单斜晶体中太赫兹波的产生和传播性质

Among many laser crystals and nonlinear crystals, there are few monoclinic crystals. The study on electromagnetic responses of monoclinic crystals is also not sufficient. Recently, we find that a new phenomenon in BaGa4Se7(BGSe) crystal: the THz radiation generated by fs lasers shows a narrow band spectrum that corresponding to phonon frequencies in the crystal, and the THz spectra and polarization states do not depend on the polarization of the optical pulses, as well as the excitation configuration. Also, in our polarization sensitive THz-TDS measurements, it is confirmed that there exists a phonon-polariton regulated propagation of THz waves. Therefore, the travelling polariton wave plays a key role in our observations. Because monoclinic crystals have unique symmetric sreucture and electromagnetic responses, the study on interaction of electromagnetic field and monoclinic crystal has significant importance in science and related applications. This proposal is trying to study the property of electromagnetic excitation in monoclinic crystals, including the mechanism of polariton generation, propagation and probable applications for development of THz functional devices.

在激光晶体和非线性光学晶体中，很少用到单斜晶体，迄今对其电磁响应特性的研究也很不充分。最近，我们在BaGa4Se7(BGSe)晶体中发现一个全新的现象：飞秒激光脉冲激发的窄带太赫兹辐射直接对应于晶体的声子极化激元振荡频率，而太赫兹发射谱分布和偏振状态基本不依赖于激发光脉冲的偏振特性、以及晶体方位角等几何安排；通过偏振敏感的太赫兹时域透射光谱测量和分析，我们进一步确认所观察到的太赫兹发射和传播特性均涉及到晶体中极化激元(polariton)的相干激发和传播。由于单斜晶体具有独特的结构和电磁响应特性，研究此类材料和电磁场的相互作用特性具有重要的科学和应用价值。本项目将研究单斜晶体中电磁激发的特性，特别是极化激元波的激发和传播机理，以及可能的太赫兹功能材料和器件应用。

本项目研究目标为探讨单斜晶体在太赫兹频率下的电磁响应机理，以及基于材料物性的太赫兹电磁场产生和传播特性。利用我们实验室自己发展的具有高灵敏度偏振测量能力的THz-TDS相干测量系统，对BGSe晶体在近红外激光以不同入射几何条件泵浦下所产生的太赫兹透射信号进行了高精度的实验测量，并结合拉曼光谱测量和分析，确认了在特定的共振激发频率下，光学声子与太赫兹电磁场之间的强耦合(极化激元)可以完全支配此类晶体中太赫兹电磁场的传播；理论分析和实验测量进一步确认了沿单斜晶体对称轴方向传播的本征模式为两个主轴相互垂直的具有相同椭圆率和手性的椭圆偏振光，表明通常描述电磁场在各向异性晶体中采用的折射率椭圆分析模型在单斜晶体的情况下不成立。这些结果有助于发展基于高效相干声子激发的光电晶体材料以及基于其中独特激发特征的太赫兹光电器件。