Researchers from the GPL Photonics Laboratory at the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, together with the University of Chinese Academy of Sciences, reported a route to unpolarized nonreciprocal thermal emitters in the IEEE Journal of Selected Topics in Quantum Electronics. The work explains why nonreciprocity—traditionally restricted to a single polarization under the transverse magneto-optical Kerr effect (TMOKE)—can be achieved for both polarizations, and it demonstrates device designs that enhance nonreciprocal radiative heat transfer.

Reciprocity is a bedrock principle in optics and thermal radiation: it links emission with absorption and makes forward and backward transport symmetric. Breaking that symmetry unlocks one-way heat flow and isolation, but conventional TMOKE devices only act on transverse-magnetic (TM) waves; their transverse-electric (TE) counterparts remain reciprocal. Prior attempts to reach dual-polarized nonreciprocity rotated the incident plane or relied on complex two-dimensional metasurfaces—useful, yet lacking a unifying explanation and often yielding disjoint angular or spectral response for each polarization. The new study formulates the “generalized transverse magneto-optical effect,” identifying transversal inhomogeneity and the transverse wave nature of light as the common mechanism that enables dual-polarized and even unpolarized nonreciprocity across many magneto-optical platforms.

The team focused on a practical class of one-dimensional gratings built from magneto-optical media, including Weyl semimetals that support strong gyrotropy near epsilon-near-zero wavelengths. In conventional “X-gratings,” TMOKE still favored only the TM polarization. By rotating the grating vector to create a “Y-grating,” they introduced a controlled transversal inhomogeneity that forced TE and TM fields to couple through Maxwell’s transversality condition. That coupling allowed TMOKE physics to manifest under TE excitation as well, enabling nonreciprocal emission for both polarizations without resorting to 2D patterning or rotated incidence. The analysis connected the effect to mode conversion at subwavelength bar sidewalls and to the mixed-polarization solutions permitted in inhomogeneous anisotropic media.

The research process proceeded in steps and used standard tools. The authors defined the magneto-optical permittivity tensor and adopted a TMOKE configuration with magnetization transverse to the incident plane. They then modeled thin films and gratings that combined high-index Ge layers with a Weyl-semimetal layer over a metallic mirror to suppress transmission. Rigorous coupled-wave analysis quantified angular and spectral emissivity, and a generalized dispersion treatment clarified when TE–TM mode conversion must occur in a transversal inhomogeneous layer. Finally, they evaluated radiative heat transfer using two figures of merit: the directional nonreciprocal radiative power (PNR) and the corresponding efficiency (ηNR), integrated over selected angles and wavelengths.

The results underscored three advantages. First, the Y-grating exhibited strong nonreciprocal thermal emission for both TM and TE polarizations, with substantial overlap in angle and spectrum. Overlap matters because unpolarized sources divide power between polarizations; if both carry nonreciprocity over the same angular-spectral region, the usable power grows. Second, the unpolarized emitter outperformed an idealized single-polarized nonreciprocal benchmark in the defined efficiency metric, indicating a practical boost for nonreciprocal radiative heat transfer. Third, the concept generalized across magneto-optical choices: Weyl semimetals were highlighted for their large gyrotropy, and prior experiments suggest thin-film deposition and thermal stability compatible with device operation.

Compared with earlier dual-polarized demonstrations that depended on rotating the incident plane or stacking resonators, this approach kept the incident plane fixed and used only a one-dimensional periodicity. The physical picture is intuitive: once a structure is inhomogeneous across the transverse direction, Maxwell’s equations tie the field components together, and magneto-optical intensity effects that normally act only on TM waves can influence TE-excited states through mode conversion. In that setting, TMOKE ceases to be “TM-only” and becomes generalized, enabling unpolarized nonreciprocity in reflection, absorption, and emission.

Potential applications are broad in thermal photonics. Unpolarized nonreciprocal emitters can channel heat preferentially without moving parts, improving thermal radiative circulators, directional heaters, and infrared camouflage systems. Because the Y-grating relies on mainstream materials processing and avoids intricate metasurface couplings, arrays could be scaled for large-area devices. The framework also provides a blueprint for designers: introduce transversal inhomogeneity that enforces TE–TM coexistence, choose magneto-optical media with strong gyrotropy, and target angular-spectral regions where the two polarizations overlap to maximize PNR and ηNR.

From a scientific standpoint, the paper clarifies how nonreciprocity, adjoint Kirchhoff’s law, and polarization interrelate in magneto-optical systems. In bulk, TMOKE breaks reciprocity only for TM waves, while TE obeys the equality of emissivity and absorptivity. In transversal inhomogeneous media, however, the TE channel acquires TM content via field transversality and boundary-induced coupling, allowing the nonreciprocal intensity effect to appear in TE-excited configurations—thereby resolving a long-standing polarization constraint. This insight sets a general principle that can guide new materials, geometries, and operational bands.