High-field high-bandwidth terahertz pulses: Generation in laser-excited gas plasmas and coherent detection in biased-air waveguides

利用飞秒放大激光系统产生和检测宽带太赫兹(THz)脉冲，可以同时获得高太赫兹峰值场和高带宽。基于这些技术的太赫兹光谱仪为观察远红外到中红外的非线性太赫兹现象和超快激发态动力学开辟了新的可能性。该项目涉及使用双色光致气体等离子体产生太赫兹，以及使用太赫兹场诱导二次谐波(TFISH)在气体中进行相干检测。有关等离子体-THz产生的基本问题仍然悬而未决，特别是在等离子体介质中的光学-THz共传播以及由此产生的相位匹配和饱和对发射THz场的影响。我们打算通过实验和理论研究的新组合来解决这些问题。对于GAS-TFISH检测方法，我们的目标是通过增加THz波导结构来增加相互作用长度来扩展其灵敏度。通过优化波导和太赫兹光束的几何结构，我们预计相对于现有的自由空间实施方案，场敏感度将至少提高一个数量级。该项目的一个关键成果将是实现优化的太赫兹光谱分析系统，其焦点太赫兹场为数百千伏/厘米，频率响应平坦，扩展范围超过数十太赫兹。

The use of femtosecond amplifier laser systems for the generation and detection of broadband terahertz (THz) pulses allows one to achieve both high THz peak fields and high bandwidth. THz spectroscopies based on these techniques are opening up new possibilities to observe non-linear THz phenomena and ultra-fast excited-state dynamics in the far- to mid-infrared. This project concerns the use of two-colour photoinduced gas plasmas for THz generation, and THz-field-induced second-harmonic generation (TFISH) in gases for coherent detection. Fundamental questions remain open concerning the physics of the plasma-THz generation, in particular the optical-THz co-propagation in the plasma medium and the resultant impact of phase-matching and saturation on the emitted THz field. We intend to address these issues using a novel combination of experimental and theoretical investigations. For the gas-TFISH detection method, we aim to extend its sensitivity by the addition of a THz waveguide structure to increase the interaction length. By optimisation of the waveguide and THz beam geometries, we anticipate at least an order of magnitude field-sensitivity improvement relative to the existing free-space implementation. A key result of this project will be the realisation of an optimised THz spectroscopy system with focal THz fields of hundreds of kV/cm and a flat frequency response extending over tens of THz.