基于强场太赫兹时域光谱系统的太赫兹非线性人工电磁媒质的研究

Investigation of material nonlinear characteristics at THz frequency employing intense (electric field of THz pulse exceeding 500 kV/cm ) THz time-domain spectroscopy (THz-TDS) is a more recent research area,which will further promote the development of THz science and expand application areas of nonlinear optics. In this proposal, we aim to employ nonlinear materials/elements in metamaterials and dynamically study the nonlinear responses of these novel THz metamaterials. Systematic research includes investigation of nonlinear responses and magnetic-electric coupling of materials such as graphene and GaAs in cryogenic enviroment with strong magnetic field utilizing our home-made wide-bandwidth intense THz time-domain spectroscopy system, employing proper nonlinear materials/elements in metamaterials, and dynamic analysis and study of nonlinear properties of these novel nonlinear metamaterials in intense THz-TDS system. The intense THz source was obtained by optical rectification. By tilting the pulse front of the femtosecond laser pump pulses, velocity matching between the group velocity of the pump pulses and the phase velocity of the THz signals can be fulfilled in nonlinear LiNbO3 crystal. The generated THz pulse energies can be scaled up to several microjoules. The electric field of the THz pulse is approaching to 1 MV/cm. In this proposal, we design and fabricate various novel nonlinear metamaterials based on numerical simulations, theoretical analyses, and traditional photoetching technique. In cryogenic environment with strong magnetic field (1.5 K, 7 T), these novel nonlinear metamaterials will also be systematically investigated. By utilizing the observed new phenomena, it is promising to exploit novel terahertz nonlinear functional devices, which will further enrich the family of terahertz nonlinear functional devices.

利用强电场（>500 kV/cm）太赫兹时域光谱技术（THz-TDS）研究材料在太赫兹波段的非线性特性是最新兴的研究方向，可以推动太赫兹科学的发展并进一步扩展非线性光学的应用领域。本项目又将非线性材料引入到人工电磁媒质中，设计新型非线性人工电磁媒质并动态分析它们的非线性响应特性。系统研究包括：在集成了温控、磁控等调制机制的强电场THz-TDS系统中，对石墨烯、砷化镓等材料的非线性响应和磁电耦合特性进行动态分析和研究；运用获得的新现象和新机理有选择地将非线性材料引入到人工电磁媒质，以数值模拟和理论分析为基础，传统光刻技术为平台，设计并制备多种新型非线性人工电磁媒质；研究这些人工电磁媒质在普通环境和超低温（1.5 K）、强磁场(7 T)环境中的非线性电磁响应，通过运用获得的新机理，有望开发新型的太赫兹非线性功能器件，包括特殊环境中应用的太赫兹非线性功能器件，丰富太赫兹新型非线性功能器件家族。

本项目基于自主搭建的强场太赫兹时域光谱系统，提出旨在利用强场太赫兹光谱技术研究新型非线性人工电磁媒质的非线性特性。通过优化系统中器件的参数设置，获得的太赫兹电场>500 kV/cm，并在此基础上引入了光泵一路，搭建成了光泵-强场太赫兹时域光谱系统，可用于表征集成了光敏材料的太赫兹超材料的光谱特性。在此过程中进行了一些有意义的研究工作，包括：1）在由四个U型金属和一个金属棒组成的微结构单元中引入硅桥，通过光泵激励实现了宽带电磁诱导透明现象的主动调控；2）利用三个金属同心矩形环结构可以实现太赫兹波段的双带透射，在微结构中巧妙引入硅材料，光激励情况下可以实现对两个传输窗口的单独调控或者同时调控；3）通过在明模和暗模谐振结构中引入硅材料，可同时操控明模和暗模的谐振频率，进而实现对电磁诱导透明谐振频率的主动调控；4）在砷化镓基片上外延生长1 μm厚n型掺杂的砷化镓薄膜，并在其上镀上一层亚波长互补的C形缺口环实现对太赫兹波的相位操控，通过在金属层和掺杂砷化镓层加偏压，可实现对异常偏折的太赫兹波振幅的动态调控；5）将VO2薄膜引入太赫兹超材料金属微结构中，通过改变VO2薄膜的温度，实现其半导体相到金属相的转变，造成自由电子浓度的迅速升高，进而实现对太赫兹波的温度调控；6）初步研究了石墨烯与太赫兹超材料的结合机制，并测试和分析了强场作用下石墨烯对太赫兹波透射的影响。