It was proved that the joint operation of electromagnetic reciprocity and n-fold (n ≥ 3) rotational symmetry would secure arbitrary polarization-independent backscattering efficiency [Phys. Rev. B 103(4), 045422 (2021) [CrossRef] ]. Here we remove the restriction of reciprocity and study the backscatterings of plane waves by rotationally symmetric magneto-optical structures, with collinear incident wavevector, rotational axis and externally applied magnetic field. It is revealed that though nonreciprocity removes the degeneracy of backscattering efficiencies for circularly-polarized incident waves of opposite handedness, the remaining rotational symmetry is sufficient to guarantee that the efficiency is related to the polarization ellipticity only, having nothing to do with the orientations of the polarization ellipses. Moreover, the backscattering efficiency reaches its extremes (maximum or minimum values) always for circularly-polarized incident waves, and for other polarizations the efficiency is their ellipticity-weighted arithmetic average. The principles we have revealed are dictated by rotational symmetries only, which are irrelevant to specific geometric or optical parameters and are intrinsically robust against any rotational-symmetry preserving perturbations. The correlations we have discovered could be further exploited for fundamental explorations in nonreciprocal photonics and practical applications including polarimetry and ellipsometry.

中文翻译：

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旋转对称非互易结构的磁手性反向散射

事实证明，电磁互易性和n倍（n≥3）旋转对称性的联合作用将确保任意与偏振无关的后向散射效率[Phys. Rev. B 103 (4), 045422 (2021) [交叉引用] ]。在这里，我们消除了互易性的限制，并研究了具有共线入射波矢、旋转轴和外加磁场的旋转对称磁光结构的平面波后向散射。结果表明，虽然非互易性消除了旋向相反的圆偏振入射波后向散射效率的简并性，但剩余的旋转对称性足以保证效率仅与偏振椭圆率有关，而与偏振方向无关。偏振椭圆。此外，对于圆偏振入射波，后向散射效率总是达到其极值（最大值或最小值），而对于其他偏振，效率是其椭圆率加权算术平均值。我们揭示的原理仅由旋转对称性决定，与特定的几何或光学参数无关，并且本质上对任何保持旋转对称性的扰动具有鲁棒性。我们发现的相关性可以进一步用于非互易光子学的基础探索和包括偏振测量和椭圆测量在内的实际应用。