Following the ultrafast photoexcitation of a semiconductor, it embodies competing dynamics among photocarriers, many-body transient states of highly energetic excitons, and electron-hole liquid. Here, we show that femtosecond optical pulse excitation induces transient excitonic shift current contributing to stronger THz emission from a single crystalline bulk MoS2 at low temperatures. The control of dominating excitonic shift current is elucidated from excitation density dependent experiments at varying temperatures. A strong decrease in the excitonic contribution beyond a critical fluence of 150microJ/cm^2 is observed at a very low temperature of 20K. This behavior suggests the formation of a new quantum condensate, i.e., the electron-hole liquid, in the regime when the exciton density is overwhelmingly large that the average spacing between exciton pairs is comparable to the exciton radius. Furthermore, the exciton density dependent THz emission at varying temperatures is consistent with the Varshni model and the crystal Debye temperature of 260K.
太赫兹光谱范围约为0.1至15 THz，是最少探索但技术变革最多的光谱区域之一。当前的一个挑战是开发具有宽带和无间隙光谱的高效和紧凑的太赫兹发射器/检测器，其可以针对各种泵浦光子能量进行定制。在这里，我们展示了有效的单周期宽带太赫兹产生，范围从约0.1至4太赫兹，从一个薄的层的开口环谐振器与几十纳米的厚度在1.5微米（200太赫兹）的电信波长泵浦。太赫兹辐射是由开口环谐振器的磁偶极共振激发产生的，在非共振泵浦下会迅速衰减。这，连同泵浦偏振依赖性和功率缩放的太赫兹发射，确定了在开口环谐振器中的光致非线性电流的作用。我们还揭示了一个巨大的片非线性极化率$\sim$ 10 $^{-16}$ m $^2$ V $^{-1}$，远远超过薄膜和散装非中心对称材料。
Phonon-related monochromatic THz radiation and its magneto-modulation in 2D ferromagnetic Cr2Ge2Te6
寻找多种类型的太赫兹辐射源是太赫兹技术的关键。在这里，通过利用一个二维（2D）的铁磁Cr2 Ge 2 Te 6晶体，我们首次展示了磁可调谐单色太赫兹辐射源。在低光子能量的宽带太赫兹泵浦下，可以产生频率约为0.9THz，带宽约为0.25THz的强太赫兹辐射，在160 K时转换效率可达2.1%。此外，有趣的是，发现这种单色THz辐射可以有效地调制低于160 K的磁场。根据实验和理论分析，出射的THz辐射被确定为声子极化激元的辐射，其温度和磁场依赖行为证实了该二维铁磁晶体中存在大的自旋-晶格耦合。这些观测结果为可调谐单色太赫兹光源的产生提供了一条新的途径，在光子学和自旋电子学器件的未来应用中可能具有巨大的实际意义。
Time-resolved broadband analysis of slow-light propagation and superluminal transmission of electromagnetic waves in three-dimensional photonic crystals
Core-level signature of long-range density-wave order and short-range excitonic correlations probed by attosecond broadband spectroscopy
University of California at Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Center for Computational Sciences, University of Tsukuba（筑波大学计算科学中心）；Max Planck Institute for the Structure and Dynamics of Matter（马普物质结构与动力学研究所）；Tsung-Dao Lee Institute and Zhangjiang Institute for Advanced Study, Shanghai Jiao Tong University（上海交通大学桐江书院与张江高等研究院）
