Symmetry plays a central role in conventional and topological phases of matter, making the ability to optically drive symmetry changes a critical step in developing future technologies that rely on such control. Topological materials, like topological semimetals, are particularly sensitive to a breaking or restoring of time-reversal and crystalline symmetries, which affect both bulk and surface electronic states. While previous studies have focused on controlling symmetry via coupling to the crystal lattice, we demonstrate here an all-electronic mechanism based on photocurrent generation. Using second harmonic generation spectroscopy as a sensitive probe of symmetry changes, we observe an ultrafast breaking of time-reversal and spatial symmetries following femtosecond optical excitation in the prototypical type-I Weyl semimetal TaAs. Our results show that optically driven photocurrents can be tailored to explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast timescales.

对称性在物质的常规相和拓扑相中起着核心作用，使得光学驱动对称性变化的能力成为开发依赖于这种控制的未来技术的关键步骤。拓扑材料，如拓扑半金属，对时间反演和晶体对称性的破坏或恢复特别敏感，这会影响体电子态和表面电子态。虽然以前的研究主要集中在通过耦合到晶格来控制对称性，但我们在这里展示了一种基于光电流产生的全电子机制。我们使用二次谐波产生光谱作为对称性变化的灵敏探针，在原型 I 型外尔半金属 TaAs 中观察到飞秒光激发后时间反转和空间对称性的超快破坏。