Achieving high-performance polarization-independent nonreciprocal thermal radiation with pattern-free heterostructures

TL;DR

This paper addresses the challenge of achieving broadband, polarization-independent nonreciprocal thermal emission without patterned nanostructures. It proposes pattern-free multilayer heterostructures combining InAs (magneto-optical) and magnetic Weyl semimetals, leveraging magneto-optical Kerr effects and ENZ physics, and optimizes layer magnetizations using NSGA-II to maximize dual-polarization contrasts across a spectral band. Key findings show that a 2-layer InAs configuration and a 6-layer 3InAs+3Weyl stack can deliver substantial nonreciprocal contrasts for both $p$- and $s$-polarized waves, with the six-layer design achieving $\eta_p$ up to $23.6\%$ and $\eta_s$ up to $0.5\%$, and unpolarized performance around $22\%$. These results establish a fabrication-friendly route to high-performance, dual-polarized nonreciprocal thermal emitters and offer design insights for tailoring polarization-conversion effects across broad infrared bandwidths.

Abstract

Many advanced energy harvesting technologies rely on advanced control of thermal emission. Recently, it has been shown that the emissivity and absorptivity of thermal emitters can be controlled independently in nonreciprocal emitters. While significant progress has been made in engineering these nonreciprocal thermal emitters, realizing a highly efficient, pattern-free emitter capable of supporting dual-polarization nonreciprocal emission remains a challenging task. Existing solutions are largely based on metamaterials and exhibit polarization-dependent behavior. This work proposes pattern-free multilayer heterostructures combining magneto-optical and magnetic Weyl semimetal materials and systematically evaluates their nonreciprocal emission performance for p- and s-polarized waves. The findings show that omnidirectional polarization-independent nonreciprocity can be achieved utilizing multilayer structures with different magnetization directions that do not follow simple vector summation. To further enhance the performance, Pareto optimization is employed to tune the key design parameters, enabling the maximization of nonreciprocal thermal emission in a given wavelength range. This approach offers a versatile strategy for designing high-performance thermal emitters tailored for multi-objective optical functionalities.

Figures (7)

• Figure 1: Design of dual-polarized nonreciprocal thermal emitters. (a) Patterned structures enables narrowband directional nonreciprocity by triggering bound state in continuum (BIC) modes wu2025midfang2024dualxia2022circular. (b) Our proposed design realizes dual-polarized broadband omnidirectional nonreciprocal emission by optimizing the magnetization configuration in a multilayer heterostructure composed of magneto-optical materials and Weyl semimetals, without requiring intricate patterns or nanostructures. $\theta$ denotes polar angle and nonreciprocity is illustrated as contrast between emissivity and absorptivity in comparison to wavelength.
• Figure 2: (a) Schematic of radiative heat exchange between two blackbodies (A and B) and an emitter (E) under thermal equilibrium. (b) A pattern-free heterostructure consisting of InAs layers on top of Weyl semimetal layers on an Ag substrate. The incident wavevector $\mathbf{k}_{\mathrm{i}}$ is characterized by an azimuthal angle $\phi$ and a polar angle $\theta$. The azimuthal angle $\phi$ is the angle between $\mathbf{k}_{\mathrm{i}}$ and the $x$-axis and varies within $[-180^{\circ}, 180^{\circ}]$. The polar angle $\theta$ is defined as the angle between $\mathbf{k}_{\mathrm{i}}$ and the $z$-axis, ranging from $[0^{\circ}, 90^{\circ}]$. The angle $\varphi$ specifies the orientation of the external magnetic field $\mathbf{B}$ (or the momentum-separation vector $2\mathbf{b}$ in a Weyl semimetal) relative to the plane of incidence, and takes values within $[-180^{\circ}, 180^{\circ}]$. Counterclockwise direction corresponds to positive values.
• Figure 3: Polarization-independent contrasts for one- and two-layer InAs structures. (a) Schematic of the one-layer InAs structure with doping level $n_{e} = 3.5 \times 10^{17}\,\mathrm{atoms/cm}^3$ and the normalized contrast for (b) $s$ and (c) $p$ polarizations. (d) Two-layer InAs structure and the corresponding normalized contrast for (e) $s$ and (f) $p$ polarizations. The doping levels for the first and the second layers are $n_{e,1} = 3.5 \times 10^{17}\,\mathrm{atoms/cm}^3$ and $n_{e,2} = 5.5 \times 10^{17}\,\mathrm{atoms/cm}^3$, respectively. Calculations are done at $\lambda=30$$\mu m$ and $\phi=0^{\circ}$. Polar angle is fixed as $\theta=45^{\circ}$ for two-layer structure.
• Figure 4: The objective spaces and solutions for single- and bi-objective maximization problems.
• Figure 5: Optimization results for the 2-layer InAs structure. (a) Pareto fronts from two optimization trials for the 2-layer InAs structure and three representative solutions: solution (1) where $\eta_p$ is maximized, solution (2) where $\eta_p$ is zero, and solution (3) where $\eta_s$ is maximized. (b), (c) Directional $s$- and $p$-polarized contrasts for solutions (1), (2), and (3), respectively. Polar angle is fixed as $\theta=45^{\circ}$.