Iguchi S, Masuda R, Seki S, Tokura Y, Takahashi Y
Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo, 113-8656, Japan.
Institute of Engineering Innovation, University of Tokyo, Tokyo, 113-0032, Japan.
Nat Commun. 2021 Nov 18;12(1):6674. doi: 10.1038/s41467-021-26953-x.
Spontaneous symmetry breaking in crystalline solid often produces exotic nonreciprocal phenomena. As one such example, the unconventional optical rotation with nonreciprocity, which is termed gyrotropic birefringence, is expected to emerge from the magnetoelectric coupling. However, the fundamental nature of gyrotropic birefringence remains to be examined. Here w`e demonstrate the gyrotropic birefringence enhanced by the dynamical magnetoelectric coupling on the electrically active magnon resonance, i.e. electromagnon, in a multiferroic helimagnet. The helical spin order having both polarity and chirality is found to cause the giant gyrotropic birefringence in addition to the conventional gyrotropy, i.e. natural optical activity. It is demonstrated that the optical rotation of gyrotropic birefringence can be viewed as the nonreciprocal rotation of the optical principal axes, while the crystallographic and magnetic anisotropies are intact. The independent control of the nonreciprocal linear (gyrotropic birefringence) and circular (natural optical activity) birefringence/dichroism paves a way for the optically active devices.
晶体固体中的自发对称性破缺常常会产生奇特的非互易现象。作为其中一个例子，由磁电耦合有望产生具有非互易性的非常规旋光性，即回转双折射。然而，回转双折射的基本性质仍有待研究。在此，我们展示了在多铁性螺旋磁体中，通过电活性磁振子共振（即电磁振子）上的动态磁电耦合增强的回转双折射。除了传统的旋光性（即自然旋光性）外，具有极性和手性的螺旋自旋序被发现会导致巨大的回转双折射。结果表明，回转双折射的旋光性可被视为光学主轴的非互易旋转，而晶体学和磁各向异性保持不变。对非互易线性（回转双折射）和圆（自然旋光性）双折射/二向色性的独立控制为光学活性器件开辟了道路。
