Effect of Cumulative Gas Hydrodynamics on Two-Colour Air-Plasma Terahertz Sources at High Repetition Rates Up to 100 kHz

Terahertz (THz) air-based two-colour plasma sources are a popular method for various applications demanding ultra-broadband THz spectra, based on the most ubiquitous non-linear medium: ambient air. In this scheme, THz emission predominantly stems from a photocurrent of free electrons in a laser-driven air plasma [1]. To efficiently drive multi-photon absorption or tunnel-ionisation, high peak powers on the order of 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup>–10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> W are required. Traditionally, regenerative amplifier systems operating at 1 kHz have been the workhorse for such THz sources. The recent progress in the development of novel ultrafast laser sources based on ytterbium (Yb), alongside non-linear temporal pulse compression techniques, however, nowadays enables us to reach these peak power levels at unprecedented repetition rates of 100s of kHz, opening the door for high-repetition-rate THz air photonics. At these high repetition rates, the short inter-pulse time difference on the μs-timescale becomes relevant and can affect the gas hydrodynamics and THz generation [2], [3]. So far however, only a few experiments have been reported in this direction [2], [4], and no experimental quantitative evidence was presented to guide the design of such high-repetition-rate two-colour air-plasma THz sources. Here, we present an experimental correlation between the optical-to-THz efficiency and the cumulative gas dynamics at up to 100 kHz of repetition rate. Our observations are found in approximate agreement with a one-dimensional photocurrent model, verifying the effect of the cumulative hydrodynamics at repetition rates of ≥20 kHz. In optimized conditions, up to 2.8 mW average power was achieved with only a minimal decrease of conversion efficiency at 100 kHz.