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Hyperbolic metamaterials (HMMs) are artificial subwavelength-structured media that exhibit excellent hyperbolic properties. The Casimir force for HMMs can be predicted by effective medium theory (EMT), which simplifies the calculations considerably. However, the conditions of EMT in the Casimir force between HMMs have not been investigated. In this work, we compare the exact results of the Casimir force calculated by the transfer matrix method (TMM) with those from EMT. The numerical results suggest that EMT can accurately calculate the Casimir force only when the gap distance is 100 times larger than the period of a unit cell. When the gap distance and period of unit cell values are comparable, EMT is not suitable for TMM, and the main error occurs in the hyperbolic band. Furthermore, the effect of the filling ratio on the applicable conditions of EMT is also studied. This work demonstrates the conditions of EMT in calculating the Casimir force between HMMs and provides help for the development of micro- and nanoelectromechanical systems. [ABSTRACT FROM AUTHOR]

Recent studies have suggested that Weyl semimetals were the promising materials to verify Kirchhoff's law for nonreciprocal materials in experiment. Nevertheless, existing designs based on Weyl semimetals could not achieve perfect nonreciprocal radiation around a wavelength of 10 µm at small angles. Therefore, it is of significant importance to design structures that can realize perfect nonreciprocal radiation at a shorter wavelength and smaller angle. Here, by using attenuated total reflection, we demonstrate that perfect nonreciprocal radiation can be realized at a wavelength of 10 µm at an angle of 30°. The difference between directional emissivity and absorptivity is as large as 0.99, which is the best result until now, as far as we know. The perfect nonreciprocal radiation is attributed to the nonreciprocal guided resonances in the Weyl semimetal film, which is confirmed by the distribution of magnetic field and dispersion relation. Such a design is promising in verifying Kirchhoff's law for nonreciprocal materials in experiment. [ABSTRACT FROM AUTHOR]

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]

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]

The effects of Bi3+ substitution on the microstructures and properties of low-temperature fired NiCuZn ferrites have been investigated. It is found that Bi3+ ions can enter into the ferrite lattice and consequently enhance the grain growth and densification in the sintering process due to the activation of the lattice, which in turn lead to an increase of quality factor and static permeability. Bi3+ substitution for Fe3+ is beneficial to forming the spinal phase and lowers the sintering temperature to about 900 °C. Pcv (power loss) was analyzed by dividing Pcv into Ph (hysteresis loss) and Pe+Pr (eddy current loss and residual loss) from the frequency dependence of the power loss. Due to the compact and uniform microstructures, and high sintering density, relatively low losses were obtained in the Bi-substituted samples. [ABSTRACT FROM AUTHOR]