotropy on radiative transfer in a macroscopic scale, a random walk scheme is thus proposed to solve the anisotropic radiative transfer problem in such medium, and the macroscopic transport mean free path describing energy diffusion process is derived. Results are further compared with those under isotropic assumption. By considering external blackbody as thermal radiation sources for the coating, we show that anisotropy of radiative properties affects the transmitted radiative heat flux across it considerably, especially at high operating temperatures and for thick coatings. On the other hand, in moderate operating conditions, the simplistic approach can also give an acceptable approximation on heat transfer for the presented particular coating microstructure. The present work provides a fundamental framework for studying radiative transport in anisotropic porous media.

Thermal barrier coatings (TBCs) are porous media in which many different pores and cracks are induced by different manufacturing procedures. Many studies have been conducted to investigate the impact of microstructures of TBCs on thermal conductivity; nevertheless, the influence of microstructures on the radiative properties of TBCs has not drawn significant attention. In addition, the working condition of thermal barrier coatings is at high temperatures at which the contribution of radiative heat transfer plays a very crucial role. Therefore, it is necessary to study the radiative properties of TBCs to characterize their insulation performance. In this work, the microstructures of air-plasma- sprayed (APS) 8wt% yttria stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) are constructed by the quartet structure generation set (QSGS) algorithm. A finite-difference-time-domain (FDTD) method is carried out to simulate radiative heat transfer through TBCs. Three parameters—average pore size, directional growth probability Di (especially horizontal growth probability D13) and porosity—have been investigated to study the microstructural effect on the radiative properties of TBCs. The reflectance of freestanding 50-μm-thick thermal barrier coatings is studied using Lumerical FDTD Solutions in the wavelength range from 1 to 6μm at normal incidence. The absorption and scattering coefficient as a function of wavelength are extracted using the four-flux model. The results will help us to characterize the radiative heat transfer process across the TBCs and provide us with a theoretical guide to design TBCs with a high thermal insulation property.

Thermal barrier coatings provide excellent thermal insulation for metal components of gas turbines. Although the relationships between microstructures and mechanical properties as well as thermal conductivity of various TBCs have been extensively studied, there still exists a deficiency of a full understanding on microstructural-related thermal radiation transport inside them, which becomes more and more crucial for advanced gas turbine applications requiring higher operating temperatures. This study aims at presenting a microstructure-based numerical investigation on radiative transfer in the air plasma sprayed (APS) TBC.In this study, the microstructures of APS TBCs are quantitatively reconstructed based on the Ultra-Small-Angle X-Ray Scattering (USAXS) measurement by the Stony Brook University group, in which the microscale interlamellar pores, intrasplat cracks and globular voids are regarded as oblate spheroids with different sizes and aspect ratios, with a specific distribution of orientations. This is a typical anisotropic medium, in which the physical properties vary with the observing direction. The anisotropic feature of radiative properties including the scattering coefficient and phase function is for the first time demonstrated using the discrete dipole approximation (DDA) method. A modified Monte Carlo method is proposed and implemented to solve the anisotropic radiative transfer problem in such medium. The spectral normal-hemispherical reflectance and transmittance of the coating are thus obtained and further compared with the experimental data from literature as well as our group to validate this numerical method. This work provides a versatile numerical framework for the study of the anisotropic radiative transfer mechanism in APS thermal barrier coatings based on microstructure charaterization.

The typical yttria-stabilized zirconia material for making the thermal barrier coatings (TBCs) is intrinsically semitransparent to thermal radiation, and the unique disordered microstructures in TBCs make them surprisingly highly scattering. To quantitatively understand the influence of disordered microstructures, this paper presents a quantitative prediction on the radiative properties, especially the transport scattering coefficient of plasma sprayed TBC based on microstructure analysis and rigorous electromagnetic theory. The impact of the porosity, shape, size, and orientation of different types of voids on transport scattering coefficient is comprehensively investigated under the discrete dipole approximation. An inverse model integrating these factors together is then proposed to quantitatively connect transport scattering coefficient with microstructural information, which is also validated by available experimental data. Afterwards, an optimization procedure is carried out based on this model to obtain the optimal size and orientation distribution of the microscale voids to achieve the maximal radiation insulation performance at different operating temperatures, providing guidelines for practical coating design and fabrication. This work suggests that the current model is effective and also efficient for connecting scattering properties to microstructures and can be implemented as a quantitative tool for further studies like non-destructive infrared imaging as well as micro/nanoscale thermal design of TBCs.

A theoretical model on radiative properties of air plasma-sprayed (APS) 8wt% yttria stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) is proposed and validated. Starting from analysis of microstructures, pores inside the coating are regarded as scatterers. Dependent scattering effects among scatterers are thoroughly considered in the framework of quasicrystalline approximation (QCA) and Percus–Yevick (P–Y) pair distribution function for sticky hard spherical particles, in which large nonspherical pores are treated as clusters of the elemental pores. Afterwards, the Monte Carlo (MC) method is used to solve the radiative transfer problem inside TBCs based on the radiative properties. The predicted optical responses including reflectance, transmittance and absorptance of TBC slabs with different thicknesses agree well with the experimental data. The widely used independent scattering assumption in conventional studies is also investigated and examined to be inapplicable in the semitransparent spectral region of TBCs, especially between around 3.2 and 5.6μm. This study provides a simple and physically robust direct prediction on radiative properties of TBCs solely based on their microstructures and bulk optical properties of the material. Moreover, the present method can be easily applied to the studies on radiative properties of other semitransparent porous media.

Thermal barrier coatings (TBCs) are not the pure bulk materials, and they contain many different micro/nano structures caused by the plasma spraying process. The microstructures in the coatings play a very important role on mechanical and thermal properties, therefore, the coating properties differ hugely from those of the corresponding bulk materials. The influence of microstructures on thermal conductivity has been investigated by many researchers, however their effect on radiative heat transfer is seldom studied. The radiative heat transfer will become increasingly important as the temperature rises, and this warrants the study on the influence of microstructures on radiative properties. In the present paper, Finite-Difference-Time-Domain (FDTD) method was employed to simulate the radiative heat transfer behaviors of TBCs with different types of microstructures. The relationship between microstructures and radiative properties of the coating is investigated. Furthermore, the parameters of microstructures such as defect size, shape coefficient, porosity and orientation angle are examined in detail. The results will help us to further understand the radiative heat transfer process across the TBCs and will provide us a theoretical guide to design TBCs with a high thermal insulation property.

In this paper the radiative properties of air plasma sprayed (APS) thermal barrier coatings are investigated by measuring the spectral normal-hemispherical reflectance and transmittance of samples with various thickness and porosities in a broad spectral band ranging from 250nm to 15μm. The spectral absorption and scattering coefficients are obtained by a four flux model based on the experimental data. The results show that the porous zirconia ceramics exhibits high scattering coefficient (>15,000m−1) and low absorption coefficient (<500m−1) in the spectral range from 0.5 to 5.5μm. Moreover, the scattering coefficient increases with increasing porosity, while the spectral absorption coefficient is not quite sensitive to the change of porosity. In order to examine the effects of microstructure of porous zirconia coatings, a theoretical model of Mie scattering is established, and a fairly good agreement with the experimental data is achieved. The results indicate that the scattering coefficient heavily depends on the microstructures of coatings, i.e., pore size, porosity and pore distribution, and this will in turn considerably affect the thermal radiation heat transfer inside the coating.