Paper detail

High-repetition-rate terahertz and ultraviolet radiation for high-throughput ultrafast electron diffraction

Scaling femtosecond terahertz (THz) and ultraviolet (UV) sources to high repetition rates is essential for high-throughput ultrafast spectroscopy and imaging applications. Yet, their efficient generation at high average power remains limited by thermal effects, phase-matching constraints, and material damage. Here, we demonstrate broadband THz and UV generation driven by a common Yb:KGW laser operating from at 40 - 600 kHz. THz radiation is produced by optical rectification in stoichiometric MgO:LiNbO$_3$ using a line-focus geometry, yielding single-cycle pulses of 55 - 92 nJ energy with peak electric fields of 37 - 90 kV/cm. Electro-optic sampling and beam-quality measurements reveal tunable control between central frequency, bandwidth and field amplitude by translating the generation region transversely within the crystal. Using shorter pump pulses preserves THz conversion efficiency, while longer pulses at 100 kHz reduce THz output by up to a factor of four due to cumulative thermal effects. Femtosecond 257.5 nm UV pulses are generated by cascaded fourth-harmonic generation in $β$-barium borate with conversion efficiencies exceeding 10% at 40 kHz and stable operation up to 600 kHz. These results demonstrate a compact, thermally robust platform for high-average-power nonlinear conversion and are directly relevant to next-generation high-repetition-rate ultrafast electron diffraction and spectroscopy systems.