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

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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.