Ultrafast laser-driven THz sources for time-domain spectroscopy are a ubiquitous tool in many fields of science and are even starting to penetrate industrial fields. Whereas enormous progress has been achieved in achieving ultra-broadband operation and very high fields in the last decades, average power has for very long remained a bottleneck for many applications in spectroscopy and sensing. this talk, we will review recent progress in the generation and application of high-average power broadband Terahertz pulsed sources at MHz repetition rates for time-domain spectroscopy.

We present the generation of broadband terahertz radiation with the two-color gas plasma scheme. Driven by a multipass cell compressed high-power fiber laser system at a repetition rate of 200 kHz an average power of 94 mW is generated. A measurement based on the air-biased coherent detection scheme is used to characterize the full bandwidth of the generated pulses. By using the full potential of the driving laser further scaling of the generated THz power towards watt-level average power can be expected in the near future.

We report on a direct all optical encoding of a free-space terahertz signal onto an optical signal probing the absorption of CdSe/CdS quantum dots. The ultrafast electro-absorption modulator is based on the quantum confined stark effect. This method demonstrates the ability for high speed modulators with transition rates in the Tbit/s range and extinction ratio exceeding 6 dB. An extreme change in transmission of more than 15% is measured, which is, to the best of our knowledge, the highest value ever reported for solution processed electro-absorption materials at room temperature.

We present the methods for controlled pulse burst amplification in a single and dual femtosecond regenerative amplifier that enable a plethora of third and higher-order nonlinear spectroscopic techniques. High-resolution time-delay scans and narrowband frequency sweeping is obtained without any mechanically moving parts.