The advancement of high-efficiency technologies to convert low-temperature thermal energy (< 600 K) is vital for optimizing energy utilization and supporting carbon mitigation. Near-field thermophotovoltaics (NF-TPV) convert medium- to high-temperature heat (> 600 K) from solar collectors into electrical energy through evanescent wave coupling, offering a promising approach to enhance the efficiency of solar thermal systems. However, the utilization of low temperature heat energy by NF-TPV system deserves further investigation. Herein, we propose an enhanced NF-TPV system based on hyperbolic metasurface for the efficient recovery of low-temperature thermal energy. Results show that NF-TPV systems utilizing calcite (CaCO3) metasurface as thermal emitters can achieve performance exceeding those of systems using CaCO3 films by up to six times. The metasurface system achieves 61.6 W/cm2 power density and an upper bound efficiency of up to 63.2 % of the Carnot limit when the emitter temperature is only 600 K. The improved performance is attributed to the strong collective near-field coupling in the metasurface. The findings contribute to understanding the strong collective near-field coupling in the hyperbolic metasurface system and provide a novel way to utilize low temperature thermal energy.

With the increasing prominence of energy issues, the radiative thermal management techniques hold great potential in sustainable energy research, which attracted much attention. In this study, a temperature-adaptive selective emission structure is proposed to control the phase transition state of doped vanadium dioxide (VO2) by the difference of daytime and nighttime temperatures to achieve all-day radiative thermal management. During the day, the ambient temperature increases. When the VO2 temperature exceeds the phase transition temperature, the structure has high reflectivity in the solar spectral band and high emissivity in the atmospheric transparent band (8-14 mu m), resulting in radiative cooling. At night, the ambient temperature decreases. When the temperature of VO2 is lower than the phase transition temperature, the structure has low emissivity in the atmospheric transparent band (8-14 mu m) and high absorptivity in the atmospheric radiative bands (5-8 and 14-16 mu m), thus realizing the warming effect. Additionally, the impact of variation in material thickness and angle of incidence on the spectral characteristics of the designed structures are also investigated, and the results indicated that the impact on the spectral characteristics of the structures are not significant. This study provides an innovative approach to regulating energy efficiency in buildings, vehicles and utilities, which can help to promote diversity in energy utilization.

The contribution of evanescent waves enables efficient thermoelectric conversion in near-field thermophotovoltaic (NFTPV), thereby offering a promising avenue for waste heat recovery. Previous researches indicate that hyperbolic materials (HMs) have promising prospects as emitters in NFTPV systems, while their potential to combine with two-dimensional materials remains to be explored. Here, we propose an NFTPV system that utilizes calcite (CaCO3) covered with black phosphorus (BP) as the emitter and InSb as the photovoltaic cell. The proposed structure significantly enhances the performance of NFTPV compared to the structure without BP, achieving an output power of 3.23 x 105 W/m2 and efficiency of 42.6 % at an emitter temperature of 900 K. The improvement in performance is attributed to the frequency matching between the radiation spectrum of emitter and interband transition of InSb, where the coupling of hyperbolic resonances in CaCO3 and surface plasmon modes in BP plays an important role. Moreover, we found that the performance will be further improved when the CaCO3 thickness is small. In addition, the sandwich structure BP/CaCO3/BP-InSb is also a desirable alternative solution. This study would be beneficial for understanding the role of resonant coupling and provides a new strategy to improve the performance of NFTPV systems.

As a natural biaxial hyperbolic material, α-phase molybdenum trioxide (α-MoO3) exhibits dielectric and metallic properties in the plane, rendering it an exceptional candidate for polarization-dependent devices. In this work, we design a lithography-free polarization-dependent absorber consisting of an α-MoO3 film, a germanium layer, and a silver substrate. The results show that a narrowband absorption of up to 0.99 is achieved at a wavelength of 12.2 µm for transverse magnetic polarization. In contrast, the absorption is only 0.06 at this wavelength for transverse electric polarization. This remarkable polarization-dependent absorption performance is attributed to the coupling of epsilon-near-zero modes and Fabry-Perot resonances, which is confirmed by the electric field and power dissipation density distributions. Furthermore, strong polarization-dependent performance could also be achieved when the crystal axis of α-MoO3 is rotated in the out-of-plane. This work demonstrates that in-plane anisotropic α-MoO3 has the potential for designing high polarization-dependent devices.

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 CaCO3-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.

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 Ge2Sb2Te5 (GST), and an upper truncated pyramid array composed of TiO2. 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 longwave infrared region. In addition, it has laser stealth at 1.06 mu m and 1.54 mu m, and provides radiative cooling in the non-atmospheric window (5-8 mu m). The dipole resonance formed by TiO2 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.

Thin films demonstrate considerable potential in the realms of radiative heat transfer and play a major role in thermal rectification and energy conversion. Hyperbolic materials can effectively promote radiative heat transfer due to the ability to excite hyperbolic polaritons in a wide range of hyperbolic bands. However, the potential of biaxial hyperbolic film in radiative heat transfer remains insufficiently explored. In this work, the radiative heat transfer between alpha-phase molybdenum trioxide (alpha-MoO3) is theoretically investigated, considering separations ranging from 20 nm to 2 mu m. When considering radiative heat flux along the [010] direction, the near-field radiative heat flux of alpha-MoO3 with a thickness of 10 nm is only 8 % less than that of the bulk material and exceeds the blackbody limit by approximately three orders of magnitude at a separation of 20 nm. This phenomenon is attributed to the excitation of hyperbolic polaritons. Conversely, when the gap distance is 1000 nm, the heat flux between films is an order of magnitude lower than that between bulk materials. These findings help study radiative heat transfer between thin films at micro- and nano-scale.

Twisted systems exhibiting unique electronic and mechanical properties are valuable in quantum physics and material science. Inspired by the concept of twistronics, we investigate the Casimir interaction in the twisted system. We delve into the behavior of the Casimir force within a system comprising layers of lithium iodate (LiIO3), separated by a composite medium of nanoparticles dispersed in a liquid. A notable observation is that the Casimir force undergoes a transition from attractive to repulsive, arising in the relative rotation of the LiIO3 at the Casimir magic twist angle. The transitions are induced by quasispindle dispersion and the competition between repulsion and attraction in wavevector space. This work combines macroscopic geometric configurations with microscopic quantum interactions, marks a step in the tunable Casimir attractive-repulsive transition, and is helpful for switchable nanoscale devices with ultralow static friction.

Near-field thermophotovoltaics (NF-TPV) offers the potential for achieving elevated power density and conversion efficiency by leveraging the amplification of thermal radiation within a nanoscale gap. Here, we propose an NF-TPV device with a sandwich emitter composed of calcite film and graphene layer. The results show that this sandwich configuration can significantly enhance output power, outperforming monolayer-graphene-covered heterostructures and the single calcite film. These are because the sandwich configuration can enhance hybrid polaritons, which are formed by the coupling between surface plasmon polaritons in graphene and hyperbolic phonon polaritons in calcite. In addition, the effects of graphene chemical potentials on the performance of NF-TPV devices are also studied. The tunable power density range of the sandwich structure can be up to 3.26 times that of other structures by altering the chemical potential of graphene. The findings presented here may unpack a promising path for enhancing and manipulating the performance of NF-TPV at the nano- and microscale.

Casimir interaction is an intriguing phenomenon that is induced by electromagnetic quantum fluctuations, which dominates the interaction between microstructures at small separations and is essential for micro- and nano-electromechanical systems (MEMS and NEMS). However, Casimir interaction driven by hyperbolic polaritons remains an unexplored frontier. In this work, we investigate the Casimir interaction between natural hyperbolic material hexagonal boron nitride from the perspective of force distribution with different optical axis orientations for the first time. The attractive Casimir force is remarkably enhanced due to the excitation of volume-confined hyperbolic polaritons (HPs). Furthermore, distinct repulsive contributions to the force are observed due to surface-confined HPs that only exist when the optical axis is in-plane. The HPs are associated with a striking thickness dependence of spectral force properties, suggesting that the discrete volume-confined HPs lead to the attractive-repulsive transition of Casimir force. This work sheds light on the relation between HPs and the vacuum fluctuation-induced force, which could offer new opportunities for the development of the MEMS and NEMS.

Relative rotation between the emitter and receiver could effectively modulate the near-field radiative heat transfer (NFRHT) in anisotropic media. Due to the strong in-plane anisotropy, natural hyperbolic materials can be used to construct near-field radiative modulators with excellent modulation effects. However, in practical applications, natural hyperbolic materials need to be deposited on the substrate, and the influence of substrate on modulation effect has not been studied yet. In this work, we investigate the influence of substrate effect on near-field radiative modulator based on α-MoO3. The results show that compared to the situation without a substrate, the presence of both lossless and lossy substrate will reduce the modulation contrast (MC) for different film thicknesses. When the real or imaginary component of the substrate permittivity increases, the mismatch of hyperbolic phonon polaritons (HPPs) weakens, resulting in a reduction in MC. By reducing the real and imaginary components of substrate permittivity, the MC can be significantly improved, reaching 4.64 for εs = 3 at t = 10 nm. This work indicates that choosing a substrate with a smaller permittivity helps to achieve a better modulation effect, and provides guidance for the application of natural hyperbolic materials in the near-field radiative modulator.

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.

Nanoparticles (NPs) have attracted much attention recently because of their excellent photothermal properties. In particular, nanofluids (NFs) based on core-shell plasmon NPs have become the key to solar thermal utilization. This work proposed an ZrC-Au core-shell NP suitable for direct absorption solar collectors (DASCs). The optical properties of ZrC-Au core-shell NP are investigated based on the finite element method (FEM). The physical mechanism of its existence can be explained by the surface plasmon resonance and localized surface plasmon resonance of ZrC-Au core-shell NP. Meanwhile, the effect of core-shell size on the NP optical properties of ZrC-Au core-shell is investigated based on electromagnetic field distribution. In addition, the effects of length (H) and mass flow ((m) over dot) on the temperature rise and efficiency of the collector (eta) are analyzed with DASC 2D simulation. Research shows that ZrC-Au core-shell NPs with t = 5 nm and r(3) = 15 nm can effectively broaden the solar spectral absorption band, increase the absorption peak value, and the photothermal conversion efficiency (f(v) = 20 ppm, h = 15 mm) reaches 96 %, which is 15.89 % higher than Au NP. Meanwhile, the eta of ZrC-Au NPs can be improved by similar to 8.86 % compared with Au NPs under specific parameters (H = 2 cm, L = 20 cm, f(v) = 20 ppm, (m) over dot = 1 g/(s m)). Combined with the preparation possibility and economy of ZrC-Au core-shell NPs, the broad application prospect of this NP in DASC and other photothermal fields was analyzed.

The coupling phenomenon of surface plasmon polaritons and hyperbolic phonon polaritons in multilayer graphene/vacuum/α-MoO3/vacuum structures is revealed.

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-MoO3 heterostructures. Compared with the individual BP and α-MoO3 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-MoO3 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.

Core-shell nanoparticles (CSNPs) are widely used in energy harvesting, conversion, and thermal management due to the excellent physical properties of different components. Because of the synergistic interaction between the core and the shell, the thermal radiative properties are expected to be further enhanced. In this work, we achieve near-field radiative heat transfer (NFRHT) enhancement between SiC@Drude CSNPs. Numerical results show that the total heat flux between NPs is 1.47 times and 9.98 times higher than homogeneous SiC and Drude NPs at the same radius when the core volume fraction is 0.76. Surface modes hybridization arising from the interfaces of the shell-core and shell-air contributes to the improved thermal radiation. The effect of shift frequency on the NFRHT between SiC@Drude CSNPs is studied, showing that the enhancement ratio of NFRHT between CSNPs can reach 4.34 at a shift frequency of 1 × 1014 rad/s, which is 38.34 times higher than the previous work. This study demonstrates that surface modes hybridization in CSNPs can significantly improve NFRHT and open a novel path for high-efficiency energy transport at the nanoscale.

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.

Thin films exhibit substantial potential in energy management and utilization as the development of micro- and nanofabrication technologies. It is well known that thermal radiation is one of the fundamental ways of energy transfer. However, the potential of hyperbolic films for radiative heat transfer is always ignored. Whether the radiative heat flux between hyperbolic films surpasses that of the bulk materials remains insufficiently explored. In this work, we theoretically investigate the radiative heat transfer between hexagonal boron nitride (hBN) at a separation from 20 nm to 2 mu m. The results show that when the optical axis of hBN is oriented in-plane, the nearfield radiative heat flux of hBN with a thickness of 10 nm exceeds that of hBN bulk by 47% and exceeds the blackbody limit by two orders of magnitude at a gap distance of 20 nm. The physical mechanism is attributed to the volume-confined hyperbolic polaritons can be excited in a higher wavevector space. Conversely, when the gap distance is 600 nm, the heat flux between films is considerably lower than that of bulk material. This work opens up potential avenues for developing hyperbolic film-dependent thermal devices and strategies for thermal management.

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 isotopeinduced 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.

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.