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As a natural van der Waals crystal, α-MoO3 has excellent in-plane hyperbolic properties and essential nanophotonics applications. However, its tunable properties are generally neglected. Here, we achieve effective modulation of spontaneous emission (SE) from a single-layer flat plate by changing the crystal directions. Numerical results and theoretical analysis show that α-MoO3 exhibits good tunability when the crystal directions of α-MoO3 are different in y–z or x–y planes. A modulation factor of more than three orders of magnitude is obtained at 634 cm−1. This phenomenon is caused by the excitation of hyperbolic phonon polaritons in α-MoO3 at specific bands. However, when the crystal directions of α-MoO3 are different in the x–z plane, the SE of the material exhibits strong angle independence. Additionally, for the semi-infinite α-MoO3 flat structure, we determine the distribution of the modulation factor of SE using the wavenumber and rotation angle. Finally, we extend the calculation results from semi-infinite media to finite thickness films. We obtain the general evolution law of the peak angle of the modulation factor with thickness, increasing the modulation factor to approximately 2000, which exceeds the maximum modulation factor observed in previous works by 48 times. We believe this work could guide the SE modulation of anisotropic materials and benefit the field of micro-/nano-lasers and quantum computing. [ABSTRACT FROM AUTHOR]

Plasmonic nanofluids, which have superior optical properties, provide a novel way to achieve efficient solar thermal utilization. Here, the optical properties and photothermal conversion performance of hollow nanopillar structures based on four plasmonic materials (Ag, Au, Cu, TiN) were investigated and compared with the corresponding solid nanopillar structures. Numerical results show that the most significant increase in photothermal conversion efficiency is the plasmonic nanofluid based on hollow Au nanopillars, with an increase of 34.14%. The underlying physical mechanism is that the local and propagating surface plasmon resonances are further promoted in the hollow structure. In contrast, the increase of the photothermal conversion efficiency is only 1.41% for the hollow TiN nanopillars. This is due to the fact that the solid TiN nanopillars already have a high photothermal conversion efficiency of 97.19%. Thus, we can consider the enhancement of the photothermal properties of the plasmonic nanofluid by the hollow structure, especially for precious metal materials with narrow-band absorption, such as Au and Ag. For plasmonic materials with intrinsic broad-spectrum absorption (TiN, Cu), the enhancement effect is not significant. It is believed that this work will provide theoretical guidance for high-performance direct absorption solar collectors. • Optical properties of hollow nanopillars based on Ag, Au, Cu, and TiN are compared. • The most significant increase (Au) in photothermal conversion efficiency is 34.14%. • Local and propagating surface plasmon resonances are promoted in hollow structure. • This work will guide designing high-performance direct absorption solar collectors. [ABSTRACT FROM AUTHOR]

• The effect of slotting on the optical properties of silver nanopillars is studied. • Slotting can enhance the resonance absorption peaks of silver nanopillars. • The best slotting method offers a 13 % improvement compared to nanopillars. • This work shows slotting has a promising prospect in solar thermal utilization. Metal nanoparticles (MNPs) have attracted enormous research interest due to their excellent plasma properties. However, in the thermal utilization of solar energy, the narrowband absorption of MNPs limits their application. The investigation of MNP absorption through adjustment in the broadband spectrum holds significant scientific and practical implications. In this work, we study the effect of slotting on the light absorption and photothermal properties of silver nanopillars. The results show that the increase in the number of slots is beneficial to reduce scattering and improving the resonance absorption of silver nanopillar. Through the calculation of photothermal conversion efficiency, it is found that different slotting methods have different levels of improvement compared with silver nanopillars. The utilization of a combination of transverse and longitudinal slotting techniques leads to a 13 % enhancement in the photothermal conversion efficiency of silver nanopillars. The electric field distribution and temperature distribution at resonant wavelengths shows that the excellent light absorption ability of the slot nanopillars is caused by the "lightning rod" effect at the tip of the slit and the excitation of multiple plasmon resonances. The existence of slots is beneficial to compensate for the photothermal response of silver nanopillars in the near-infrared region. Our study proves that slotting helps enhance the application of silver nanopillars in solar thermal utilization. [ABSTRACT FROM AUTHOR]

• The near-field radiative heat transfer between BP/α-MoO 3 heterostructures is studied. • The anisotropic polariton hybridization generates topological reconfiguration and transition. • The heat flux between multilayer heterostructures is further enhanced. • This work provides guidance for designing near-field thermal radiation devices. The effect of polariton hybridization in different materials have been proved to enhance near-field radiative heat transfer (NFRHT). However, the hybrid effect between anisotropic polaritons has not been studied yet. Here, we investigate the NFRHT between two black phosphorus (BP)/α-MoO 3 heterostructures. Compared with the individual BP and α-MoO 3 configurations, the NFRHT between heterostructures is significantly enhanced and can reach twice that of individual BP sheets. We find that the polariton hybridization in heterostructure exhibits complementary and competitive mechanisms, which leads to significant topological reconfiguration of hybrid polaritons, and modulates the intensity of photon tunneling. With the increase of α-MoO 3 film thickness, we observe that the topological structure of hybrid polaritons will undergo a transition of quasi-elliptic, quasi-hyperbolic and multiple resonance states. Moreover, we analyze the NFRHT between multilayer heterostructures, as the periodic cell number increases, the hybrid polaritons of multilayer heterostructures also exhibit obvious topological transitions, resulting in nearly twice the enhancement of NFRHT. Our work reveals the abundant phenomena of photon tunneling reconfiguration caused by anisotropic polariton hybridization and the accompanying enhancement of NFRHT, which may benefit the applications of near-field thermal management. [ABSTRACT FROM AUTHOR]

Concentrating photovoltaic (CPV) technology is a promising approach for collecting solar energy and converting it into electricity through photovoltaic cells, with high conversion efficiency. Compared to conventional flat panel photovoltaic systems, CPV systems use concentrators solar energy from a larger area into a smaller one, resulting in a higher density of solar radiation and increased electrical output. However, the use of concentrators can lead to nonuniform radiation and high temperatures that may damage the solar cells. Therefore, implementing a suitable thermal management solution is crucial to ensure optimal performance of CPV systems. This review article aims to provide a comprehensive overview of recent research and technical challenges in solar concentrators, trackers, and cooling systems for mitigating temperature effects and enhancing the efficiency of CPV cells. It will explore the causes and potential solutions for temperature effects in CPV systems, particularly focusing on the components involved. [ABSTRACT FROM AUTHOR]

• The NFRHT between α -MoO 3 films with different substrate permittivities is studied. • The substrate effect on the NFRHT highly relies on film thickness and gap distance. • Anisotropic hyperbolic polaritons can explain the underlying physical mechanism. • This study provides guidance for designing near-field thermal radiation devices. Near-field radiative heat transfer (NFRHT) has promising prospects in modern nanotechnology, such as near-field thermal microscopy, nanoscale non-contact thermal management, and information processing. Experimentally, supported substrates are crucial in ensuring structural stability, especially in ultrathin structures. However, the effect of the substrate on the NFRHT has seldom been explored. Here, the NFRHT between α -MoO 3 films with different permittivities of substrate is studied. For lossless substrates, the NFRHT is suppressed as the permittivity of the substrate increases when the heat transfer is along the [010] and [100] crystal directions of α -MoO 3. When the NFRHT is along the [001] crystal direction of α -MoO 3 , high-permittivity substrates suppress the NFRHT when the film is thin (< 10 nm), while enhancing the NFRHT in thicker films (> 10 nm). Moreover, we find that the effect of the substrate on the NFRHT highly relies on film thickness. The effect of lossy substrate on NFRHT is also discussed. We find that the loss of the substrate is more significant for the enhancement of heat flux at a small gap distance (20 nm). However, when the gap distance is large (100 nm), the excessive loss suppresses the NFRHT. Hyperbolic polaritons (HPs) can effectively explain the above phenomenon, as confirmed by energy transmission coefficients distribution and dispersion relation in wavevector space. In particular, the volume-confined hyperbolic polaritons play a dominant role in the flux variation between the thin films (1 nm and 10 nm). For the 100-nm film, surface-confined hyperbolic polaritons are more important. This study sheds light on the effect of substrate on HPs and provides theoretical guidance for designing near-field thermal radiation devices. [ABSTRACT FROM AUTHOR]

• Enhancement and modulation of NFRHT in α -MoO 3 three-body NPs system is studied. • An enhancement ratio of over 20,000 is achieved. • The physical mechanism is attributed to the high directionality of the LHPs. • This study demonstrates the potential of anisotropic HPs in NFRHT modulation. Anisotropic hyperbolic polaritons (HPs) provide novel opportunities to control light at nanoscale because of their promising ability of subdiffractional confinement, low losses, and polarization sensitivities. However, it has not been explored in the modulation of many-body near-field radiative heat transfer (NFRHT). In this work, we investigate the NFRHT in an α -phase molybdenum trioxide three-body nanoparticles (NPs) system. The results show that the near-field radiative conductance (NFRHC) can be significantly enhanced by adding the third NP between two NPs due to the excitation of the localized hyperbolic polaritons (LHPs). The enhancement effect can be actively modulated by rotating the intermediate NP, which is attributed to the high directionality of the LHPs. In addition, the effects of interparticle distance and vertical displacement on the NFRHC are also studied. We achieved an enhancement ratio of over 20,000 when the interparticle distance and vertical displacement is 4 times and 1.2 times the radius, 38 times higher than that of previous works. This work demonstrates the potential of anisotropic HPs in NFRHT modulation within the many-body system. [ABSTRACT FROM AUTHOR]

• We investigate the radiative heat transfer between stacked structures consisting of two different hyperbolic materials in the near-field and far-field. • In the far-field, the heat flux is suppressed by covering a bulk hyperbolic material with different hyperbolic film. • The near-field radiative heat flux decreases by 80% when bulk α -MoO 3 is covered with a 20-nm hBN film, while a 4-fold enhancement of heat flux is reached when bulk hBN is covered with a 20-nm α -MoO 3 film. • This work will guide us in designing the emitter and receiver based on hyperbolic materials when considering the radiative heat transfer in both near-field and far-field. Hyperbolic materials have attracted much attention in nanophotonics and thermal radiation because they can support high-wavevector modes. The near-field radiative heat transfer between single hyperbolic materials has been studied extensively, however, the radiative heat transfer (RHT) between multilayer hyperbolic materials in both near-field and far-field has seldomly been investigated. In this paper, we investigate the RHT between stacked structures consisting of two different hyperbolic materials in the near-field and far-field, taking uniaxial hBN, and biaxial α -MoO 3 for instance. The influence of the arrangement and thicknesses of the two materials on the RHT have been fully studied. The numerical results demonstrate that the RHT is dominated by the substrate material of the stacked structures in the far-field, and the contribution of the hyperbolic film is limited. However, in the near-field, the RHT is dominated by the hyperbolic film. Besides, when the substrate material is hBN, the addition of α -MoO 3 will greatly enhance the RHT. When the substrate is α -MoO 3 , the addition of hBN will be greatly suppressed. The performance of RHT in different cases is attributed to the coupling of hyperbolic polaritons supported by hBN and α -MoO 3 , which is verified by investigating the energy transmission coefficients. Our results in this work may guide us in designing the emitter and receiver based on hyperbolic materials when considering the RHT in both near-field and far-field. [ABSTRACT FROM AUTHOR]

The mid-infrared spectral range is critical for bio-sensing and gas detection. Existing methods for designing wavelength-selective absorbers involve two-dimensional (2D) and three-dimensional (3D) nanofabrication, which is exceedingly expensive, rendering these methods impracticable. The application of a plane multilayer configuration using Fabry–Perot (FP) resonance can help overcome these drawbacks. This paper presents the numerical analysis of an innovative narrowband absorber with a VO 2 -graphene-based Fabry–Perot (VGFP) multilayer structure using thermally and electrically tunable methods to regulate light absorption in the mid-infrared region. The maximum thermal modulation of spectral absorbance ranges from 0.068 to 0.999. The FP resonance is the primary cause of the strong light absorption and is explained in detail using the multiple reflection interference principle and the impedance transformation method. The absorption ratio of each layer was obtained from the analysis of its electric field and energy. The spectral selectivity of the absorber was varied by changing the dimensions of each layer of the absorber. Additionally, the effect of the incidence angle and gate voltage on the spectral absorbance of the multilayer structures was analyzed. Finally, the sensing performance of the VGFP multilayer structure was investigated. The findings of this study can be referred to for designing tunable high-performance optoelectronic devices in the future. • A narrowband absorber with a VO 2 -graphene-based multilayer structure is proposed. • The maximum thermal modulation of the spectral absorbance is 0.068–0.999. • With gate voltage graphene chemical potential and thus spectral selectivity vary. • Fabry–Perot resonance is the primary cause of the strong light absorption. • The findings enable development of tunable high-performance optoelectronic devices. [ABSTRACT FROM AUTHOR]

• Multi-layer Ni-Al2O3 solar absorber with metal-insulator-metal structure is presented. • Absorbs solar energy from ultraviolet to near-infrared solar radiation band. • Spectral absorption rate approximately 11% higher than traditional photonic crystal. • Beneficial in areas such as solar thermal utilization and thermal photovoltaics. Efficient absorption of solar spectral radiation is a key requirement in solar heat utilization. In an effort to achieve this goal, this study investigated slow light effect and magnetic polaritons, and analyzed other physical model coupling mechanisms in photonic crystals. To construct a class of two-dimensional layers of Ni-Al 2 O 3 as a pyramid array solar energy absorber, the high melting points of Ni and Al 2 O 3 were utilized to create an arrangement of absorbers with good thermal stability, low sensitivity to angle of incidence, and polarization independence. Based on AM1.5 solar spectral data, a simulation study of the photothermal absorption characteristics of this type of absorber in the 0.3–2.5 µm band was conducted. The results showed that the photothermal conversion efficiency is as high as 96.45%. Compared with the traditional single physical effect design, the conversion efficiency of this photonic crystal structure is increased by ~11%. Furthermore, increasing the concentration factor allows the absorber to maintain high efficiency even at high temperatures, which provides theoretical evidence for the efficient use of solar radiation. This work will be beneficial in several areas, including solar thermal utilization, thermal photovoltaics, absorber design, and radiator design in radiant refrigeration systems. [ABSTRACT FROM AUTHOR]

• Multi-layer Ni-Al2O3 solar absorber with metal-insulator-metal structure is presented. • Absorbs solar energy from ultraviolet to near-infrared solar radiation band. • Spectral absorption rate approximately 11% higher than traditional photonic crystal. • Beneficial in areas such as solar thermal utilization and thermal photovoltaics. Efficient absorption of solar spectral radiation is a key requirement in solar heat utilization. In an effort to achieve this goal, this study investigated slow light effect and magnetic polaritons, and analyzed other physical model coupling mechanisms in photonic crystals. To construct a class of two-dimensional layers of Ni-Al 2 O 3 as a pyramid array solar energy absorber, the high melting points of Ni and Al 2 O 3 were utilized to create an arrangement of absorbers with good thermal stability, low sensitivity to angle of incidence, and polarization independence. Based on AM1.5 solar spectral data, a simulation study of the photothermal absorption characteristics of this type of absorber in the 0.3–2.5 µm band was conducted. The results showed that the photothermal conversion efficiency is as high as 96.45%. Compared with the traditional single physical effect design, the conversion efficiency of this photonic crystal structure is increased by ~11%. Furthermore, increasing the concentration factor allows the absorber to maintain high efficiency even at high temperatures, which provides theoretical evidence for the efficient use of solar radiation. This work will be beneficial in several areas, including solar thermal utilization, thermal photovoltaics, absorber design, and radiator design in radiant refrigeration systems. [ABSTRACT FROM AUTHOR]

Titanium nitride (TiN) serves as an ideal reagent for photothermal therapy due to its cost-effectiveness, stability, and biocompatibility. This study investigates the optical and thermal properties of TiN nanoparticles in the near-infrared (NIR) region. Specifically, we employ the COMSOL Multiphysics software to solve Maxwell's equations using a finite element method, enabling us to investigate the photothermal characteristics of individual TiN nanorods. To ensure accurate photothermal coupling calculations, we utilize the Fourier steady-state heat conduction equation. The results demonstrate that the nanorod photothermal properties exhibit significantly enhanced effects when the incident light is polarized along the long axis of the rod, while the thermal properties of the nanorods are primarily influenced by their optical absorption efficiency. The tunability investigation of the nanorods demonstrates that an increase in aspect ratio (AR) results in a red-shift of the absorption peaks. Concurrently, there is an expansion in the bandwidth corresponding to the absorption peak, accompanied by an augmentation in its peak value. It is evident that at AR = 5, the TiN nanorods exhibit enhanced photothermal properties in the NIR region, resulting in a temperature increase of 43 K in the surrounding medium through efficient heat transfer. Consequently, this accomplishment enables achieving the desired temperature range for effective photothermal therapy. The significance of this study lies in providing a valuable reference and theoretical guidance for exploring the utilization of TiN nanorods in photothermal therapy, with a specific focus on attaining a more precise understanding of temperature elevation and distribution during the treatment process. [ABSTRACT FROM AUTHOR]

Optimal optical absorption properties are imperative for the utilization of nanofluids in solar energy applications. Excellent optical absorption facilitates the augmentation of solar energy utilization efficiency. The base fluid contributes significantly to the optical properties of the nanofluid. However, the effect of base fluid on the nanofluid optical properties has yet to be explored. This work investigates the effect of three base fluids, deionized water, ethylene glycol, and 1,2-propanediol, on the optical absorption properties of titanium nitride (TiN) nanofluids. The results show differences in the optical properties of the TiN nanofluids prepared for different sonication times. With the absence of surfactant addition, the stability of TiN nanofluids based on ethylene glycol-based and 1,2-Propanediol-based surpasses that of deionized water-based TiN nanofluids. However, in comparison to ethylene glycol-based and 1,2-Propanediol-based, deionized water-based TiN nanofluids exhibit commendable light absorption capacity. Finally, the photothermal conversion efficiency is calculated for different base fluids. The photothermal conversion efficiency of the deionized water-based TiN nanofluid is 50.82%, which is 13.91% and 15.98% higher than that of the ethylene glycol-based and 1,2-propanediol-based TiN nanofluids under the same conditions, respectively. Thus, deionized water-based TiN shows excellent optical absorption properties, essential for enhanced solar energy thermal utilization. This work will help guide the application of TiN nanofluids. [ABSTRACT FROM AUTHOR]

• The NF-TPV device using a periodic multilayer HM structures thermal emitter is investigated. • Two-cell structures achieve best 1.22 × 104 W/m2, surpassing CaCO 3 -InSb and hBN-lnSb structures by 2.17 and 1.12 times, respectively. • The improved performance owes credit to the strong coupling of HPhPs above the bandgap of PV cells. • The study offers a new approach to enhance the performance of NF-TPV systems. The near-field thermophotovoltaics (NF-TPV), as a solid-state energy converter, enables thermal-to-electric energy conversion efficiently due to the coupling of evanescent waves, holding great potential in waste heat recovery. Studies suggest that hyperbolic phonon polaritons (HPhPs) excited in natural hyperbolic materials (HMs) have promising prospects to improve NF-TPV performance. However, the impact of coupling of HPhPs within multilayer HM structures on NF-TPV systems remains unexplored. Here, we study an NF-TPV system in which a periodic multilayer structure composed of different HMs serves as the thermal emitter, with InSb PV cell acting as the receiver. The numerical results show that the Two-cell structure achieves an output power of 1.22 × 104 W/m2 at a temperature of 900 K, surpassing the output power of the CaCO 3 -InSb and hBN-InSb structures by 2.17 and 1.12 times, respectively. The improved performance owes credit to the strong coupling of HPhPs above the band gap of the PV cell. The study offers a new approach to enhance the performance of NF-TPV systems in waste heat recovery and the utilization of renewable energy. [ABSTRACT FROM AUTHOR]

Isotope engineering, which has received much attention for significantly reducing the optical loss of phonon polaritons, opens new doors for thermal radiation manipulation. Recent studies have shown that the shift and broadening of phonon lines caused by isotopic mass effects significantly affect near-field radiative heat transfer. Near-field thermophotovoltaics (NF-TPV) is an efficient thermal-electric energy conversion device based on radiative heat transfer at the nano/micro scale. However, the impact of isotopes on NF-TPV remains to be further studied. Here, we have theoretically studied the performance of NF-TPV systems with hBN thermal emitters of distinct isotope compositions. The numerical results show that the 98.7% 10B hBN isotope enrichment configuration reaches 67.8 kW/m2 output power and 34.7% efficiency when the emitter temperature is 900 K, surpassing the natural hBN by 8.3 kW/m2 and 1.20%. Nevertheless, the performance is suppressed in 99.2% 11B hBN structure, reduced by 1.8 kW/m2 and 0.64% compared to natural hBN. This is attributed to the isotope-induced shift and broadening of the region of hyperbolic phonon polaritons. This study can provide theoretical guidance for designing NF-TPV systems based on isotope engineering and has great application prospects in renewable energy and energy storage fields. • The impact of isotope on performances of NF-TPV systems with hBN thermal emitters is investigated. • The 98.7% 10B hBN shows the best performance (67.8 kW/m2 and 34.7%), surpassing other isotope composition devices. • The varied performance is due to the isotope-induced shift and broadening of the region of HPhPs. • This study provides theoretical guidance for designing NF-TPV systems via isotope engineering. [ABSTRACT FROM AUTHOR]

Solar absorber is the key component to realize efficient utilization of solar energy. In this paper, only titanium nitride (TiN) is used to create a broadband, wide-angle, polarization insensitivity solar absorber. The average absorption of the absorber in the spectral range of 300-2500 nm is 91.3%. The distributions of electric and magnetic fields indicate that the strong absorption is caused by the coupling effect of surface plasmon resonance and guided mode resonance. Investigate the effects of the geometric parameters of the absorber on the spectral absorption performance. Moreover, we also discuss the absorption performance of the solar absorber at different incidence angles. It is demonstrated that even at a large incidence angle of 60°, the average spectral absorption can exceed more than 70%. The proposed solar energy absorber is also insensitive to polarization. The research results in this work could benefit the applications in solar thermophotovoltaic system. [ABSTRACT FROM AUTHOR]

Smart radiation devices (SRDs) for spacecraft, which utilize thermochromic films with adaptive emissivity switching capability and minimal resource consumption, have garnered attention. However, there remains a lack of systematic strategies for more efficient SRDs design. In this study, we focus on a typical SRD, namely VO 2 /BaF 2 /Ag, to exemplify and investigate two optimization methods: the quarter-wave condition and the traversal method. By employing the quarter-wave condition, the intermediate dielectric layer thickness required to induce Fabry-Perot resonance is determined as 1.81 μm. In this case, the emission modulation is 0.62, and the solar absorption is 0.29. However, utilizing the traversal method, optimized thicknesses for VO 2 and BaF 2 are found to be 0.02 μm and 1.60 μm, respectively. It is found that the emission modulation can reach 0.64, while the solar absorption remains unchanged at 0.29. Notably, the traversal method yields superior emission modulation of SRD compared to the quarter-wave condition, while there is no difference in solar absorption of SRD obtained by the two optimization methods. Consequently, the quarter-wave condition can provide an approximate range for the thickness of the dielectric layer in SRD designs, with further optimization achieved through the traversal method to enhance SRD performance. Overall, our research supplies valuable theoretical guidance for SRD design and aims to propel advancements in spacecraft thermal control. [ABSTRACT FROM AUTHOR]

• The HTC between α -MoO 3 films along different crystal directions is studied. • The HTC and MC are strongly related to the crystal direction. • The excitation and mismatch of HPhPs can explain the underlying physical mechanism. • This work paves novel way to design non-contact thermal modulators based on HMs. Near-field radiative heat transfer (NFRHT) has great potential in thermal management and energy harvesting due to the giant radiative heat flux. Modulation of NFRHT is another important research direction, where relative rotation between the emitter and receiver is a typical method. However, the enhancement of modulation contrast (MC) is mainly concerned in previous works, while large heat flux is not taken into account. In this work, we investigate the heat transfer coefficient (HTC) between α -MoO 3 films along different crystal directions of α -MoO 3. We take account of the modulator with high heat flux and high MC. The results show that the HTC and MC can reach 223.2 W/(m2·K) (36 times of blackbody limit) and 6.33 at a gap distance of 100 nm when NFRHT is along [010] crystal direction of α -MoO 3 , respectively. These are attributed to the excitation and mismatch of hyperbolic phonon polaritons (HPhPs). Furthermore, when NFRHT is along [100] and [001] crystal directions of α -MoO 3 , MCs reach 5.84 and 2.38, respectively. The results are strongly related to the crystal direction. In addition, the impact of film thickness on the MC for different crystal directions is investigated. This work is conducive to the applications of hyperbolic materials in non-contact thermal modulators. [ABSTRACT FROM AUTHOR]

Solar absorber is a key device that converts solar irradiation into heat via photothermal conversion, which has potential applications in solar thermal power systems, and seawater desalination. In this paper, we propose a wide-angle, broadband and polarization insensitivity metamaterial solar absorber based on titanium nitride (TiN). Numerical results show that this solar absorber has a total average spectrum absorption of up to 97.5% at wavelength of 300–2500 nm. Based on the distributions of the electric and magnetic field, it is believed that the coupling resonance of surface plasmon resonance, guided mode resonance and cavity resonance is responsible for its excellent absorption of solar energy. Furthermore, we discuss the effects of incidence angle on the solar absorber and demonstrate that the average spectrum absorption of more than 85% can be obtained even at a large incidence angle of 60°, and it is insensitive to the change of polarization angle. The effect of geometric parameters of the solar absorber on spectrum absorption performance was investigated. This research will provide new ideas for the design of solar thermophotovoltaic and thermal conversion systems. [ABSTRACT FROM AUTHOR]

With the development of space technology, the miniaturisation of satellites has posed a challenge to thermal control technology. As an essential spacecraft thermal control technology, variable emission thermal control devices are important for reducing the load and size of spacecraft and adapting to the complex and changing thermal environment of space. In this paper, a vanadium dioxide (VO 2)-based multilayer films structure for thermal control device is proposed and analyzed. The influence of the thicknesses of the films on the emission of the structure and the solar absorption has been systematically investigated. Two commonly used optical dielectric layers, SiO 2 and Al 2 O 3 , are selected for the study. Besides, regardless of whether the intermediate medium is SiO 2 or Al 2 O 3 , the emission regulation can reach 0.47, and the solar absorption is smaller than 0.38. Furthermore, the high emission at high temperature is more conducive to realizing when the intermediate medium is SiO 2 , while Al 2 O 3 has more potential to maintain low emission at low temperature. In addition, through analyzing the distributions of the electric field and normalized power dissipation density, it is found that the absorption enhancement originates from the intrinsic loss of the medium. Furthermore, the proposed structure is robust to the angle of incidence and the polarization. This work will provide important theoretical guidance for the design of spacecraft smart radiation devices. [ABSTRACT FROM AUTHOR]

• We theoretically investigate the near-field radiative heat transfer between α -MoO 3 nanoparticles mediated by α -MoO 3 planar substrate. • Numerical results show that the max modulation contrast could reach 60 when the planar substrate is a semi-infinite medium. • Underlying mechanisms can be interpreted as the strong directionality of the localized hyperbolic polaritons and volume-confined hyperbolic polaritons. • This work will guide the design of high-performance near-field radiative modulators based on anisotropic many-body systems. α -phase molybdenum trioxide (α -MoO 3) is an excellent biaxial hyperbolic material, whose thermal radiation properties depend highly on the crystal directions. The directional properties of thermal radiation can be controlled by changing the crystal directions of α -MoO 3 , which provides a novel path for the modulation of thermal radiation. In this work, we find that the near-field radiative heat transfer (NFRHT) between two α -MoO 3 nanoparticles (NPs) mediated by an α -MoO 3 planar substrate clearly depends on the crystal directions of α -MoO 3. Due to the strong directionality of the localized hyperbolic polaritons in the NPs and the volume-confined hyperbolic polaritons in the substrate, we can achieve an effective modulation of the NFRHT by mechanical rotation. Numerical results show that the max modulation contrast could reach 60 when the planar substrate is a semi-infinite medium. The contribution of the vacuum, scattering, and cross terms under the influence of anisotropic NPs are analyzed separately to reveal the mechanism. Moreover, we investigate the effects of rotation angle and thickness of planar substrate on the modulation of NFRHT. Finally, we discuss the effects of rotating NPs on the performance of near-field radiative modulator. This work will guide the design of high-performance thermal radiative modulators based on anisotropic many-body systems. [ABSTRACT FROM AUTHOR]

Near-field radiative heat transfer (NFRHT) could surpass the blackbody limit defined by Stefan-Bolzmann's law by several orders of magnitude, which has potential applications in thermal switching, thermal management, and photovoltaics. To further develop the NFRHT from theory to application, the substrate, which could enhance the stability of the structure, is a critical factor not to be ignored. However, the substrate effect on the NFRHT is still rarely discussed. In this work, we investigate the NFRHT between hexagonal boron nitride (hBN) films with different permittivities of the substrate. Results demonstrate that when the thickness of the film is 1 nm, increasing the permittivity of the substrate will suppress the NFRHT. In contrast, when the thickness of the film is larger (>2 nm), the high-permittivity substrate could enhance the NFRHT. The spectral heat flux (SHF) corresponding to substrates with different permittivities was investigated. The SHF in Type I hyperbolic band of hBN increases with the increase in the permittivity of the substrate, while that in Type II hyperbolic band is completely opposite. This competitive relationship leads to the above-mentioned phenomenon of NFRHT. The underlying physics mechanism can also be explained by the hyperbolic phonon polaritons (HPPs), which are analyzed by the energy transmission coefficients and dispersion relations. The findings in this work will deepen the understanding of the substrate on HPPs and pave a novel way for near-field radiation devices based on hyperbolic materials. [ABSTRACT FROM AUTHOR]

• We have demonstrated a THz device with strong chirality and asymmetric transmission effect based on a twisted bilayer α-MoO 3. • It is found that the proposed structure achieves a giant circular dichroism value of 0.81 at 9.8 THz. • The strong chirality originates from the symmetry breaking of the structure. • The asymmetric transmission for circularly polarized wave can reach 0.55 at 9.2 THz. Chirality and asymmetric transmission have wide-ranging applications in fields such as optics, chemical synthesis, pharmaceutical research, biological analysis, materials science and communication technology. In this work, we have demonstrated a terahertz (THz) device with strong chirality and asymmetric transmission effect based on a twisted bilayer α-MoO 3. The unique optical response can be realized by adjusting structural parameters such as the thicknesses of the layers as well as the angle of relative rotation. It is found that the proposed structure achieves a giant circular dichroism (CD) value of 0.81 at 9.8 THz, which originates from the symmetry breaking of the structure. The phenomenon has been approved by the polarization conversion and the distributions of the electric field. In addition, the device exhibits the asymmetric transmission effect for circularly polarized wave incidence. The results show that the forward and backward transmissivity of circularly polarized waves with different handedness exhibit a significant difference of up to 0.55 at 9.2 THz. This work simultaneously achieves chirality and asymmetric transmission effect in the terahertz band, which paves the way for developing terahertz devices capable of chirality manipulation and optical isolation. [ABSTRACT FROM AUTHOR]

With the integrated application of multiple detection technologies, multi-band compatible stealth is crucial to improve the counter-detection capability of targets. However, there are still significant challenges in achieving stealth that is compatible with different bands. This paper presents a wavelength-selective emitter with a structure consisting of a layer composed of a high-temperature-resistant material and a phase change material Ge 2 Sb 2 Te 5 (GST), and an upper truncated pyramid array composed of TiO 2. The structure can transition between "stealthy" and "non-stealthy" states by controlling the phase transition process of the GST. The structure in stealth mode has low reflectivity in the visible (VIS) region, low emissivity in the mid-wave infrared and long-wave infrared region. In addition, it has laser stealth at 1.06 μm and 1.54 μm, and provides radiative cooling in the non-atmospheric window (5–8 μm). The dipole resonance formed by TiO 2 on the side gives the structure lower reflectivity in the VIS band and at two laser wavelengths. The high emission in the non-atmospheric window is mainly attributed to the Fabry-Perot resonance supported in the structure. In addition, the effects of incidence angle and temperature on the stealth performance of the structure are investigated. This research has promising applications in the field of multi-band compatible stealth. • We propose a GST-based multi-band compatible stealth structure. • The structure can switch between "stealthy" and "non-stealthy" states. • This structure can realize stealth in the visible, infrared band and laser wavelength. • The structure achieves multi-band stealth at different angles and temperatures. [ABSTRACT FROM AUTHOR]

Spinning thermal radiation is a unique phenomenon in the symmetry-broken system, which is of both fundamental and applied interest. Its potential applications include chiral nanoparticle probing and radiation detection. However, these applications are often restricted by limited circular dichroism and quality factor. In this paper, the spinning thermal radiation of twisted hyperbolic heterostructures is investigated. By adjusting the twist angle, we demonstrate the near-perfect circular dichroism (CD = 0.99) and the high quality factor at a wavelength of 20 μm. The epsilon-near-zero mode of α-phase molybdenum trioxide allows for excellent spin-selective thermal radiation. The co-polarization and cross-polarization have proven it in the reflected wave and the distribution of the electric field components. The strong spinning thermal radiation originates from the cross-polarization conversion between the left-handed circularly polarized and right-handed circularly polarized waves. This work provides a general approach to spinning thermal radiation for future energy applications. [ABSTRACT FROM AUTHOR]

MXene has received a lot of attention from researchers in the fields of photothermal and photovoltaic due to its excellent optical properties. However, research on MXene-based solar absorbers has rarely been carried out. In this work, we proposed a MXene-based solar absorber with absorption broadband, wide-angle, and polarization insensitive. The absorber has excellent light capture capability, with its average absorption in the solar band as high as 97.8 % and an energy absorption ratio of 98.9 % in the AM1.5 solar radiation spectrum. The electric and magnetic field distributions indicate that the unique hierarchical nanohole design can effectively excite multiple resonance couplings of surface plasmon resonance, cavity resonance, and guided mode resonance. The proposed absorber can still maintain the broadband absorption characteristics within a certain manufacturing tolerances. Moreover, the absorber is insensitive to polarization and has a high light absorption under a large incident angle. We believe that this work holds great promise for solar thermal conversion and heat utilization. [ABSTRACT FROM AUTHOR]

Solar absorber, which is widely used in concentrated solar thermal systems and solar photovoltaics, has attracted great attention in recent years. Concentrated solar irradiation not only saves production costs, but also effectively improves the photothermal conversion efficiency of the system. However, the huge radiative heat flux will cause the system temperature to rise rapidly, which poses significant challenges for designing solar absorbers. Here, a metasurface absorber based on cylinder array structure is proposed for efficient solar energy harvesting, using the high-temperature resistant materials TiN and SiO 2. The total solar absorption of the absorber is up to 0.94 at wavelengths of 300–2500 nm. The high absorption performance of the metasurface absorber can be explained as the coupling effect of surface plasmon resonance and Fabry-Pérot resonance, which is confirmed by the electric field distribution. Moreover, the effect of geometric parameters on absorption performance is analyzed. Finally, we discuss the influence of incident angle on the solar absorber. We believe this work could deepen the understanding of coupling resonance mode and guide the design of high-temperature solar absorbers. [ABSTRACT FROM AUTHOR]

• The near-field radiative heat transfer between multilayer structures composed of different hyperbolic materials is investigated. • The total heat flux between multilayer structures is enhanced compared to that between hBN (19-fold) and α -MoO 3 (1.46-fold) films. • The underlying physical mechanism is that vacuum layers promote the strong coupling of hyperbolic phonon polaritons supported by different hyperbolic materials. Near-field radiative heat transfer (NFRHT) has drawn significant interest in the recent years, including thermal management and energy harvesting. The NFRHT between single hyperbolic materials (HMs) has been thoroughly investigated. However, the research on the NFRHT between periodic multilayer structures composed of different HMs is rarely discussed. Moreover, the coupling effect of hyperbolic phonon polaritons (HPPs) supported by different HMs is still unclear. In this work, we investigated the NFRHT between multilayer structures composed of hexagonal boron nitride (hBN) and α -phase molybdenum trioxide (α -MoO 3), separated by vacuum layers. The influence of the gap distance, the unit-cell number, the thickness of the HMs and thickness of the vacuum layer on the NFRHT are discussed. The numerical results show that the maximal total heat flux (THF) between six-cell multilayer structures is 19 times compared to the THF between hBN films at the gap distance of 50 nm and is 1.46 times compared to the THF between α -MoO 3 films at the gap distance of 30 nm. Such enhancement can also be found at other similar gap distances. The vacuum layer promotes the coupling of HPPs supported by different HMs, which can be elucidated by the energy transmission coefficients. This work could benefit the application of near-field thermal radiative devices based on HMs. [ABSTRACT FROM AUTHOR]

Nanofluids are one of the keys to improving solar energy utilization efficiency. Metal nanoparticles (MNPs) have narrow-band absorption limitations in photothermal conversion. Recent studies have shown that it can overcome narrow-band absorption by adding slots on the nanoparticles (NPs) to improve photothermal conversion efficiency. In this work, we investigated the effect of curved slots on the optical properties of Ag nanorods by the finite element method. The results show that the presence of the curved slot enables the Ag nanorods to excite multiple resonance absorption peaks in the wavelength range of 300–1100 nm. The curved slotting reduces the overall scattering of the Ag nanorods and enhances its spectral absorption in the visible and near-infrared regions. In addition, we also calculated and compared the photothermal conversion efficiency of nanorods and nanorods with curved slots. At f v = 1 × 10−5 and h = 20 mm, the photothermal conversion efficiency of the curved slot Ag nanorods is increased by 6.8% over that of the Ag nanorods. In the measured volume fraction and fluid depth range, when the photothermal conversion efficiency reaches about 90%, the h and f v required for nanorods with curved slots are about 0.64 times that of nanorods. Nanorods with curved slots only need a meagre volume fraction and fluid depth to achieve high photothermal conversion efficiency. We believe that the proposed curved slot metal nanorods can provide a feasible solution for solving the narrow-band absorption of nanorods and improving the efficiency of solar energy utilization. [ABSTRACT FROM AUTHOR]

• A model of random distributed flake particles system is established. • The spectral radiation transfer model of the Al/acrylic resin composite coatings is built by the MCRT method. • The effects of the size, volume fraction and orientation of particles on infrared radiation characteristics is studied. The composite functional coating containing micro-nano flake particles is a kind of classic thin-layer structural material, which has excellent photothermal properties. In this paper, a model of random distributed flake particles system is established, and then combining with the geometrical optics theory, the spectral radiation transfer model of the composite functional coating containing micro-nano flake particles is built by the Monte Carlo ray-tracing method (MCRT). After verifying the reliability of the model, the effect of the distribution characteristics, particle size parameters, volume fraction and other parameters of random distributed flake particles system on the spectral radiation characteristics of the coating in 8∼14 μm infrared window is investigated. The results indicate that the spectral emissivity of the coating enhances about 9 % at most when particle radius increases from 5 μm to 15 μm, decreases about 60 % at most when volume fraction of particle system increases from 10 % to 30 %, and goes up about 74.5 % when the flake particles change form horizontal arrangement to orientation range is 0 ° ∼ 60 °. [ABSTRACT FROM AUTHOR]