Abstract

The wide application possibilities of terahertz time domain spectroscopy (THz-TDS) has led to a rapid progress in the performance of table-top phase-stable, few-cycle THz sources, which keep setting new records in terms of bandwidth, tunability and pulse energy. One aspect, however, that has so far remained difficult to tackle is their average power, which typically remains in the µW range. This is because using traditional ultrafast driving lasers such as Ti:Sa lasers, an inevitable compromise between pulse energy and repetition rate needs to be met. However, many applications suffering from low signal-to-noise-ratio and long measurement times, both in linear and nonlinear spectroscopy, would largely benefit from having both high-field and high-repetition rate – i.e. high average power. Increasing the average power of THz-TDS sources calls for higher driving powers than provided by commonly used Ti:Sa lasers. Yb-doped, diode-pumped ultrafast solid-state lasers with power levels from hundreds of watts up to several kilowatts with sub-ps pulse duration are now readily available, however THz generation with more than a few tens of watts remains widely unexplored. In a recent proof-of-principle experiment, we demonstrated THz generation by optical rectification (OR) using a one-box Yb:YAG modelocked thin-disk oscillator with an average power of more than 100 W [1]. We obtained a THz power of 78 µW, mainly limited by the long 580 fs pulses of our laser. Here, we increase the conversion efficiency by an order of magnitude by using 88 fs long pulses nonlinearly compressed in a multi-pass cell (MPC) [2]. We measure an average power of 1.35 mW, which is to the best of our knowledge the highest THz power so far achieved at MHz repetition rate, improving the current state-of-the-art by more than a factor of 4 [3].

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