Epsilon-near-zero characteristics of near-field radiative heat transfer between α-MoO3 slabs.

Epsilon-near-zero (ENZ) materials, which manifest a wealth of exotic optical characteristics, have attracted significant research interest in recent years. However, these characteristics have rarely been considered in the study of near-field radiative heat transfer (NFRHT). In this work, we investigated the ENZ characteristics of the NFRHT between two symmetric biaxial α-MoO3 slabs. The numerical results show that the NFRHT is greatly enhanced around the ENZ frequency of 1.604 × 1014 rad s-1. Notably, near the other two ENZ frequencies (1.832 × 1014 rad s-1 and 1.891 × 1014 rad s-1), only the NFRHT between α-MoO3 slabs of certain thicknesses is enhanced. The reasons can be explained by the fact that the VHPs can be excited in almost all azimuthal angles at angular frequencies of 1.604 × 1014 rad s-1 and 1.891 × 1014 rad s-1. For the ENZ frequency of 1.832 × 1014 rad s-1, the VHPs can be excited at the top and bottom, while the SHPs excited at the left and right sides. It is worth noting that both the hyperbolic and ENZ characteristics affect the NFRHT between α-MoO3 slabs. Moreover, the excitation is strongly dependent on the thickness of the slab. Our findings contribute to understanding the physical mechanisms underlying the characteristics of the NFRHT at ENZ frequencies.

Metasurfaces Assisted Twisted α-MoO3 for Spinning Thermal Radiation.

Spinning thermal radiation has demonstrated applications in engineering, such as radiation detection and biosensing. In this paper, we propose a new spin thermal radiation emitter composed of the twisted bilayer α-MoO3 metasurface; in our study, it provided more degrees of freedom to control circular dichroism by artificially modifying the filling factor of the metasurface. In addition, circular dichroism was significantly enhanced by introducing a new degree of freedom (filling factor), with a value that could reach 0.9. Strong-spin thermal radiation resulted from the polarization conversion of circularly polarized waves using the α-MoO3 metasurface and selective transmission of linearly polarized waves by the substrate. This allowed for extra flexible control of spinning thermal radiation and significantly enhanced circular dichroism, which promises applications in biosensing and radiation detection. As a result of their unique properties, hyperbolic materials have applications not only in spin thermal radiation, but also in areas such as near-field thermal radiation. In this study, hyperbolic materials were combined with metasurfaces to offer a new idea regarding modulating near-field radiative heat transfer.

Optical axis-driven modulation of near-field radiative heat transfer between two calcite parallel structures.

Recently, the increasing research on the anisotropic optical axis (OA) has provided a novel way to control light. However, this method is rarely applied to modulate the near-field radiative heat transfer (NFRHT). In this work, we investigate the influences of the OA orientation of calcite on the NFRHT between two calcite parallel structures. The numerical results demonstrate that the near-field radiative heat flux is larger when the OA is along the z-axis than that when the OA is along the x-axis. This is because when the OA is along the z-axis, the excited hyperbolic polaritons exhibit a full range of angles in the type I hyperbolic band. In contrast, the excited hyperbolic polaritons exhibit a finite angle when the OA is along the x-axis. Moreover, it is further investigated that the thickness has a significant impact on the NFRHT between calcite slabs. Our findings may highlight the promising role of calcite in manipulating NFRHT.

Tunable high-quality-factor absorption in a graphene monolayer based on quasi-bound states in the continuum.

A tunable graphene absorber, composed of a graphene monolayer and a substrate spaced by a subwavelength dielectric grating, is proposed and investigated. Strong light absorption in the graphene monolayer is achieved due to the formation of embedded optical quasi-bound states in the continuum in the subwavelength dielectric grating. The physical origin of the absorption with high quality factor is examined by investigating the electromagnetic field distributions. Interestingly, we found that the proposed absorber possesses high spatial directivity and performs similar to an antenna, which can also be utilized as a thermal emitter. Besides, the spectral position of the absorption peak can not only be adjusted by changing the geometrical parameters of dielectric grating, but it is also tunable by a small change in the Fermi level of the graphene sheet. This novel scheme to tune the absorption of graphene may find potential applications for the realization of ultrasensitive biosensors, photodetectors, and narrow-band filters.

Pore-Level Study of Syngas Production From Fuel-Rich Partial Oxidation in a Simplified Two-Layer Burner.

We performed pore-level simulation of fuel-rich partial oxidation of a CO2/CH4 mixture in a two-dimensional porous burner with staggered arrangement of discrete particles. The chemistry was treated with detailed chemical kinetics GRI-Mech 1.2, and surface-to-surface radiation was taken into account by the discrete ordinates (DO) model. The predicted results were validated against the available experimental data and results by the volume-averaged method. The predicted main syngas products (CO, H2, and CO2) agreed well with the experimental data for the whole investigation range; it indicated that the pore-level simulation could precisely predict syngas productions from fuel-rich partial oxidation in a two-layer burner with the simplified arrangement of porous media. Variations of species, temperature, and velocity within the pores were presented and discussed. The predicted molar fractions of CO, H2, CO2, H2O, etc. over the pores between particles were highly two-dimensional; the flame thickness was on the order of the particle diameter (2.5 mm) and smaller than the particle diameter. The predicted area-weighted average temperatures were greater than the experiments due to the ignorance of the heat loss to the surroundings through burner walls. The effect of CO2 adding on syngas production is examined.