  We present, to the best of our knowledge, the highest average power from a
laser-driven single-cycle THz source demonstrated so far, using optical
rectification in the titled pulse-front geometry in cryogenically cooled
lithium niobate, pumped by a commercially available 500 W ultrafast thin-disk
Yb-amplifier. We study repetition rate dependent effects in our setup at 100
kHz and 40 kHz at this high average power, revealing different optimal fluence
conditions for efficient conversion. The demonstrated sources with multi-100 mW
average power at these high repetition rates combine high THz pulse energies
and high repetition rate and is thus ideally suited for nonlinear THz
spectroscopy experiments with significantly reduced measurement times. The
presented result is a first benchmark for high average power THz time domain
spectroscopy systems for nonlinear spectroscopy, driven by very high average
power ultrafast Yb lasers.
