Tunable Near-Field Radiative Heat Transfer between Graphene-Coated Magneto-Optical Metasurfaces.

The highly structured design of metasurfaces greatly facilitates the manipulation of near-field radiative heat transfer (NFRHT). In this study, we incorporate magneto-optical materials into metasurfaces to theoretically explore the mechanism for controlling NFRHT between anisotropic magneto-optical metasurfaces. Our findings indicate that the interaction between the magnetization-induced modes, arising from interband transitions of graphene, and the surface modes of InSb under a magnetic field leads to a transition in the heat transfer spectrum from a dual band to a triple band. The modification of the distribution and magnitude of transmission wave vectors in surface electromagnetic modes by magnetic fields serves to modulate the radiative heat flux. By combining active control by a magnetic field with passive structural design of metasurfaces, the regulation of heat flux can be increased by more than 8-fold compared with the planar configuration. Additionally, the magnetic field amplifies the anisotropy of the photon energy distribution induced by the symmetry breaking of the metasurface structure. This study is anticipated to provide a pathway for achieving flexible tuning of NFRHT by combining active and passive regulation. It also opens up possibilities for multiband information transmission and for improving the performance of energy conversion devices.

NFRHT modulation between graphene/SiC core-shell and hBN plate through strain.

We numerically investigate the near-field radiative heat transfer (NFRHT) between a graphene/SiC core-shell (GSCS) nanoparticle and a hexagonal boron nitride (hBN) plate. By applying a compressive strain to the hBN plate, its hyperbolic modes can be tuned. Consequently, the hyperbolic phonon polaritons (HPPs) of hBN and the high-frequency localized surface resonance (LSR) of GSCS nanoparticle can couple and decouple, thus allowing for the active control of NFRHT. Furthermore, we predict that, combining with the effect of the chemical potential of graphene shell on NFRHT, a thermal rectification ratio of up to 13.6 can be achieved. This work enriches the phonon-polariton coupling mechanism and also facilitates dynamic thermal management at the nanoscale.

Enhanced Near-Field Radiative Heat Transfer between Graphene/hBN Systems.

Near-field radiative heat transfer (NFRHT) can exceed the blackbody radiation limit owing to the coupled evanescent waves, implying a significant potential for energy conversion and thermal management. Coupled surface plasmon polaritons (SPPs) and hyperbolic phonon polaritons (HPPs) with small ohmic losses enable a long propagation wavelength that is essential in NFRHT. However, so far, there still lacks knowledge about the experimental investigation of the coupling of SPPs and HPPs in terms of NFRHT. In this study, the NFRHT between graphene/hexagonal boron nitride (hBN) systems that can be readily transferred onto various substrates, with a gap space of ≈400 nm is measured. NFRHT enhancements in the order of three and six times higher than the blackbody limit for graphene/hBN heterostructures and graphene/hBN/graphene multilayers, respectively are demonstrated. In addition, the largest ever radiative heat flux using graphene/hBN/graphene multilayers under similar gap space of 400 nm is obtained. Consequently, analyzing the photon tunneling modes reveal that these phenomena are consequences of coupled SPPs of graphene and HPPs of hBN.

Magnetically tunable dual-band terahertz absorption based on guided-mode resonance.

We propose a magnetically tunable dual-band terahertz (THz) absorber by using an InAs substrate with a subwavelength zero-contrast germanium grating. The results demonstrate that the absorption peaks in this absorber can be dynamically tuned by changing the intensity and the rotation angle of the applied transverse magnetic field, which is achievable at a moderate order of magnitude of 0.1 Tesla. In addition, we investigate the distribution of magnetic field intensity and find that the magnetically tunable absorption originates from the combination of the magneto-optical effect and the guided-mode resonance effect, where the absorption peaks shift in different directions at normal incidence and oblique incidence. Furthermore, the absorption intensity of the proposed structure could reach 99% with an ultra-high Q-factor of 258. This work paves the way for actively adjustable high-resolution THz absorption or a nonreciprocal thermal emitter.

Many-body near-field radiative heat transfer: methods, functionalities and applications.

Near-field radiative heat transfer (NFRHT) governed by evanescent waves, provides a platform to thoroughly understand the transport behavior of nonradiative photons, and also has great potential in high-efficiency energy harvesting and thermal management at the nanoscale. It is more usual in nature that objects participate in heat transfer process in many-body form rather than the frequently-considered two-body scenarios, and the inborn mutual interactions among objects are important to be understood and utilized for practical applications. The last decade has witnessed considerable achievements on many-body NFRHT, ranging from the establishment of different calculation methods to various unprecedented heat transport phenomena that are distinct from two-body systems. In this invited review, we introduce concisely the basic physics of NFRHT, lay out various theoretical methods to deal with many-body NFRHT, and highlight unique functionalities realized in many-body systems and the resulting applications. At last, the key challenges and opportunities of many-body NFRHT in terms of fundamental physics, experimental validations, and potential applications are outlined and discussed.

Dual-band tunable narrowband near-infrared light trapping control based on a hybrid grating-based Fabry-Perot structure.

A hybrid grating-based Fabry-Perot structure is proposed to investigate light manipulation in the near-infrared wavelength region. It is found that the electromagnetic energy can be easily trapped in different parts of the system at different polarization states. For TM polarization, numerical results show that two remarkable narrowband absorptance peaks appear owing to the excitation of critical coupling with guided mode resonance and Fabry-Perot resonance. While for TE polarization, only one narrowband absorptance peak is generated because only Fabry-Perot resonance is excited. The near-infrared spectral selectivity of the system can be tuned by changing the geometrical parameters. In addition, the spectral absorptance of the system can be optimized by applying gate voltage on graphene sheet to change graphene chemical potential. This valuable dual-band tunable narrowband absorber is a potential application for high-performance optoelectronic devices.

Tunable narrowband shortwave-infrared absorber made of a nanodisk-based metasurface and a phase-change material Ge2Sb2Te5 layer.

A tunable absorber made of a nanodisk-based metasurface is proposed to realize a narrowband shortwave-infrared (SWIR) perfect absorption. By introducing a phase-change material Ge2Sb2Te5 (GST) layer, we produce a selective and active control of the optical response. It is found that the narrowband absorption of 99.9% can be achieved for amorphous GST (aGST) with a modulation depth of 54.6% at 1931 nm, which is attributed to the strong electric dipole resonance in the germanium nanodisks. Moreover, under the aGST state, the full width at half-maximum of 22 nm can be acquired for a normal TM-polarized wave, and such a nanodisk-based absorber enables a tunable operating wavelength by adjusting the geometrical parameters to realize the spectral selectivity. In addition, the nanodisk-based metasurface nanostructure, combined with a dielectric Bragg reflector with alternately stacked SiO2 and TiO2 layers, can realize the SWIR dual-band absorption for aGST and single-band absorption for crystalline GST through the adjustment of electric and magnetic resonances. The designed absorbers have the potential applications in tunable absorption filter, thermal sensing, and optical signal processing.

Multichannel tunable narrowband mid-infrared optical filter based on phase-change material Ge2Sb2Te5 defect layers.

The defect mode, which exists at the defect layer of a one-dimensional photonic crystal (1DPhC) heterostructure, provides the possibility of realizing narrowband transmission owing to its strong electromagnetic field localized effects. In this study, we numerically investigate the single- and multichannel narrowband filters in the mid-infrared region based on the defect mode by adding the monolayer and multilayer phase-change material Ge2Sb2Te5 (GST) defect layer in 1DPhC. It can provide a promising avenue to tune the transmission spectra by changing the crystallization fraction X of the GST defect layer. The remarkable narrowband transmission enhancement can be acquired for both TM and TE polarizations in spite of the large oblique incident angle. Such a defect-mode-based 1DPhC heterostructure enables tunable operating wavelength by adjusting geometrical parameters to realize the spectral selectivity of the filter in the mid-infrared region. The significant improvement and tunability of the designed single- or multichannel filters can be applied to biochemical sensing and material characterization.

Plasmon-enhanced broadband absorption of MoS2-based structure using Au nanoparticles.

The light absorption of a hybrid novel MoS2-based nanostructure is theoretically investigated by using the finite-difference time-domain (FDTD) simulations, and high-efficiency broadband absorption is achieved in the visible wavelength region. The enhancement of localized electromagnetic field owing to that localized surface plasmon resonances (LSPRs) supported by Au nanoparticles (NPs) can be used to enhance the absorption of MoS2, and the localized absorption of monolayer MoS2 are remarkably enhanced up from about 18.3% and 4.6% to about 55.2% and 84.8% at the resonant wavelengths of 467.7 nm and 557.8 nm, respectively. Furthermore, the effects of radii of Au NPs, period of Au NPs array, Au@Si NPs core-shell ratios, period numbers of the distributed Bragg mirror (DBR), and incident angle on the absorption of the proposed nanostructure have been systematically investigated. The similar design idea to enhance the light-MoS2 interaction can also be applied to other transition-metal dichalcogenides (TMDCs). This work will contribute to the design of TMDCs-based nanophotonic and optoelectronic devices.

Highly efficient narrow-band absorption of a graphene-based Fabry-Perot structure at telecommunication wavelengths.

We numerically investigate a novel and competitive graphene-based Fabry-Perot (GFP) structure to enhance the light-matter interaction of graphene at telecommunication wavelengths, and highly efficient narrow-band absorption is achieved. The absorptance of the GFP structure can reach near-unity by optimizing the position of graphene in the dielectric layer, and the localized absorptance of graphene at telecommunication wavelengths can be improved from 2.3% to 83.2%, which is attributed to the strong field confinement of Fabry-Perot resonance in the dielectric layer. The remarkable enhancement of graphene absorption can be acquired for both TM and TE polarizations. Such a graphene-based structure enables a tunable operating wavelength by adjusting geometrical parameters to realize the spectral selectivity of the system in the near-infrared range. Furthermore, the optimized GFP structure possesses excellent spectral selectivity with the full width at half-maximum of 33 nm. The meaningful improvement and tunability of graphene absorption can provide a promising prospect for the realization of high-performance graphene-based optoelectronic devices.

Two-dimensional trilayer grating with a metal/insulator/metal structure as a thermophotovoltaic emitter.

A thermophotovoltaic system that converts thermal energy into electricity has considerable potential for applications in energy utilization fields. However, intensive emission in a wide spectral and angular range remains a challenge in improving system efficiency. This study proposes the use of a 2D trilayer grating with a tungsten/silica/tungsten (W/SiO2/W) structure on a tungsten substrate as a thermophotovoltaic emitter. The finite-difference time-domain method is employed to simulate the radiative properties of the proposed structure. A broadband high emittance with an average spectral emittance of 0.953 between 600 and 1800 nm can be obtained for both transverse magnetic and transverse electric polarized waves. On the basis of the inductance-capacitance circuit model and dispersion relation analyses, this phenomenon is mainly considered as the combined contribution of surface plasmon polaritons and magnetic polaritons. A parametric study is also conducted on the emittance spectrum of the proposed structure, considering geometric parameters, polar angles, and azimuthal angles for both TM and TE waves. The study demonstrates that the emitter has good wavelength selectivity and polarization insensitivity in a wide geometric and angular range.

Experimental Studies on the Combustion Characteristics of Multisource Organic Solid Waste for Collaborative Disposal Using Municipal Solid Waste Incinerators.

This study investigated the evolution of furnace conditions during the heat conversion process of multisource organic solid waste. To achieve this, combustion tests involving different sludge mixing ratios, variable load operation, and multisource organic solid waste collaborative disposal were performed on a 750 t/d new municipal solid waste incineration grate furnace. The test results revealed that as the sludge mixing ratios increased from 0 to 10 and 20%, the temperature level in the furnace decreased and the fuel-type NOx emission increased. Moreover, the sludge featured poor combustion stability under low-load conditions owing to fluctuations in its calorific value and moisture content. Field tests of multisource organic solid waste revealed that after mixing waste cloth strips and papermaking waste, the temperature level in the furnace increased. Additionally, the emissivity distribution was positively correlated with the furnace flame temperature distribution, and NOx emissions also increased. The overall results indicated the feasibility of controlling the mixing rate of different organic solid wastes in the municipal solid waste incinerator within a reasonable range for cooperative incineration.

Effects of Acetylene Addition to the Fuel Stream on Soot Formation and Flame Properties in an Axisymmetric Laminar Coflow Ethylene/Air Diffusion Flame.

The effects of adding acetylene to the fuel stream on soot formation and flame properties were investigated numerically in a laminar axisymmetric coflow ethylene/air diffusion flame using the open-source flame code Co-Flame in conjunction with an elementary gas-phase chemistry scheme and detailed transport and thermodynamic database. Radiation heat transfer of the radiating gases (H2O, C2H2, CO, and CO2) and soot was calculated using a statistical narrow-band correlated-k-based wide band model coupled with the discrete-ordinates method. The soot formation was described by the consecutive steps of soot nucleation, surface growth of soot particles via polycyclic aromatic hydrocarbons (PAHs)-soot condensation or the hydrogen abstraction acetylene addition (HACA) mechanism, and soot oxidation. The added acetylene affected the flame structure and soot concentration through not only chemical reactions among different species but also radiation effects. The chemical effect due to the added acetylene had a significant impact on soot formation. Specifically, it was confirmed that the addition of 10% acetylene caused an increase in the peak soot volumetric fraction (SVF) by 14.9% and the peak particle number density by about 21.1% (z = 1.5 cm). Furthermore, increasing acetylene concentration led to higher concentrations of propargyl, benzene, and PAHs and consequently directly enhanced soot nucleation rates. In addition, the increased H mole fractions also accentuated the soot surface growth. In contrast, the radiation effect of the addition of 10% acetylene was much weaker, resulting in slightly lower flame temperature and SVF, which in turn reduced the radiant heat loss.