Ultrathin Nonreciprocal Thermal Absorbers Enabled by Anomalous Hall Effect and Fizeau Drag in Dirac and Weyl Semimetals

Fundamental limits of thermal radiation are imposed by Kirchhoff’s law, which assumes the electromagnetic reciprocity of a material or material system. Thus, breaking reciprocity can enable breaking barriers in thermal efficiency engineering¹. We present 1D photonic crystals composed of Weyl and Dirac semimetal and dielectric layers, whose structures are optimized to maximize the nonreciprocity of infrared radiation absorptance and emittance in planar and compact designs. Two different mechanisms to enable nonreciprocal infrared absorbers are simulated and compared – the anomalous Hall effect in Weyl semimetals² and electric-current-induced Fizeau drag in either Dirac or Weyl semimetals³. To engineer an ultra-compact absorber structure that does not require gratings or prisms to couple light, we used a genetic algorithm (GA) to maximize nonreciprocity in the design globally, followed by the application of the numerical gradient ascent (GAGA) algorithm as a local optimization to further enhance the design⁴. The first absorber design takes advantage of the intrinsic nonreciprocity of time-reversal symmetry (TRS) breaking Weyl semimetals due to their pseudomagnetic field in momentum space. We then apply the GAGA methodology to design and optimize a flat absorber using inversion (I)-symmetry breaking Weyl and Dirac semimetals as active layers, in which tunable nonreciprocity is induced through an applied DC current bias. This momentum bias imparts plasmon Fizeau drag, the drag of an electrical current on propagating surface plasmon polaritons (SPPs). We use a semi-classical theory recently developed in our lab to model SPP transport along interfaces of 3D semimetals under Fizeau drag³. Lastly, in both designs the optimization algorithm accounts for both s- and p-polarized absorptance spectra in the mid- to far-IR to create a final design suitable for thermal applications, which maximizes the nonreciprocal absorptance of p-polarized light and simultaneously minimizes the parasitic, reciprocal absorptance of s-polarized light.<br/><br/>1. Boriskina, S. V, Blevins, M. & Pajovic, S. The Nonreciprocal Adventures of Light. Opt. Photonics News 33, 46–53 (2022).<br/>2. Tsurimaki, Y., Qian, X., Pajovic, S., Han, F., Li, M. & Chen, G. Large nonreciprocal absorption and emission of radiation in type-I Weyl semimetals with time reversal symmetry breaking. Phys. Rev. B 101, 165426 (2020).<br/>3. Blevins, M. G. & Boriskina, S. V. Plasmon Fizeau Drag in 3D Dirac and Weyl Semimetals. ACS Photonics 11, 537–549 (2024).<br/>4. Gold, H., Pajovic, S., Mukherjee, A. & Boriskina, S. V. GAGA for nonreciprocal emitters: genetic algorithm gradient ascent optimization of compact magnetophotonic crystals. Nanophotonics, 13(5), 773–792 (2024).