In a solid, the electronic subsystem can exhibit incipient order with lower point group symmetry than the crystal lattice. Ultrafast external fields that couple exclusively to electronic order parameters have rarely been investigated, however, despite their potential importance in inducing exotic effects. Here we show that when inversion symmetry is broken by the antiferromagnetic order in Cr₂O₃, transmitting a linearly polarized light pulse through the crystal gives rise to an in-plane rotational symmetry-breaking (from C₃ to C₁) via optical rectification. Using interferometric time-resolved second harmonic generation, we show that the ultrafast timescale of the symmetry reduction is indicative of a purely electronic response; the underlying spin and crystal structures remain unaffected. The symmetry-broken state exhibits a dipole moment, and its polar axis can be controlled with the incident light. Our results establish a coherent nonlinear optical protocol by which to break electronic symmetries and produce unconventional electronic effects in solids.

在固体中，电子子系统可以表现出具有比晶格更低的点群对称性的初始有序性。然而，尽管超快外场在诱发奇异效应方面具有潜在的重要性，但专门与电子序参数耦合的超快外场却很少被研究。在这里，我们表明，当 Cr ₂ O ₃中的反铁磁序破坏反演对称性时，通过晶体传输线偏振光脉冲会通过光学整流产生面内旋转对称性破缺（从C ₃到C ₁） 。使用干涉时间分辨二次谐波产生，我们表明对称性降低的超快时间尺度表明了纯电子响应；底层的自旋和晶体结构不受影响。对称破缺态表现出偶极矩，其极轴可以通过入射光控制。我们的研究结果建立了一种相干非线性光学协议，通过该协议可以打破电子对称性并在固体中产生非常规电子效应。