Four-state programmable nonreciprocal thermal emission in Weyl-semimetal-assisted phase-change multilayers

Programmable nonreciprocal thermal emitters are promising for reconfigurable infrared sources, thermal camouflage, and direction-selective radiative control, yet realizing multiple stable nonreciprocal emission states in a compact passive platform remains challenging. Here, we numerically demonstrate a four-state nonreciprocal thermal emitter based on a Weyl-semimetal-assisted phase-change multilayer composed of an air spacer, Ge2Sb2Te5 (GST), BaF2, Sb2S3, a Weyl semimetal, and a Mo back reflector. Four nonvolatile optical states are encoded by independently switching GST and Sb2S3 between their amorphous and crystalline phases. The phase configuration modulates both the spectral position and the magnitude of the directional emissivity contrast across the 8–14 μm atmospheric infrared window. At the nominal incidence condition, the peak contrast reaches 0.779 at 13.12 μm for amorphous-GST/amorphous-Sb2S3, 0.916 at 9.70 μm for crystalline-GST/amorphous-Sb2S3, and 0.764 at 10.63 μm for crystalline-GST/crystalline-Sb2S3, while amorphous-GST/crystalline-Sb2S3 intentionally provides a spectrally identifiable low-contrast state at this angle. Angular and azimuthal maps further show that the same phase configuration can recover strong directionality at other observation angles, with angularly optimized contrasts exceeding 0.86 for all four states. The analysis clarifies the mechanism, robustness, and practical limitations of the proposed theory-guided design.