The charge-neutral exciton has been predicted to carry genuine photocurrent due to the geometric Berry phase of the electronic bands, if the inversion symmetry in a crystal is broken. We detect such exciton shift current in a prototypical polar semiconductor CdS by using terahertz emission spectroscopy. A distinct peak emerges in the photocurrent spectra at the energy of the exciton resonance, which is demonstrated to result from the distinct displacements of electrons and holes in real space within the excitons to produce a finite transient charge current at subpicosecond time scale. Our findings elucidate the Berry phase physics of the charge-neutral photoexcitations and also shed light on the novel energy harvesting mechanism by exciton generation.

中文翻译：

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非中心对称半导体 CdS 中超快激子位移电流的太赫兹发射光谱

如果晶体中的反转对称性被破坏，则由于电子带的几何 Berry 相，预计电荷中性激子将携带真正的光电流。我们通过使用太赫兹发射光谱检测了原型极性半导体 CdS 中的这种激子位移电流。在激子共振的能量下，光电流谱中出现了一个明显的峰值，这被证明是由于激子内真实空间中电子和空穴的不同位移导致在亚皮秒时间尺度上产生有限的瞬态充电电流。我们的研究结果阐明了电荷中性光激发的浆果相物理学，并阐明了通过激子产生的新型能量收集机制。